composite structures

ECCS CECM E K S

EUROPEAN CONVENTION FOR CONSTRUCTIONAL STEELWORK CONVENTION EUROPEENNE DE LA CONSTRUCTION METALLIQUE E U R O P A I S C H E K O N V E N T I O N F U R S T A H L B A U

ECCS - Joint Committee on Composite Structures

I 1 Composite Structures

1981 No 28

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior permission of the Copyright owner:

ECCS General Secretariat CECM EKS

ECCS assumes no liability with respect to the use for any application of the material and information contained in this publication.

AV. Louise, 326, bte 52 B - 1050 BRUSSELS (Belgium)

Composite Structures

European Convention for Constructional Steelwork Convention Europbenne de la Construction Mktallique EuropaisctpKonvention fur Stahlbau

prepared by the Technical General Secretariat of the ECCS \ .

THE CONSTRUCTION PRESS LONDON AND NEWYORK

All rights reserved. No part of this publication may be .

reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior permission of the Copyright owner :

ECCS' General Secretariat, CECM A V . Louise 326, bte 5 2 EKS B - 1 0 5 0 Brussels (Belgium)

ECCS assumes no liability with respect to the use for any application of the material and information contained in this publication.

Contents

Section 1

Section 2

Section 3

Section 4

Section 5

Section 6

Section 7

Section 8

Introduction

General 1.1 Scope; 1.2 Assessment of safety Definitions and symbols 2.1 Definitions; 2.2 Symbols Design - general 3.1 General; 3.2 Limit states; 3.3 Representative values of actions; 3.4 Properties of materials; 3.5 Method of partial coefficients - general; 3.6 Design for the ultimate limit states; 3.7 Design-for the serviceability limit states; 3.8 Static equilibrium; 3.9 Prestressed structures; 3.1 0 Design requirements for composite beams Analysis of structures 4.1 General; 4.2 Effective span; 4.3 Stability; 4.4 Distri- bution of bending moments and vertical shear forces at the serviceability limit state; 4.5 Distribution of bending moments and vertical shear forces at the ultimate limit state

Analysis of cross-sections 5.1 General; 5.2 Definitions; 5.3 Compact beams - ultimate limit state; 5.4 Slender beams - ultimate limit state; 5.5 Serviceability limit state Design of the shear connection - general 6.1 General; 6.2 Limit state requirements; 63 Properties of shear connectors; 6.4 Design strength of shear connectors; 6.5 Detailing of shear connection; 6.6 Tests on shear connectors; 6.7 Friction grip bolts Design of the shear connection - ultimate limit state 7.1 Critical cross-sections; 7.2 Maximum loads per connector; 7.3 Longitudinal shear; 7.4 Complete shear connection; 7.5 Partial shear connection; 7.6 Transverse reinforcement Design of the shear connection - serviceability limit state 8.1 Longitudinal shear; 8.2 Maximum loads per connector - static loading; 83 Design requirements - static loading; 8.4 Design for fatigue

.

5

9

11

17

37

45

61

95

109

3

Sectign 9

Section 10

Section 1 1

8

Section 12

Section 13

Section 14

Section 15

Section 16

Section 17 Section 18

4

Temperature, shrinkage and creep 9.1 Temperature effects; 9.2 Shrinkage and creep; Control of cracking 10.1 General Deflect ions 1 1.1 General; 1 1.2 Calculation of deflections; 11.3 Deflections of simply supported beams with incomplete connection; 1 1.4 Limitations on deflections Prestressing in composite construction 12.1 General; 12.2 Methods of prestressing; 12.3 Degree of prestressing; 12.4 Limit state requirements; 12.5 Service- ability; 12.6 Ultimate limit state; 12.7 Control of cracking Vibration 13.1 General; 13.2 Beams for buildings; 133 Beams for bridges Composite beam with precast slab 14.1 General; 14.2 Joint between steel beam and concrete slab; 143 Shear connection; 14.4 Transverse reinforcement; 14.5 Concrete deck as diaphragm; 14.6 Shrinkage and creep

Composite floors with profiled steel sheet 15.1 Scope; 15.2 Materials; 15 3 Design methods - shuttering; 15.4 Design and testing of composite slabs

Composite columns 16.1 Scope; 16.2 Materials; 16.3 Composite column cross- sections; 16.4 Loadcarrying capacity analysis; 16.5 Design method; 16.6 The need to provide mechanical shear connection; 16.7 Serviceability limit state Framed structures for buildings Workmanship and construction 18.1 Responsibility; 18.2 Sequence of construction; 18.3 Stability of steelwork; 18.4 Support conditions during construction; 18.5 Temperature effects during construction; 18.6 Anchorage of prestressing cables; 18.7 Construction accuracy and quality control of materials; 18.8 Shear connectors; 18.9 Recast concrete slabs forming the flanges of composite beams; 18.10 Composite floors with profiled steel sheets; 18.1 1 Construction of columns

113

117

119

123 :

127

129

133

149

175

177

Introduction ‘ .

The “Joint Committee on Composite Structures’’ was formed in 1971 under the auspices of the Liaison Committee of International Associations for Civil Engineering, with the active participation of the following organisations: ’ 3 VV

- Euro-International Committee for Concrete (CEB) - European Convention for Constructional Steelwork (ECCS) - International Federation for Prestressing (FIP) - International Association for Bridge and Structural Engineering (IABSE).

Its essential task was to prepare a technical document for the design of composite (steel and concrete) structures and structural parts, apt to be used as a common basis or reference for national and international codes or specifications.

The activity of this Committee was first devoted to a preliminary examination and discussion of the current situation and practice in the various countries and to the survey of basic aspects of this technical field, with due consideration to recent orientations and research findings. This activity has resulted in an (unpublished) internal report.

The second phase of the activity has been concentrated on the drafting of this “Model Code”, which has been very carefully prepared thanks to the active participation of most Committee members, including leading authorities in this field with similar responsibilities in their own countries.

The Committee has endeavoured to cover the basic aspects of the practical design of composite structures in agreement with the latest knowledge resulting from research and constructional practice, by constantly keeping in mind the need to pre- serve an openness for further developments and progress in knowledge and practice.

This Model Code has been prepared in consistency with the Recommendations of the participating international bodies for steel structures and for reinforced and prestressed concrete structures, to the extent compatible with the specific character and behaviour of the composite material.

Particular attention has been given to the application of the general principles of structural safety in consistency with the provisions adopted for steel and concrete structures separately, but by considering the composite material as a distinct one.

5

The code format chosen for this document is intended to facilitate its use for and its conversion into national or international official rules, as well as to allow its adoption as a basis for the practical design of composite structures for which this may be authorised, eg, for certain international contests.

Thanks are due to the members of this Committee for the considerable work performed in the full sessions held twice every year as well as in the Task Groups formed for the drafting of various parts of this document. Special mention is due to the members who have assumed responsibilities as Task Group Chairmen and/or as reporters for specific chapters: Messrs Breitschaft, Dittmann, Dowling, Johnson, O’Leary, Roik and Stark. These acknowledgements would be incomplete without a mention of the outstanding contribution of Professor Johnson to the advancement and the co-ordination of the whole work.

h s t but not least, homage must be expressed to the memory of two Committee members, now deceased, who have significantly contributed to the success of this action: Yves Guyon and Paul Lorin.

by the CEB-ECCS-FIP-IABSE Joint Committee is now released to the competent bodies and professionals, as a step towards international unification and technical

making authorities and thus fulfil its role within the framework of documents governing structural engineering practice.

Commit tee.

With the above remarks, the Draft Model Code for Composite Structures prepared

I progress, with the hope that it will meet the attention of code and specification-

Any possible comments and remarks on its contents will be welcomed by the

D Sfintesco Chairman, Joint Committee on Composite Structures

6

. . .

JOINT COMMITTEE ON COMPOSITE STRUCTURES

Arch, W H Avram, C* Badoux, J C Batanero, J Breitschaft, G Buckby, R J Chiorino, M A Crisinel, M Delesques, R Dittman, G Dobruszkes, A Dowling, P J Dubas, P Godfrey, G B Huber, K Janss, J Johnson, R P Kakko, H Martinez Calz6n Meseguer, A G* - O'Leary, D C* Roik, K Rowe, R E Saillard, Y Sfin tesco , D (Chairman) Siebke, H Sontag, H Stark, J W Trost, H Winand, A Walfel, E Y a m , L C P * = corresponding member.

United Kingdom Rumania Switzerland Spain Germany United Kingdom Italy Switzerland France Germany Belgium United Kingdom Switzerland United Kingdom Switzerland Belgium United Kingdom Finland Spain Spain United Kingdom Germany United Kingdom France France Germany Germany Netherlands Germany Belgium Germany United Kingdom

7

COMMENTARY

I 1.1 I

I

I The aim of these recommendations is to provide comprehensiv design meth I

~

for composite members in buildings and bridges. It is recognised that for certain d

structures not all the recommendations are applicable. For beams, guidance is given in Clause 3.10, in the form of Clause references, on the recommendations that should be followed, depending on the type of structure and its intended use.

1.2 This semi-probabilistic approach, the Level 1 method of Volume I, is charac-

terised by the use of partial safety factors applying to actions, action effects, and resis tances.

8

I -

RECOMMENDATIONS

Section 1. General

1.1 SCOPE These recommendations apply to structures and members (beams, slabs, or

columns) consisting of a steel component and a reinforced or prestressed concrete component mechanically interconnected so as to act together to resist the load. Recommendations are given for composite floors with profiled steel sheets and for beams with haunches, but not for encased composite beams.

The steel component may be either a rolled or a fabricated section. Concrete may have normal-density or lightweight aggregate.

The recommendations refer generally to concrete cast in situ. Composite beams with precast concrete slabs are considered in Section 14, and prestressed composite structures in Section 12.

1.2 ASSESSMENT OF SAFETY Structural safety is treated according to Volume I of the Joint Committee on

Structural Safety (JCSS) of the International System of Unified Standard Codes, which applies to all structural materials and all types of construction. It is referred to here simply as ‘Volume I.’ Limit state design philosophy is used, ahd safety is assessed by a semi-probabilistic approach.

To determine the numerical values, code-making authorities, master builders or consulting engineers should consider each particular case, taking account of the general principles given in Volume I and the more detailed recommendations of Section 3.

9

COMMENTARY

...

Degree of interaction In practice some slip will always occur and the term f i l l interaction is used

where it has been shown that the effects of slip between the concrete flange and the steel beam can safely be neglected in the design.

Degree of shear connection The term ‘partial’should not be considered to imply in any way that the con-

nectors are unsatisfactory for the purpose for which they are designed. The use of partial connection is of interest where the full bending strength of the section need not be fully utilised, for example where the size of the steel member is governed by the load carried by the steel beam alone in unpropped construction, or where the size of the member is determined by serviceability criteria rather than strength. The definition of complete connection is equally applicable to slender or compact beams, whether propped or unpropped during construction.

10

RECOMMENDATIONS

Section 2. Definitions and symbols

2.1 DEFINITIONS

to fulfil the function for which it was designed.

unable to sustain any further increase in load.

A limit state is a condition beyond which a structure or part of a structure ceases

The ultimate limit state denotes the state at which any part of the structure is

The serviceability limit state denotes the state when remedial action is necessary

Partial safety factors are the factors applied to the characteristic loads, prestress-

to enable the structure to continue to fulfil its design function.

ing forces, imposed deformations and strengths and properties of materials to take account of possible unusual increases in load or deformation beyond those considered in deriving the characteristic values and possible variations in material strength between the characteristic strength and the strength of the material in the actual structure.

The design actions and strengths shall be determined in accordance with Section 3 and international or national Codes for nominal or characteristic values of permanent and variable actions and strengths of materials.

Degree of interaction

steel beam.

flange and the steel beam, and so causes a discontinuity of strain that has to be taken into account in the analyses.

Degree of shear connection in compact beams Complete shear connection is achieved in a beam that is compact (see 5.2.1)

throughout its length when the beam has a bending strength at critical cross-sections (see 7.1) that would not be increased by the addition of further connectors.

less than the number required for complete connection.

Full interaction implies that no slip occurs between the concrete slab and the

Partial interaction implies that slip occurs at the interface between the concrete

Partial shear connection occurs when the number of shear connectors provided is

11

COMMENTARY

It is important to distinguish between the following properties of the shear

- strength Pu, - deformation capacity Smm.

connection:

-Ss,ax-

Figure C2.1 Typical loadlslip relationship for connectors.

Where the shear connection is ‘%omplete’: failure will depend on the bending strength a t sections I or II as shown in Figure C2.2. Only the strength of the connectors - Pu - is of importance. Where the number of connectors provided on plane 111 is not sufficient to enable the beam to achieve its full bending strength, the connection is “partial’: In this case, the ultimate bending strength depends essentially on the shape of the load/slip diagram of the connectors, the span of the beam und the method of construction. Section IV will be critical for vertical shear in the case of short beams with relatively high loads. A t section V, interaction will occur between vertical shear and bending moment.

1 1 I I I V I

I I l l - - ! - - - I l l I

A ’

Figure C2.2

COMMENTARY

Deformation capaciw of connectors (1) For flexible connectors, the defomtion capacity must not on& satisfy (2)

below, but must also be higher, due to the additional slip that occurs before failure in beams designed with partial connection.

(2) For all stiff connectors, the deformation capacity must be large enough to permit redistribution of the longitudinal shear in beams with complete connection to the extent that the mean load per connector at longitudinal shear failure is not less than its design strength.

The deformation capacities required for both are different and it is important to distinguish between the two requirements.

Load

pu Flexible connector

Stiff connector

Figure C2.3 Typical load/slip relationship for flexible and stiff connectors.

Previous page is blank

14

RECOMMENDATIONS

Deformation capacity of connectors (1) Shear connectors may be considered as flexible if their deformation capacity is

such that at the ultimate limit state sufficient slip can occur between the concrete flange and the steel beam without reduction of shear strength to justify the assumption that the connection behaves in an ideal elastic plastic manner.

Headed stud connectors of the proportions specified in Section 6.2.2 may be considered as flexible.

(2) All other types of connector should be regarded as stiff unless shown by tests or analysis to satisfy the definition of flexible connectors given above.

2.2 SYMBOLS The symbols used in this draft are generally consistent with the recommenda-

tions given in Draft International Standard ISO/DIS 3890, August 1975. Symbols concerning properties and behaviour of concrete, reinforcement and

prestressing steels are consistent with the Model Code for Concrete Structures, 3rd Draft (Bulletin 117 E - CEB).

Symbols concerning properties and behaviour of structural steels are consistent with the 1978 Europeah Recommendations for Steel Construction.

15

~ _____ __ ~

- -

COMMENTARY

3.1 The recommendations refer to existing methods such as ‘elastic design ’and

‘plastic design They can be applied only by qualified engineers who know the assumptions adopted in these methods, and can assess which sources of error can safely be neglected.

methods of calculation; but the engineer must then prove the reliability of the safety assessment. This proof is not required when the present recommendations are followed.

Other approaches may also be used, such as direct experimentation or different

I 3.2.1 Established inelastic methods of checking the ultimate load caving capacity of

a ) a defined degree of redistribution of bending moments, or of bending and shear stresses, due to yielding of some parts of the structure, or

b) attainment of the design strength of shear connectors, with or without redistribution of longitudinal shear.

Local buckling or yielding has to be considered as an ultimate limit state on& i f

composite structures may be used. For example, they may be based on:

it leads to collapse.

16

RECOMMENDATIONS

Section 3. Design - general

3.1 GENERAL

the recommendations of Section 10 of Volume I, ‘The method of partial coefficients’.

This section gives general guidance on the application to composite structures of

The following Clauses are concerned with the limit state verifications, which are based on design values of actions and combinations of actions, and of strengths of materials and resistance of members, obtained by applying various rcoefficients (partial safety factors) to representative values.

3.2 LIMIT STATES According to Section 2 of Volume I, there are two categories of limit states:

ultimate limit states and serviceability limit states.

3.2.1 ULTIMATE LIMIT STATES These limit states and typical events that cause them to be reached are defined in

Clause 2.2 of Volume I. They should be checked by elastic, inelastic, or plastic analysis as appropriate.

3.2.1.1 Elastic analysis The use of elastic analysis implies that an ultimate limit state can be reached due

to the attainment, even at a single point in the structure, of a defined level of stress. Appropriate stress levels are given in these recommendations, for use in the calculation by elastic theory of the resistance of members and cross-sections.

17

COMMENTARY

3.2.1.3

Fatigue as a phenomenon affects the strength of materials and hence that of cross-sections, and is due to repetitions of actions under serviceability conditions. For practical reasons, it may be considered in design for the serviceability limit state. The load factors for ultimate load given in Section 3 are not applicable,

3.2.2 Local buckling or yielding, perhaps in conjunction with fatigue failure, may lead

to a serviceability limit state, i f the consequence could be remedied by repair.

3.3.1

more of the following apply: Most permanent actions may be represented by a unique value because one or

a ) their variability is small, b) their influence on the total action effect is small, and c) it is obvious which of two representative actions governs for all parts of the

structure. There are some actions for which two representative values (maximum and

a) non-structural permanent surfacing in certain bridges, where the minimum weight may be taken as zero; and

b) earth or liquid pressure, in structures where use of a low value may increase the severity of a limit state.

Restressing forces should normally be considered at two ages, by taking account

minimum) should be defined. For example:

of time-dependent losses, but with only one value for each age.

3.3.2.1 The loading regulations are being worked out (see for example Appendix III of

Volume I). Meanwhile, national regulations can be applied, provided that they are based on assumptions comparable with those of Appendix II of Volume I. Sug- gested values for J/ are given in Section I0 of Volume I and in the ECCS Recom- mendations for Steel Construction.

18

RECOMMENDATIONS

3.2.1.2 Order of verification

the structure, the procedure is known as first-order verification, and is acceptable only if the possible errors due to change of geometry can be considered to be negligible. Otherwise, second-order verification should be used, in which account is taken of non-linear effects of loading due to displacement of the structure.

When it is assumed in analysis that loading causes no change in the geometry of

3.2.1.3 Fatigue

Fatigue may lead to an ultimate limit state, if the consequence should be collapse. Design for fatigue in structural steel should be in accordance with the ECCS Recommendations for Steel Construction. Commentary on the design of shear connection for repeated loading is given in 8.4.

3.2.2 SERVICEABILITY LIMIT STATES Serviceability limit states are defined, with examples, in Clause 2.3 of Volume I.

3.3 REPRESENTATIVE VALUES OF ACTIONS

3.3.1 PERMANENT ACTIONS ~

G; for example: Permanent actions can usually be represented by a single characteristic value

a) self weight of the structure (calculated from the nominal dimensions and the mean density of concrete and steel, assuming a mean percentage of reinforce- men t) ,

b) weight of non-structural permanent finishes;

c) actions resulting from a practically constant water level; and d) imposed deformations due to shrinkage of concrete or unintended move-

ments of supports.

Prestress due to tendons or intended deformations imposed during construction, as defined in 3.9.2, can be represented by a single characteristic value P.

3 3.2 VARIABLE ACTIONS

3.3.2.1 Representative values The representative values Q,, of variable actions and the combination factors

3/o, J I 1 , and J / z are defined in Section 10 of Volume I. Their numerical values are fixed by the loading regulations on the basis of experience and available statistical information.

19

I

COMMENTARY

For indirect actions, the representative values are related to deformations which give rise to internal forces in the structure.

In special cases, Q k and $& may also have to be considered at the ultimate limit state.

Q, is the nominal maximum value associated with the serviceability limit state. I t is frequently the same as Qk.

The definition of the frequent value depends on the type of structure. I t is suggested that for buildings it should be taken as that which is likely to be exceeded during only S% of the design life of the structure, or which may occur at least 100 000 times during that life.

Unless a more precise estimate is made, the effects of creep (see Volume I I ) are studied under permanent and quasi-permanent actions, considered as constant loads of long duration.

For some loadings, such as wind, the minimum may be negative.

20

. RECOMMENDATIONS

For the ultimate h i t state, the maximum representative values are normally:

- the characteristic Value, Qk -the combination value, $, Qk.

For the serviceability limit state the maximum representative values are normally: - the service value, Qsr - the frequent value, $ Qk - the quasi-permanent value, $2 Qk.

The minimum value of a variable action is in general zero. For some loadings, such as water pressure, the minimum may be positive, and should then be considered when the action is favourable. For simplification, it may be possible to define a single minimum representative value.

3.3.2.2 Temperature effects .

as representative values, nominal values agreed with the client. For imposed strains and deformations due to temperature, the designer may use,

3.3.2.3 Natural forces

For special structures, the designer may determine representative values of natural forces from available statistical information, provided that this information is considered to be sufficient by the competent public authority.

3.3.2.4 Erection loads

consultation with the contractor and agreed with the client. Where necessary, nominal values for erection loads should be determined in

3.3.3 ACCIDENTAL ACTIONS These values are normally defined by the competent public authority.

3.4 PROPERTIES OF MATERIALS

should be determined in accordance with the Model Code for Concrete Structures (V.olume 11); and those for structural steel in accordance with the ECCS Recom- mendations for Steel Construction.

Characteristic and design strengths of shear connectors are given in Section 6.

The characteristic properties of concrete, reinforcing steel, and prestressing steel

21

COMMENTARY

3.4. I I t may sometimes be necessary to use separate values for concrete and for steel,

because the coefficient of thermal expansion for lightweight concrete and limestone aggregate concrete can be as low as 7 x 10" per "C

3.5 A general statement of this method is given in Section 10 of Volume I.

3.5.1 Behaviour is over-proportional when increase of an action results in a relatively

greater increase in the action effects (Figure C3.1). Superposition of effects is then not possible, and Equation (3. I ) obviously applies.

Behaviour is under-proportional when increase of an action results in a relatively smaller increase in the action effects (eg, rope net constructions, suspension bridges). The partial safety factor rf Should then be sub-divided (Equation (3.3.)).

If both types of behaviour occur within a single system it may be necessary to investigate both cases.

4

@ Linear

@ Overproportional

@ Underproportional

Figure C3.1

3.5.2 An additive safety element 6 may be: a) an additional action or geometrical imperfection, such as additive dead

b) an additional action effect such as additive bending moment at a point o f weight or shifting of bearings; or

22

RECOMMENDATIONS

3.4.1 COEFFICIENT OF THERMAL EXPANSION

be taken as 10 x 10-6 per "C. The coefficient of thermal expansion for both concrete and steel may normally

3.5 METHOD OF PARTIAL COEFFICIENTS - GENERAL

3.5.1 DETERMINATION OF DESIGN ACTION EFFECTS Account should be taken of the various combinations of actions when calcula-

ting the most unfavourable effect on each member and cross-section. The use of the partial safety factors depends on the nature of the relationship

between action effects and actions. This may be over-proportional, linear, or under- proportional. Design action effects Sd should be calculated from characteristic actions Fk, partial safety factors yf,i, and combination factors J/i using the appropriate equation, as follows:;

for over-proportional or linear behaviour,

s d = s [g(Yf,i; J/i;6fyi;Fk,i)l (3.1)

s d = [7fyiS(J/i;6f,i; Fk,i)l +6S, (3 -2)

Sd = [rf3,iS(yfi,i;J/i;6f,i;Fk,i)1+6S, (3 -3)

which for linear behaviour can be used in the form

and for under-proportional behaviour,

where

and the additive safety elements 6 f,i and 6 s are explained in 3.5.2.

'Yf1.i 'Yf3,i = 'Yf,i

3.5.2 ADDITIVE SAFETY ELEMENTS If action effects are strongly influenced by external imperfections (eg, by un-

intended eccentricity relevant to buckling or overturning, or by variation of dead weight exceeding the tolerance limits associated with the characteristic value), additive safety elements (6f i and 6s in Equations (3.1) to (3.3)) should be used instead of (or in addition to) the coefficients yf.

23

COMMENTARY

contraflexure (relevant to minimum reinforcement) or additive shear force where the calculated shear is zero (relevant to minimum shear connection).

3.6 The ultimate limit state of loss of static equilibrium is considered in Clause 3.8.

3.6.2 Various load cases are possible for an individual action; for example, the floor

A member in a building that canies the variable loads from a large area of floor, loading in a multi-storey multi-bay structure.

whether at one or several levels; can normally be designed for a variable load less than the total calculated from the unit variable load and the area of floor. This reduction is not included in Equations (3.5) to (3.7).

Equation (3.5) should be used in design for stability, and may be used with elastic or simple plastic analysis.

Equation (3.6) is suitable only for use with linear elastic analysis. Equation (3.7) is used only for special structures, such as suspension bridges. When maximum and minimum values of y, are given for a permanent action,

there are two possible methods for obtaining the most severe design condition: a) The two values of y, are used in alternate spans. This can be done by treat-

ing as a free action that part of the permanent action that exceeds the level corresponding to the minimum value of yr

b) The bending moment distribution is calculated with y, = 1, and factored by the two values of y, in turn. At each cross-section, the more severe action effect is used.

Both methods should be associated with appropriate detailing rules, particularly in relation to points of contraflexure.

When action effects are strongly influenced by the difference between two permanent actions of the same origin (eg, in balanced cantilever construction), method ( a ) should be used. This always applies to checks on static equilibrium (Clause 3.8).

24

RECOMMENDATIONS

3.6 DESIGN FOR THE ULTIMATE LIMIT STATES

3.6.1 DESIGN PRINCIPLE

action effects Sd obtained in accordance with 3.5.1 should be compared with the corresponding design resistance Rd of the member or section, obtained in accordance with 3.6.4, and Equation (3.4) should be satisfied:

At different cross-sections of the structural element under consideration, design

Sd < Rd. (3 -4)

3.6.2 FUNDAMENTAL COMBINATIONS OF ACTIONS The symbolic Equations (3.5) to (3.7) give the combinations of actions of

different origin that should be considered. Checks on the static equilibrium of the complete structure should be in accordance with 3.8.

Design action effects should be calculated as follows: for over-proportional or linear behaviour,

Sd = s ['YgG t 'Ypp t Tq(Qi k t

Sd = 'Ygs(G) t 'Yps@') + 'YqS(Qik t

$ o i Q d I , (3.5) i > i

which for linear behaviour can be used in the form

J/oiQik), (3 -6) i>i

and for under-proportional behaviour,

Sd = 'Yf3 S [Ygi G t Ypi P + 'Yqi (Qi k t iF1 J/oi Qik) I, (3.7)

where - - Yf3 'Ygi - Tg, 7f3 7p1 = Yp, and 'Yf3 'Yq1 - 'Yq,

P is as defined in 3.9.2, and Ql k is the basic variable action in the combination.

I Each variable action should be considered in turn as the basic action except those for which it is obvious that the resulting combination cannot be critical.

25

~~ ~~~

COMMENTARY

3.6.2.1

of one origin, using the ‘unfavourable’or ‘faVourable’value of 7, (Table 3.1) as appropriate.

Using the values for yfi given in Volume I and yf as given in Table 3. I , yf3 is found to lie between I . 0 7 and 1.125, depending on the nature of the action.

The value yg = 1.35 is a mean value. Clause 10.3.1 of Volume I indicates when variations from it may be justified.

For vectorial action effects, all y factors applied to any favourable component should be reduced by 20%.

For the condition of erection, special consideration should be given to the choice of partial safety factors.

In design to Clause 3.6, method (b) above is normally used for permanen t actions

3.6.2.2

It will rarely be necessary to include more than two in addition to the permanent action.

For floor loading in buildings it4s generally adequate to consider two cases on&: adjacent spans loaded and alternate spans loaded. These are deemed to give the most unfavourable conditions.

It is normally obvious in design how many individual actions should be considered,

For Equations (3.6) and (3.7) the simplification is analogous.

3.6.3 For accidental combinations, the relevance of the $-values needs careful con-

sideration. Only that part of each variable action likely to be present a t the same time as the accidental action or situation needs to be included, so that engineering judgement should be used.

26

. RECOMMENDATIONS

3.6.2.1 Partial safety fictors

Values for the partial safety factors yg, y,, and yq are given in Table 3.1. These include the factor yf3.

Combination Effect of the action 'Ys

Fundamental Unfavourable 1.35 1.2 1.5 (Clause 3.6.2) Favourable 1 .o 0.9 0 < rq < 0.9 Accidental Unfavourable 1.1 1 .o As relevant (Clause 3.6.3) Favourable 0.9 1 .o As relevant

Table 3.1 Numerical values for 'yf.

3.6.2.2 Simplified method

method is possible, in which Equation (3.5) is replaced by: For the majority of structures for buildings and for some bridges a simpler

n

i= I sd = s (1 -35 G + Tq Qik) (3 -8)

and, when minimum dead load is more critical,

(3 -9) n

Sd = S (1.OG + yq I= Qik) 1= 1

where yq = 1.5 for n = 1 and ys = 1.35 for n 2 2. Prestressing actions, when present, are treated as in Equations (3.5) to (3.7).

3.6.3 ACCIDENTAL COMBINATIONS OF ACTIONS AND ACCIDENTAL SITUATIONS

An accidental combination consists of only one accidental action Qa, accom- panied by the permanent actions and the appropriate variable actions. The design values of these should be taken as $ Qlk for the basic variable action and J/zi Qik for the others, thus:

Sd = SLY$ + TaQa + 'Yq ($1 Qlk + iFl 9 ~ i Q i k ) I (3.10)

Unless other values are specified, the partial safety factors should be:

rg = 1.1 or 0.9, whichever is the less favourable, and

This clause is applicable also to accidental situations in which there is no acci-

- 'Ya - Tq = 1.0.

dental action.

27

COMMENTARY

3.6.4 The resistance R , can also be determined by direct experiment. In Equation (3.11) f d designates equally the design strength of steel or concrete,

The value yc = 1.5 is based on the assumption that the concrete is mixed on the in tension or in compression.

site or in the works, and that its production is controlled in accordance with Section 23 of Volume II. If the standard of control is lower, yc should be increased and if it is higher, 'yc may be reduced (note to Clause 6.4.2.3 of Volume II).

When assessing deformations of the structure as a whole to treat buckling, it may be more accurate to use a stress-strain relation for concrete related to the mean strength, as proposed in a note to Clause 6.4.1 of Volume II.

For firther information on ?a, particularly where there is danger of instability, reference should be made to the European Recommendations for Steel Construc- tion.

In the absence of better information, ym for profiled steel sheeting should be taken as equal to ?a.

3.7.1

structure. Appropriate requirements for each stmcture should be defined by the engineer responsible for the design in agreement with the client (for example, limiting crack widths or strain limits that allow for the behaviour of the finishes and adjacent elements).

Where cracking is to be prevented, a check in accordance with Equation (3.4) may be appropriate, taking account of the partial sa f ev factors given in 3.7.2 and 3.7.3.

The funct,mal requirements can vary considerably depending upon the type of

3.7.2 The relevant combinations of actions are as follows:

in frequent -' G ' + P k + Qlk (Or Qser) + x ($ i i Qik) r> 1

frequent

quasi-permanent : G + p k + ($zi Qik) i> I

(3.12)

(3.13)

(3.14)

3.7.3

tensile strength of concrete; for example, in partially prestressed mem ben. The value yc = 1.3 is used in calculations for cracking that take account of the

28

RECOMMENDATIONS

3.6.4 DESIGN RESISTANCE The resistance Rd is determined as a function of the design stress-strain curves

obtained from the characteristic curves by dividing all stresses by rm. In particular, the design strength of a material is defined by:

fd = fk /7m (3.1 1)

where fk is the characteristic strength of the material and Tm is the partial safety factor given in Table 3.2. Values for shear connectors are given in 6.3.1 and 6.7.2.1.

Reinforcing steel and Structural prestressing steel steel Combination Concrete

T C 7s Ta ~~ ~

Fundamental 1.5 Accidental 1.3

1.15 1 .o* 1 .o 1 .o

*Provisional value, given in the European Recommendations for Steel Construction, 1978.

Table 3.2 Numerical values for Tm.

3.7 DESIGN FOR THE SERVICEABILITY LIMIT STATES

3.7.1 DESIGN PRINCIPLE The functional requirements are aimed principally at the limitation of cracking

and of strains in the elements of structures in service. Direct comparisons are made between calculated values and the relevant criteria and associated limitations. Thus, appropriate checks should be made that:

a) calculated stresses or crack widths in concrete do not reach certain specified

b) strains or deflections derived from analysis of the structure are less than values, and

acceptable limiting values.

3.7.2 COMBINATION OF ACTIONS

assessed and on whether i t is related to the service, frequent, or quasi-permanent values of the various actions.

The combinations of actions to be considered depend on the criterion being

The coefficient rf should be taken as 1 .O.

3.7.3 PROPERTIES OF MATERIALS

on characteristic values, with Y~ = 1. The design properties of materials are based, as appropriate, on mean values or

In certain checks for the cracking of concrete, rC is taken as 1.3.

29

COMMENTARY

3.8

forces are induced by staticall’ indeterminate effects.

in to account.

Prestress can normally be imored when checking equilibrium, except when

If second-order effects are important, strains of the ‘rigid’ body should be taken

3.9. I Beams prestressed by method (a) are rarely subjected to statically indeterminate

actions due to prestressing (eg, changes in forces at supports). There are discontinui- ties in strain in the section which should be considered as action effects in beanis where the steel strain nowhere exceeds the yield strain (ie, for all beams at the serviceability limit state, and for slender beams also at the ultimate limit state if elastic analysis is used). In beams when the steel strain exceeds the design yield strain, the strain distribution due to this method of prestressing should be considered - i f relevant - in the determinahbn of the resistance of the section.

The use of method (b ) creates statically indeterminate prestressing actions, which have always to be considered as actions or action effects.

The use of method (c ) always creates statically determinate effects (due to the different strain in the tendons and in the concrete). Statically indeterminate actions occur only in continuous beams and frames. The statically determinate part should be treated as for method ( a ) and the statically indeterminate part as action or action effect as for method (b).

The use of method ( d ) creates statically indeterminate actions, which should be treated as for method (b).

I

30

RECOMMENDATIONS

3.8 STATIC EQUILIBRIUM The checking of static equilibrium relates to the stability of the whole structure

considered as a rigid body. The check can also be applied to a structure with some elements removed, to test its stability in a simulated damaged condition.

The loads to be considered are the absolute values of: a) permanent stabilising loads G, (and PS)¶ b) permanent non-stabilising loads Gn (and Pn), and c) variable nonstabilising loads Qik. The following condition should be satisfied:

Sd = S [0.9Gs- 1.1Gn- 1.5 (Q1k t Z 3/oi Qik)] 2 0 (3.15) i > I

3.8.1 SIMPLIFIED METHOD

method is possible, in which Equation (3.1 5 ) is replaced by: For the majority of structures for buildings and for some bridges a simpler

sd=s [0.9Gs- 1.1Gn- 1.SQIk- 1.35 Z: Qik] 2 0 (3.1 6) i > I

3.9 PRESTRESSED STRUCTURES

3.9.1 SCOPE

Consideration is given to the following methods of prestressing composite

a) Stressing the steel beam by means of preliminary supports and removing these supports after hardening of the concrete.

structures.

b) Raising the inner supports of a continuous beam before casting of the concrete¶ and subsequently lowering them to their final level.

c) Prestressing the concrete part of a composite section by-tendons that are bonded to the concrete by grouting after stressing.

d) Prestressing the structure by unbonded internal or external cables (eg, cable

Each of these 'four methods of prestressing can be applied to a structure separ-

. stayed bridges).

ately or in conjunction one with another.

31

COMMENTARY

3.9.2

simple plastic theory. For inelastic analysis, actions due to prestressing may be multiplied by 7,. For elastic analysis, the action effects may be multiplied by yp .

Actions due to prestressing can be neglected if design is based on analysis by the

Loss of prestress is considered in 3.3. I .

3.9.3

given in this code.

permanent load acts in an unfavourable way and the prestress in a favourable way, and vice versa.

For simplicity, the same values of y p are given for prestressing by tendons and by imposed deformations, although different factors affect the variation of prestress:

- for prestressing by tendons: deviations in the position of the tendons and in their draw-in a t the anchorage, and variations in the stiffness of the coricrete part of the section due to cracking;

.- for prestressing by imposed deformations: deviations in the deformation or in the jacking force (depending on the method of control) and in the stiffness of the composite member, and variations in the stiffness of the concrete part due to cracking.

No detailed values of partial safety factors for prestressing by method ( d ) are

The prestress P is independent of the permanent load G, so it is possible that the

The values yp giben in Table 3.1 are primarily for trial and comparison calculations. Other values can be chosen, i f these are justified by more sophisticated methods. Further study is needed for a better understanding of the real relationships.

32

RECOMMENDATIONS

3.9.2 DEFINITION O F PRESTRESS

The symbol P in Equations (3.5) to (3.7) represents the characteristic value of the statically indeterminate actions due to the prestressing of tendons and to deformations imposed on the structure by jacking or by deliberate movement of supports. The statically determinate part of prestressing should be considered as action as well, if the strain in the steel does not exceed that corresponding to the characteristic yield stress.

3.9.3 PARTIAL SAFETY FACTORS Partial safety factors yp for the actions or action effects due to prestressing by

methods (a) to (c) are given in Table 3.1, for use in Equations (33) and (3.6). Imposed deformations may or may not be relevant at the ultimate limit state,

depending on their origin or cause. If they are, the partial safety factors given in Table 3.1 may be applied to their actions or action effects.

3.10 DESIGN REQUIREMENTS FOR COMPOSITE BEAMS

must be considered in the design of composite beams whether propped or unpropped during construction.

Where composite beams of compact cross-section are subject to repeated loading, Clause 5.5 and Section 8 are applicable, in addition to those given in column (1) of Table 3.3.

Where beams of slender cross-section are subject only to predominantly static loading, the requirements of column (2) Table 3.3 are applicable but the requirements of Section 8 need not be considered.

All parts of steel structure, other than those which are prevented from buckling by the shear connection to the concrete flange, should be checked for resistance to buckling at all stages during construction (including prestressing where appropriate) in accordance with the ECCS Recommendations for Steel Construction.

The entries in Table 3.3 refer to particular clauses in this Code of Practice which

33

COMMENTARY ' I

34

RECOMMENDATIONS

Slende mess limit a tions Distributions of bending moment and vertical shear force at ULS Distributions of bending moment and vertical shear force at SLS Stability Analysis of cross-section at ULS Analysis of cross-section at SLS Shear connection - general Design of shear connection at ULS Design of shear connection at SLS Temperature, shrinkage and creep Crack control Deflections Prestressing Vibration Workmanship and construction

Compact * beams Slender beams

repeated loading

subject to subject to

static loading

(1 1 5.2.1

4.5.1 or 4.5.2

- 3.6,4.3, 18.3

5.3

6 7

9 10 11 12 13 18

-

-

5.2.2

4.5.1

4.4 3.6,4.3, 18.3

5.4 5.5 6

see 6.2.1 8 9 10 11 12 13 18

Note: ULS denotes ultimate limit state. SLS denotes serviceability limit state. *As defined in 5.2.1.

Table 3.3 Design requirements for composite beams.

35

COMMENTARY

4.2

The effective span of a simply-supported or continuous beam may be taken as: 1. Longitudinal beams. The distance between the centres of bearing plates or

rocker pins.

2. Cantilevers. The effective length of a cantilever should be taken as its length from the free end to the face of the support plus half its effective depth except where it forms the end of a continuous beam, where the length to the centre of the support should be used.

I I 4.3.1

I as appropriate.

, I I

The EECS Recommendations for elastic or for plastic design should be applied

36

RECOMMENDATIONS

Section 4. Analysis of structures

4.1 GENERAL The concrete slab may be considered to act simultaneously as the flange of a

composite beam and as a slab spanning in a direction transverse to the axis of the steel beam.

4.2 EFFECTIVE SPAN

the restraint (torsional or flexural) afforded to the ends of such members. In calculating the effective span of a member proper account should be taken of

4.3

4.3.

STAB1 LITY

LATERAL BUCKLING In a composite beam, lateral buckling of the upper flange of the steel beam is

In negative (hogging) moment regions however, lateral torsional buckling of the effectively prevented by the shear connection to the concrete slab.

compression flange must be controlled in accordance with the European Recom- mendations for Steel Construction.

4.3.2 COMPOSITE SECTIONS

should be investigated at the ultimate limit state by second order theory, taking proper account of the rotational and directional restraint afforded at the ends of the member.

Where a composite beam resists significant axial compression, the overall stability

37

COMMENTARY

4.4 Shear lag has little effect on the distribution of bending moments and vertical shear forces in continuous beams, so it is permissible to use either the actual flange breadth or the effective flange breadth (Clause 5.1 .I) in stiffness calculations. Clearly, the relative stiffness of adjacent spans does not change very much whichever value is used for the breadth of the concrete flange. Alternatively the values given in CEBIFIP Recommendations may be used.

2. fck is the characteristic cylinder strength of the concrete in compression.

3. As an alternative, the maximum design sagging moments in each span adjacent to each support so affected may be increased by 30 fctlfc. per cent, to allow for redistribution of moments caused by transverse cracking of the concrete at the support. In this case the more unfavourable of the two moment distributions at any section should be taken for the particular aspect of serviceability being considered.

4.5.1

factored load (with yf > 1 .P) is applied initially, as would occur (for example) i f the actual density of a material were greater than expected. Use of yf = 1.1 for weight of steel (for example) does not imply that an extra 10% of the weight of the steel is added to the structure after it is in service.

In analysis for the ultimate limit state it should be assumed that the whole of the

38

RECOMMENDATIONS

4.4 DISTRIBUTION OF BENQINC MOMENTS AND VERTICAL SHEAR FORCES AT THE SERVICEABILITY LIMIT STATE

For slender beams subject to repeated loading, the distributions of bending moments and vertical shear forces due to loading on the composite member may be calculated by elastic theory using the elementary theory of bending and the properties of the transformed composite cross-section obtained by considering a breadth of concrete flange acting compositely with the steel section equal to either:

a) The actual breadth of the concrete flange, or b) an appropriate effective breadth assumed constant along the span. The concrete may be assumed to be uncracked and unreinforced, both longitu-

dinally and transversely. 'Account should be taken of the influence of significant transverse cracking of

the concrete over interior supports due to hogging bending moments in the longitudinal direction of the beam. The following procedure may be used.

1. The maximum hogging bending moments over the interior supports are first calculated assuming that the slab is uncracked.

2. At internal supports where the maximum tensile stress fct at the top of the slab is less than 0.1 5 fck, the influence of cracking need not be taken into account in the calculation of the bending moment distribution.

3. At each support where fct exceeds 0.15 fck, the region with fct > 0.15 fck should be determined using the moment envelope calculated with the slab assumed uncracked.

4. In this region the stiffening effect of the concrete should be neglected, and the new stiffness distribution should be used to recalculate the bending moments for all kinds of loads.

'

For prestressed composite beams, see Section 12.

4.5 DISTRIBUTION OF BENDING MOMENTS AND VERTICAL SHEAR FORCES AT THE ULTIMATE LIMIT STATE

4.5.1 SLENDER BEAMS

4.5.1 :1 General

proper account has to be taken of: In calculating the distribution of bending moments and vertical shear forces,

1) isostatic and hyperstatic effects due to creep and shrinkage of concrete, prestressing, and jacking; and

39

COMMENTARY

4.5.1.2

I f the steel beam is stressed by tendons, jacking, or loading before the development of composite action, for example by the weight,of wet concrete in unpropped I

construction, then stress resultants and stresses have to be determined separately for the steel beam, and added to those for the composite member.

4.5. I .3

Hyperstatic (secondaty) effects of shrinkage, temperature, prestressing, jacking of supports, etc, may be taken into account as calculated using elastic analysis. They may be assumed to decrease with the growth of plastified zones, and to vanish when a plastic hinge mechanism forms. Thus they need not be considered when plastic design is used.

I n general it is not required to carry out a non-linear analysis exactly. The designer has to decide in each different case what to assume in order to get a solution of the required accuracy. Reliable approximations may be used, as for example moment-curvature relationships that include the influences of local buckling, vertical shearing forces, etc.

Where the steel member carries loads prior to the development of composite action, the resulting strains due to the factored loads should be assumed to be already present in calculating the response of the composite member (of which the steel member forms a part) to the loads applied to it.

4.5.2.2

Research has shown that the limitations on steel slenderness given in 5.2.1 may not always ensure sufficient rotation capacity in continuous beams subject to heavy

40

RECOMMENDATIONS

2) the actual construction sequence and the particular loading history. Methods of prestressing and jacking are considered in Section 12.2. The analysis of the structure has to be carried out for the factored loads and the

factored actions due to prestress (as defined in 3.9.2), with respect to the most unfavourable load combinations, without any safety factors associated with creep, shrinkage and loss of prestress.

4.5.1.2 Elastic analysis

The envelopes of longitudinal moments, vertical shear forces and axial loads due to the whole of any particular combination of design loads applied to the composite section may be found by elastic analysis using the elementary theory of bending and the stiffness of the full composite section, assuming an uncracked slab of effective breadth determined in accordance with 4.4 (a) or (b).

This applies also to prestressed continuous composite beams with class I and I1 concrete members according to the CEB/FIP Recommendations; no moment redistribution due to concrete cracking in hogging moment regions need be considered.

4.5.1 3 Inelastic analvsis

Inelastic behaviour arises mainly from cracking of the concrete flange in hogging moment regions, yielding in the steel beam, and local buckling of compressed parts of the steel beam. Whether a linear elastic or inelastic analysis has to be carried out depends mainly on how the strengths of cross-sections are determined (Clause 5.4.1) and to what extent cross-sections are stressed.

Alternatively, for non-prestressed continuous composite beams, a linear elastic analysis may be carried out in which the stiffening effect of the concrete over 15 per cent of the span on each side of each internal support is neglected. The reinforcement may be taken into account.

Determination of bending moment envelopes should be based on nearly the same model of analysis as that used in calculating stresses and strains at cross- sections. For example, redistribution of bending moments should be taken into account, when strength calculations are based on a partially or fully plastified cross-section .

l

4.5.2 COMPACT BEAMS

4.5.2.1 General

For beams with compact cross-sections, elastic or plastic design may be used. For elastic analysis, 4.5.1.1 and 4.5.1.2 apply.

4.5.2.2 Simple plastic analysis

The distribution of bending moments at the ultimate limit state may be chosen arbitrarily in continuous beams, provided :

41

COMMENTARY

concentrated loads or where the end span differs significantly in length from the adjacent span (Johnson, R P, and Hope-Gill, M C Ypplicabili@ of simple plastic theory to continuous composite beams’: Proceedings of the Institution of Civil Engineers, Part 2, Volume 61, March 1976).

42

3)

RECOMMENDATIONS

the internal force resultants are in equilibrium with the most unfavourable combination of factored loads,

the steel section is compact as defined in 5.2.1, or if slender can still develop adequate rotation without loss of strength due to local buckling,

no two adjacent internal spans differ in length by more than 45% of the shorter one,

the end span is not less than 70% and not more than 115% of the adjacent span 9

not more than half of the design ultimate load for any space is concentrated within any length of Q/S, where P is the effective span.

43

COMMENTARY

5.1.1

The use of effective breadths of concrete flanges derived by elastic theory is conservative at load levels approaching collapse. A simpler alternative, for positive moment regions only, Is to use effective breadths of flange equal to L/6 on each side of thesteel web, but notgreater than half thedistance to the next adjacent web, nor, for edge beams greater than the projection of the cantilever slab ( L is equal to the length of the positive moment region and may be taken as two-thirds of the span of continuous beams). Otherwise, reference should be made to the CEBIFIP Recommendations.

5. I .2 Methods of designing composite members composed of steel beanis and solid

concrete slabs are well established. There are, however, additional considerations which arise when profiled steel sheets are employed. These considerations may depend upon the directions in which the ribs run relative to the steel beam.

b b A L b

I ” I” Case 1 Deck ribs parallel to the beam Case 2 Deck ribs perpendicular to the beam

Figure C 5.1

kl- e = depth of ribs w = mean width of concrete ribs

Figure C5.2

44 I

RECOMMENDATIONS

Section 5.. Analysis of cross-sections

5.1 GENERAL

5.1.1 EFFECTIVE BREADTH

The effects of shear lag should be considered in calculations of flexural stress and strength; for example, by the use of an effective flange breadth less than the actual breadth.

5.1.2 CONCRETE SLAB CONSTRUCTED USING PROFILED STEEL SHEETING

Tbe recommendations may be applied to composite construction of concrete slab on profiled steel sheeting connected to steel beams provided the following con- di tions are met :

5.1.2.1 General limitation - The nominal depth of ribs shall be not greater than 80 mm. - The mean width of concrete rib or haunch w shall be not less than 50 mm, but

shall not be taken in calculations as more than the minimum clear width near the top of the steel deck.

-- Cover over the ribs shall be not less than 50 mm. - The concrete slab shall be connected to the steel beam with welded stud shear

- The shear connection should be designed in accordance with 6.3.2.5 and 6.4.4. connectors with a diameter not greater than 19 mm.

5.1.2.2 Deck ribs oriented parallel to the supporting beams - Concrete below the top of the steel deck may be included in determining

section properties. - Steel deck ribs over supporting beams may be split longitudinally and

separated to form a concrete haunch. - Wien the nominal depth of steel deck is 40 mm or greater the mean width w

of the supported haunch or rib shall be not less than 50 mm for the first stud in the transverse row plus 4 stud diameters for each additional stud.

45

COMMENTARY

5.2. I

ey = # =yield strain of steel.

For the use of plastic design the bmcing requirements for compressed flanges are more restrictive than for elastic design.

46

i

RECOMMENDATIONS

5.1.2.3 Deck ribs oriented transverse to the supporting beams

- Concrete below the top of the steel deck shall be neglected in determining section properties.

5.1.3 COMPOSITE ACTION I I

I Where the cross-section is compact as defined in 5.2.1, composite action may be I assumed to exist for the whole of the loading at the ultimate limit state, even when

the steel section is unpropped during pouring of the concrete slab, provided that the shear connection is designed for the corresponding shear.

Where the cross-section is slender and therefore does not satisfy the requirements of 5.2.1 account should be taken of the effects of loads applied to the steel section prior to the development of composite action. This applies equally at the serviceability and ultimate limit states.

5.2 DEFINITIONS

5.2.1 COMPACT CROSS-SECTIONS

Cross-sections may be considered as compact when the web and compression flange possess sufficient stiffness to enable full plasticity and adequate rotation to be developed without loss of strength due to local buckling. Sections which satisfy the requirements of 1) or 2) following may be considered as compact:

(1) in simply-supported composite beams and positive (sagging) moment regions of continuous composite beams, provided the plastic neutral axis does not lie within the web of the steel section, or

(2) when the slenderness of all steel plates or sections that contribute to the strength of the members is less than the relevant limiting values for plastic design as follows:

a) the portion of the web subjected to longitudinal compression, ie, the depth of the web between the plastic neutral axis and the extreme compressive edge of the web, shall not exceed 1.1 5 f i times the web thickness.

b) the overhang of the compression flan e of an I or [ section beyond the web surface shall not exceed 0.25 sp ey times the flange thickness.

c) the width of the compression flange of rectangular hollow sections or boxes and the width of reinforcing plates between longitudinal bolt lines or weld seams shall not exceed 0 . 8 6 times the plate thickness.

d) the unbraced length of the compression flanges in hogging moment regions shall be not less than required for use of plastic design for steel structures as given in the section on plastic design in the ECCS Recommendations.

5.2.2 SLENDER CROSS-SECTIONS Slender cross-sections are those in which the steel section is not compact, as

defined in 5.2.1.

47

COMMENTARY

3) This method differs from the rectangular stress block method given in the CEBIFIP Recommendations, which for normal-density concrete uses a uniform compressive stress in the concrete of 0.85 fcklym but over a depth of 0.8 x, where x is the neutral axis depth determined from equilibrium considerations based on the strain distribution through the section. For lightweight concrete, 0.75 f&l’)in and 0.75 x are proposed.

5.3.3 Tests on compact composite beams have shown that the longitudinal slab rein-

forcement can increase the shear strength of the negative (hogging) moment region above the ultimate shear strength of the web even when simultaneously subjected to negative (hogging) bending moments exceeding the ultimate moment of the resistance of the composite section calculated by simple plastic theory. When the amount of slab reinforcement satisfies the condition given, no reduction need therefore be made for the effects of vertical shear in calculating the ultimate moment of resistance of the composite section in negative (hogging) bending, provided the vertical shear does not exceed the design ultimate shear strength of the web. Further research is needed to determine whether this assumption is applicable also in positive (sagging) moment regions, and to beams where the steel section is not symmetrical.

The bendinglshear interaction diagram proposed for negative moment regions is shown below with the test results plotted (Johnson, R Pand Willimington, R T, “Vertical shear in continuous composite beams’: Proceedings of the Institution of

48

RECOMMENDATIONS

5.3 COMPACT BEAMS’- ULTIMATE LIMIT STATE

5.3.1 ULTIMATE MOMENT OF RESISTANCE WITH COMPLETE SHEAR CONNECTION

Where the shear connection is ‘completey, as defined in 2.1 , the ultimate moments of resistance in both positive (sagging) bending, Mu , and negative (hogging) bending, M; , may be determined by simple plastic theory assuming full interaction between the concrete slab and steel beam and in accordance with the following:

Subject to the requirements of 4) below, the whole of the area of the steel member and of the longitudinal reinforcement within the effective breadth of the concrete flange is stressed to the design yield strength in tension or compression.

The strength of the concrete on the tension side of the plastic neutral axis is neglected. The area of concrete on the compression side of the plastic neutral axis is stressed uniformly to its design compressive strength, which may be taken as 0.8 fck/’Ym , where fck is the characteristic 28day cylinder strength. Where necessary, allowance should be made for the influence of vertical shear on the ultimate moment of resistance by the method given in 5.3.3.

VERTICAL SHEAR The design ultimate shear strength of a compact composite section in the absence

of bending moment should be calculated on the assumption that the effective area of the web of the steel section is stressed uniformly to its design yield strength in shear, (ie, fyk/rmd3). The effective area of the web may be taken as the product of the overall depth of the steel section and the thickness of the web. The contribution of the concrete slab and any concrete haunch should be neglected.

5.3.3 INFLUENCE OF VERTICAL SHEAR ON ULTIMATE MOMENT OF RESISTANCE

In members where the steel cross-section is symmetrical about both axes, no reduction for the effects of vertical shear need be made in calculating the ultimate moment of resistance of a composite section:

a) if the vertical shear at the ultimate limit state is less than 30% of the design ultimate shear strength of the web, or

b) in negative (hogging) moment regions when the design ultimate shear strength of the web is not exceeded, and the cross-sectional area of the longitudinal reinforcement within the effective breadth of the slab exceeds 0.1 5 times the total cross-sectional area of the steel member and the design yield stress for this reinforcement is not less than that for the steel member.

.

If these provisions are not met allowance should be made for the influence of vertical shear on the ultimate moment of resistance.

49

COMMENTARY

Civil Engineers, London, 53,'189-205, September 1972). The ultimate shear strength of the web in the absence of bending moment, Vo, has been calculated in accordance with 5.3.2.

M - M O

1 .o

t If 5.3.3(b) is not satisfied, and for plain steel I-sections

A A

1 , A y A ~ If 5.3.3(b) is satisfied

1- - \

I I

I I I I

I 1 1 ! 1 I 1 V -

0.33 1 .o vo

Figure CS.3 Bending shear interaction for negative moment regions.

5.4.1.1

Special attention should be paid to the isostatic (primary) effects of shrinkage, temperature, prestressing, jacking of supports, etc. In linear elastic calculations, they are fully effective. In fully plastified cross-sections they may be assumed to be zero.

50

RECOMMENDATIONS

Normal and shear stresses may be assumed to be distributed over the section in any conventional manner that is statically admissible and does not violate the von Mises yield criterion.

5.3.4 ULTIMATE MOMENT OF RESISTANCE OF COMPACT BEAMS WITH PARTIAL SHEAR CONNECTION

Where the design loadings are such that the required design ultimate bending moment for a sagging (positive) moment region, M,, is less than the ultimate moment of resistance Mu calculated in accordance with 5 3 .l, partial shear connection can be used provided that the conditions given in 7.5 are satisfied. The degree of shear connection provided can be so chosen, in accordance with 7.5, that the calculated ultimate moment of resistance is greater than or equal to M,.

5.4 SLENDER BEAMS - ULTIMATE LIMIT STATE

5.4.1 ULTIMATE MOUENT OF RESISTANCE

5.4.1.1 General

In calculating section properties, the effective breadth of the concrete flange may be determined in accordance with 4.4(b). The modulus of elasticity should be in accordance with 3.4, and the tensile strength of concrete should be neglected. Fully anchored reinforcement and prestressing steel fully bonded to the concrete member, placed within the effective breadth, may be taken into account when calculating section properties.

51

I

COMMENTARY

5.4.1.3 The reference to the ECCS Recommendations for limiting compressive strains

and stress-strain relationships for shuctural steel applies also to structures prestressed by tendons or jacking, in which loads act before the development of composite action.

52

RECOMMENDATIONS

5.4.1.2 Elastic analysis

The ultimate moment of resistance of a slender cross-section, as defined in 5.2.2, may be determined by elastic theory assuming full interaction between the concrete slab and the steel beam, and plane sections to remain plane.

The strains (or stresses) in the structural steelwork should nowhere exceed the limits given in the ECCS Recommendations for Steel Construction, having due regard to the possibility of buckling of unrestrained members in compression. The maximum compressive stress in the concrete due to flexure should not exceed the design cylinder strength, fed.

The stresses in the reinforcement should not exceed the design yield strength in tension or compression given in the CEB/FIP Recommendations.

5.4.1.3 Inelastic analysis

The cross-section strength may be calculated by means of a non-linear elastic- plastic analysis, provided the influence on the bending moment distribution (4.5.1.3) has been accounted for. Whether and to what extent the cross-section considered can be assumed to be plastified, depends mainly on the local stability of compressed parts of the steel member.

The ultimate moment of resistance of a slender cross-section may be determined by assuming full interaction between the steel and concrete and a linear strain variation across the section with the stresses in the concrete in compression derived from either of the stressstrain curves given in Figure 5.1. The compressive strain in the outermost fibre, ec, should not be less than -0.0035, except that where the composite section is wholly under compression,

where ea is the strain at the least compressed edge of the composite member, compressive strains being taken as negative. To ensure adequate shear transfer, the tensile strain in the concrete should no-

where exceed 0.01.

The stress-strain relationship for reinforcement and prestressing steel may be taken from the CEB/FIP Recommendations.

For structural steel, reference should be made to the ECCS Recommendations for Steel Construction for limiting compressive strains (having due regard to buckling under the actual stress state) and stress-strain relationships.

ec <-0.0035 - 0.75 ea

53

COhWENTARY

5.4.2

Where the plastic neutml axis lies in the web of the steel section, a possible ultimate strength method of design which is being considered is to assume that the depth of web in compression and an equal area in tension on the other side of the plastic neutral axis are ineffective. The resulting cross-section is shown below:

d Stress diagram

.Figure CS.4

Recommendations on the use of plastic design for beams where some cross-sections are slender are given for simply-supported beams only. However, under certain conditions (for example, spans equal or nearly equal, and a high ratio of moving loads to fmed loads) the ultimate moment of resistance of a composite section in positive (sagging) moment regions of continuous beams may be calculated using simple plastic analysis of the cross-section.

I 54

RECOMMENDATIONS

. 7 Second degree parabola fc = 850 f,dE (2506, - 1 I

A f Jfcd

0.85 -

i I EC

-0.0035 *

-0.001 35 -0.002

Figure 5.1 Design stress-strain curve for concrete in compression.

5.4.2 ULTIMATE STRENGTH OF SLENDER BEAMS BASED ON SIMPLE PLASTIC THEORY

In simply-supported beams, the ultimate moment of resistance Mu of a slender beam may be determined by simple plastic theory in accordance with 5.3.1 or 5.3.4 provided the plastic neutral axis does not lie within the web of the steel beam.

5.4.3. VERTICAL SHEAR

accordance with the ECCS Recommendations for Steel Construction: Vertical shear should be assumed to be resisted by the steel section alone in

5.4.4' COMBINATION OF VERTICAL SHEAR AND BENDING MOMENT In regions subject to combinations of vertical shear and bending moment,

reference should be made to the ECCS Recommendations for Steel Construction.

5 5

COMMENTARY

5.5.

ject to predominantly static loading (see Table 3, p 35). The requirements of this clause need not be considered for compact beams sub-

5.5.3

The procedure of superimposing stresses calculated on an elastic basis due to global and local wheel load effects is conservative. Can this be improved? The question of whether local wheel load effects should be considered to coexist with theglobal effects of vehicle loading depends on whether the magnitude of the specified wheel load is a true representation of the wheel loads that would occur in the actual vehicle loading. In some countries wheel loads and vehicle loads are not considered to coexist, presumably because the magnitude of the specified wheel loads has been artificially increased to allow this.

When considering the coexistent stresses in a deck slab, which also forms the flange of a composite beam, account may be taken of the effects of shear lag to reduce the longitudinal bending stress in regions of the flange remote from the web/ flange junction. The stress f x , at any point in the flange may be calculated from:

f x =fnlax [ k4 + 15J, - 1 ) (1 - k 4 M 1 where

J, = be/b for portions between webs, or bClO.85b for cantilever projections,

be is the effective breadth of flange determined in accordance with the CEBIFIP Recommendations and Supplement and the remaining terms are as defined in Figure C5.5.

I f the calculated value of f x is negative, it should be taken as zero, i f its effect is to reduce the coexistent stresses in the flange.

56

RECOMMENDATIONS

5.5. SERVICEABILITY LIMIT STATE

5.5.1 CALCULATION OF STRESSES ,

Stresses due to bending moments, prestressing and vertical shear forces may be calculated by elastic theory, using the elastic properties given in 3.4 as appropriate, assuming full interaction between the steel beam and concrete in compression. Vertical shear should be assumed to be resisted by the steel section alone and the tensile strength of concrete should be neglected except as provided in Section 12 for prestressed composite beams.

I 5.5.2 EFFECTNE BREADTH OF CONCRETE FLANGE In the absence of a rigorous analysis, allowance for in-plane shear flexibility in

the flange (shear lag effects) should be made in stress calculations based on the elementary theory of bending by replacing the actual flange breadth by an effective breadth of flange determined in accordance with the CEB/FIP Recommendations and Supplements.

5.5.3 COEXISTENT STRESSES Where a deck slab is required to resist the effects of local wheel loading acting

directly on it and the effects of loading the composite beam of which it forms a part, the effects should be considered separately and where they arise together, in conjunction.

57

COMMENTARY

Centreline between 1 adjacent webs, I /f

or free edge of slab

I I 1

Figure C5.5 Distribution of longitudinal stress in the concrete flange of a com- posit e beam.

RECOMMENDATIONS

5.5.4. STRESS LIMITATIONS AT SERVICEABILITY LIMIT STATE 1. For beams subject to repeated loading, where the fatigue life is based on stress

(or load) range, the static stresses at the serviceability limit state should comply with the following limitations:

a) the maximum tensile stress in the structural steel should not exceed 0.9 times the characteristic yield strength divided by rm, nor should the maximum equivalent stress in the steel exceed the chracteristic yield strength divided by rm, residual stresses due to welding or rolling being neglected;

b) the stress in the reinforcement, whether in tension or compression should not exceed the characteristic yield strength divided by rm ;

c) the calculated compressive stress in the concrete due to flexure should not exceed 0.6 fck where fck is the characteristic cylinder strength of the concrete.

In the absence of more precise methods of analysis, eg, yield line theory, the limitation in (lc) above may be applied to elastic methods of analysis when considering the effects discussed in 5 .S 3. 2. Stress limitations for prestressed composite beams are given in Section 12.

59

COMMENTARY

Complete

6.2.1

Dependent on the slenderness of the beam, type of loading, and the ductility of the shear connectors, different design methods for shear connection are recommended. A schematic presentation of these methods is given in the following diagrams.

Partial Complete Partial

Flexible connectors Stiff connectors

I .

ULTIMATE LIMIT STATE

connection connection . connection b . connection .h

60

I

Ultimate strength Simple method analysis. Connec: based on full tors to develop interaction longitudinal force (7.5.2.1 1 or in slab at Mu (7.4) interaction

plastic partial-

method (7.5.2.21 and 7.5.3

I I

Ultimate General strength method analysis. (7.5.2.3) Connectors and 7.5.3 to develop force in slab a t MU (7.4) - -

STATE No check necessary for beams subjected to static loading only. For beams subject to repeated loading, check shear connection for static loading and for fatigue in accordance with Section 8.

RECOMMENDATIONS

Section 6. Design of the shear connection - general

6.1 GENERAL Shear connectors and transverse reinforcement should be provided throughout

the length of the beam to transmit the longitudinal shear force between the concrete slab and the steel beam, ignoring the effect of bond between the two. I

The shear connection should be designed to satisfy the limit state requirements given in 6.2.

Recommendations for high strength friction grip bolts used as shear connectors are given in 6.7.

6.2. LIMIT STATE REQUIREMENTS

6.2.1 SHEAR CONNECTION (a) In compact beams (see 5.2.1) subject to predominantly static loading, the

shear connection should be designed to satisfy the requirements at the ultimate limit stage given in Section 7.

Where a compact beam is subject to repeated loading the recommendations of Section 8 on design for both static loading and for fatigue should also be considered.

(b) In slender beams, shear connectors should be designed initially to satisfy the requirements for static loading at the serviceability limit state given in Section 8.

In beams subject to repeated loading, the recommendations of Section 8 on

Where appropriate, the shear connection should also be checked at the ultimate design for fatigue should also be considered.

limit state in accordance with Section 7.

6.2.2 TRANSVERSE REINFORCEMENT Transverse reinforcement provided to prevent longitudinal shear failure of the

slab in the vicinity of the shear connectors should be designed for the ultimate limit state in accordance with 7.6.

61

COMMENTARY

. . ULTIMATE LIMIT [ SLENDERBEAMS 1 STATE

, 4%,,S

e .

Predominantly static loading and repeated loading 1 I

Simplyiwpported beam with neutral

I axis that satisfies I 5.4.2

other

connection onlv, to 7.4

A check on the shear connection may be necessary, as discussed below

SERVICEABILITY LIMIT STATE to 8.4

Elastic analysis at serviceability limit state, as in Section 8. Effects of temperature, shrinkage, creep, and method o f construction to be considered.

In slender beams in bridges the connectors are usually spaced in proportion to the elastic distribution of shear ( V%y/I). This gives structures which perform satisfactorily in service, and usually it gives sufficient shear strength at ultimate loads. But in some circumstances a check on the shear connection at the ultimate limit state should be made; for example, when unpropped construction is used or when the strength of a cross-section is checked using the plastic theory.

6.3 Further commentary on the significance of the two properties of shear con-

I nectors relevant for design, strength and deformation capacity, is given in 2.1.

6.3. I I

The characteristic strength of connectors can be given in terms of both the concrete cylinder strength f c k and the yield strength f y of the steel o f which the connector is made or of the weld.

ym for concrete and steel in the expression. However, since local crushing of con- The design strength can therefore be obtained by inserting appropriate values of

62

RECOMMENDATIONS

6.3 PROPERTIES OF SHEAR CONNECTORS

6.3.1 STRENGTH OF CONNECTORS The strength of a connector is the maximum load in the load direction considered

(in most cases parallel to the interface between concrete flange and steel beam) that can be carried by the connector before failure.

The characteristic strength is the specified strength below which not more than 5% of test results may be expected to fall. When a guranteed minimum value of strength is specified this may be considered as the characteristic strength.

The design strength is the characteristic strength divided by the partial safety factor 7,.

63

t , COMMENTARY. ’

Crete around one connector would not cause failure of the beam the value of Y,,, for concrete is taken less than the value 1.5 given in 3.6.4 for normal use.

6.3.2 DEFORM TION CAPACITY OF CONNECTORS

strength) is important: The deformation capacity of a connector (maximum slip at characteristic

(a) in beams where connectors are spaced not exactly in accordance with the shear force distribution, and

(b) in beams with incomplete connection where the connection is designed by ultimate strength methods assuming that the shear connectors behave in an ideal plastic manner (flexible connectors).

1

1

The deformation capacity required in both cases is different as has been stated in commentary 2.1.

In the requirements for flexible connectors an - upper limit for the characteristic cylinder strength of concrete is given because with increasing concrete strength the deformation capacity decreases.

. .

64

, . . . . . . . . , . . . . .

RECOMMENDATIONS

- For the serviceability limit state the value of 7, may be taken as 1 .O. - For the ultimate limit state the value of 7, depends on the failure mode, and

should be taken as 1.3 for crushing of concrete and as 1 .O for yielding of steel.

6.3.2 DEFORMATION CAPACITY OF CONNECTORS Connectors may be considered as flexible provided: - The shear connectors are headed studs with a diameter not exceeding 22 mm

- The specified characteristic 28-day cylinder strength of the concrete is not

and an overall length not less than 4 times the diameter.

greater than 30 N/mm2.

All othe;types of connectors should be considered as stiff unless it has been proved by tests that the deformation capacity is satisfactory for the assumption of ideal plastic behaviour.

6.4 DESIGN STRENGTH OF SHEAR CONNECTORS

6.4.1 GENERAL The strength of shear connectors may be determined either by calculation

(Clauses 6.4.4 to 6.4.7) or experimentally by push-out tests (Clause 6.6), subject to the following conditions:

6.4.2 STATIC STRENGTH

(a) Where the concrete slab is unhaunched, or the haunch satisfies the requirements of 6.5 . l , the design static strength of shear connectors embedded in normal density or lightweight aggregate concrete (density greater than 1400 kg/m3) may be calculated from the equations given in Clauses 6.4.4 t o 6.4.7. Alternatively, the design static strength may be determined in accordance with 6.3.1 using characteristic strengths determined experimentally from standard push-out tests in accordance with 6.6.1 and 6.63.

(b) Where the concrete density or haunch dimensions do not satisfy the requirements of 6.4.2(a), the design strength should be determined in accordance with 6.3 .I using characteristic strengths determined from special push-out tests in accordance with 6.6.2 and 6.6.3.

65

..F

COMMENTARY

6.4.4

The relation between connector strength and concrete strength is shown in the subjoined graph (Figure C6.1). A t higher concrete strength the ultimate strength of , the connector is constant and not dependent on concrete quality. The strength is . then equal to the pure shear strength of the connector material (or of the weld) and should therefore be Pw = A s (0.7 fu) . However, for design purposes the value is equalised to the design value of a normal bolt in shear in accordance with the ECCS Recommendations for Steel Construction.

So& is replaced with f r . The partial safety factory,,, may then be taken as unity. I t has been shown that the great scatter in test results is to some extent caused

by the influence of the dimensions of the weld and its strength. So it is possible that with certain welding procedures higher values are obtained than specified in the recommendations.

If the welding procedure is well specified and the dimensions and properties of the weld are guaranteed by the manufacturer the characteristic strength may alternatively be based upon experimental data of standard push-out tests (see Clause 6.4.2).

- f, NJmrn'

0 -*- 10 40 f, Nlmm'

Figure C6.1 Qualitative presentation of Equation (6.1).

RECOMMENDATIONS

6.4.3 FATIGUE STRENGTHS

in accordance with 6.4.2(a) are given in 6.4.4.4. For other connectors and other types of slab, the characteristic ranges of shear load AV should be determined for different numbers of cycles N from constant-amplitude alternatingload push-out tests, in accordance with the appropriate recommendations of 6.6.

Design fatigue strengths for headed stud connectors set in concrete slabs that are

6.4.4. STUD CONNECTORS

6.4.4.1 Headed studs - static shear load

the following equations: The design shear strength Pd of headed stud connectors may be calculated from

h a= 3.0

where ymc is the partial factor of safety on concrete strength, taken as 1.3 at the ulti-

rms is the partial factor of safety on steel strength, taken as 1 .O as both the ulti-

Between h/d = 4.2 and h/d = 3 .O linear interpolation is permitted,

where h = overall length of the stud, d = diameter of the stud, f d = characteristic cylinder strength of concrete at age considered, E, = short-term modulus of elasticity of concrete, fy = design yield strength of the connector material (= fo.2.) but not greater

If - spirals with dimensions as specified in 6.5.2 are placed round the studs the design shear strength according to formulae 6.1 and 6.2 may be multiplied by a factor 1.15, provided:

mate limit state and 1 .O at the serviceability limit state, and

mate and serviceability limit states.

than 0.8 fu.

67

COMMENTARY

I I

6.4.4.3

ing Laboratoly Report 200.71.438.2, Lehigh University 1971). Values of the characteristic tensile strength of concrete as a function of the characteristic compressive strength are given in the CEBIFIP Recommendations.

Formula 6.3 is based on tests reported by McKackin and others (Fritz Engineer- ,.

The expression for C, is based on the assumption that for QQ 2 2h no inter-

Furthermore a linear interpolation between Q, = 0 and Qsp = 2h as shown in action occurs.

Figure C6.2 is assumed.

&in = 4d or 5d (Clause 6.5.2)

Figure C6.2

68

RECOMMENDATIONS

Suitable tests should be made to ensure that the concrete can be adequately compacted into the space between spiral and stud.

6.4.4.2 Studs without head - static shear load Equations (6.1) and (6.2) may also be used for studs without heads, provided

uplift pf the slab is prevented. The ties which resist uplift should be designed at the ultimate limit state for a tension force I' of at least 0.1 Pd,

6.4.43 Headed studs - static tensile load

Where the shear connectors are subject to direct tension then additional ties suitably anchored should be provided to resist these forces or alternatively headed studs may be used. These should be checked for the ultimate limit state. The design tensile strength Td should be calculated from the following equation:

where rmc rms h, d, fy = as defined in 6.4.4.1, fct C = 3.0 for normal density concrete,

= material safety factor for concrete, taken as 1.3, = material safety factor for steel, taken as 1 .O,

= characteristic tensile strength of concrete,

= 2.25 for lightweight aggregate concrete, = reduction factor when the connector spacing is less than 2h:

1 1 P c, = 2+4f G1.0.

When both the longitudinal and transverse connector spacing is less than 2h a further reduction is necessary. The reduction value should then be based on suitable tests.

If the connector is loaded by combined tension and shear, the worst combination of coexistent forces at the ultimate limit state should satisfy the following equation:

where P actual shear load, Pd = the design shear strength in the absence of a tensile load, T = actual tensile load, T d = the design tensile strength in the absence of shear load.

The effect of axial tension may be neglected if the reduction in P is less than 10%.

69

COMMENTARY

Where headed studs are used primarily to resist direct tension, appropriate local rein forcement should be provided.

, . _.. . . . . . . 4. ..

I . . .

. j:. ' . .

70

RECOMMENDATIONS

6.4.4.4 Headed studs - alternating load

cycles N, or conversely gives values of N for different shear ranges AT where: Table 6.1 gives design ranges of mean shear strew AT for different numbers of

AT is the difference between the maximum and minimum mean shear stress

N is the maximum allowable number of occurrences of that cycle.

(4P/nd2) during any given load cycle,

- Number of cycles, N 104 105 s x 105 2x106 1 0 7 108 , Shear range AT in N/mm2 160 1 15 90 70 60 50

Table 6.1 Fatigue strength of stud shear connectors.

The fatigue life of a connector subject to random loading may be determined from Miner’s linear cumulative damage rule:

where ni = number of cycles applied at a given stress range, Ni = allowable number of cycles for which the given stress range would be

allowed.

6.4.4.5 Headed studs used with profiled steel sheeting

Deck ribs oriented Darallel to the sumortinr! beams

The design shear strength of a stud connector shall be the value stipulated in Clause 6.4.4.1 except that when w/e is less than 1 .5, the design shear strength shall be multiplied by the following reduction factor:

0.6-(-)<l.O W h d - e e e

where w = mean width of concrete ribs (see Clause 5.1.2), e = depth of ribs (see Clause S.1.2), hd = overall length of the stud but not greater than e + 75 mm.

Deck ribs oriented transverse to the supporting beams

Clause 6.4.4.1 multiplied by the following reduction factor: The design shear strength of a stud connector shall be the value given in

71

COMMENTARY

6.4.5 BLOCK-TYPE CONNECTORS A bar is beyond doubt a block-type connector but also the T, the [ and the

horseshoe may be considered as so, when the plate components are thick enough.

Bar connector Tconnector [-connector

1:5 A f I I

Figure C6.3

Forces on the welds: Shear force: Smax = Pu

Moment: M = Pu E 2

H or sash oe

Figure C6.4

12

I RECOMMENDATIONS

where n, = number of stud connectors in one rib at a beam intersection, not to

exceed 3 in computations, although more than 3 studs may be installed.

When the shear connectors are provided to produce composite action both for the beam and for the deck the resultant ultimate force acting on the stud shall be calculated using the following equation:

where P = & F

P = resulting force on the connector, PQ = longitudinal force caused by beam composite action, P, = transverse force caused by deck composite action (see Section 15).

6.4.5 BLOCK-TYPE CONNECTORS Connectors may be designed as block-type connectors when the front is so stiff

that it can reasonably be assumed that the concrete pressure in front of the connector at failure is uniformly distributed.

The characteristic strength may be taken as:

. P,=fblAs

where A, = the area of the front surface, - fbl = the characteristic value of the contact pressure in rront of the connector

Af = the area of the front surface of the connector enlarged at a slope of 1 :5 to the back side of the adjacent connector (Figure C6.3). Only the parts of Af falling within the concrete section may be taken into account.

The design shear strength of the connector is therefore: .

The welds between the connector and the steel beam should be designed for a shear force and a moment due to the force P, acting at the centre of gravity of the front surface. To prevent uplift on the connectors additional ties suitably anchored should be provided. These ties should be designed for a tension force T of at least 0.1 P,.

73

COMMENTARY

6.4.6

force T and a shear force D.

from tests on bolts and can be calculated from:

The shear force working on the inclined anchor bar can be resolved into a tensile

The design strength of a round solid bar loaded in tension and shear follows

d m 2 =Afy

with 1 Porn which Equation (6.9) can easily be derived. T = Pu cos a

D = Pu sin a

Detail

PU

D = P, sin a - T = P , / cos, a

Figure C6.5

6.4.7

, L - . __ . . - _. - . -. - . .. . . . .- . - I . 1 _ _ - ._. . . _. - . - - I -

I Block type connector Block type connector

. with anchor bar with hoop

/ Figure C6.6

The characteristic strength of block type connectors and anchors may not simpb beadded up because of the essential difference of deformation capacity between the two types.

74

RECOMMENDATIONS

6.4.6 ANCHORS AND HOOPS

The characteristic strength of an anchor may be taken as:

Pu = AfY Jmm <

where A = the area of the cross-section of the bar, f y = the design yield strength of the connector material, a = the angle between the anchor bar and the flange of the beam. If a does not exceed 45" the value of Pu may arbitrarily be calculated from:

Af P U =d (6.10)

If fillet welds are used, these should be designed for the shear force Pu.

6.4.7 BLOCK TYPE CONNECTORS COMBINED WITH ANCHORS OR HOOPS

characteristic strength of the combination follows from: If block type connectors are used in combination with anchors or hoops the

- block type connector with anchor bars:

pu comb = pu block t- 0.5 pu anchors

- block type connector with hoop:

pu comb = pu block +os7 pu hoop

The welds however, should be designed for the full value of the characteristic strength.

75

COMMENTARY

t "U block

'U anchor

t 'U comb < 'U block + 'U anchor

Figure C6.7

6.5.1

(a) These recommendations should also be adopted when using (for example) hollow planks or other prefabricated floor components. When studs are used this leads to the following design rules.

The underside of the stud should extend not less than 30 mm above the reinforcement in the concrete cover. The distance between the edge of the hollow plank and the stud should be sufficient to allow the placing of good quality concrete in between.

Reinforcement

. .

Figure C6.8

76

I

Slip

RECOMMENDATIONS

6.5 DETAILING OF SHEAR CONNECTION

6.5.1 GENERAL REQUIREMENTS (a) The surface of a connector that resists separation forces (ie, the inside of a

hoop or the underside of the head of a stud) should extend not less than 30 mm clear above the bottom reinfdrcement. The concrete cover over the connector should be not less than 20 mm. If concrete cover is not required as protection against corrosion the top of the connector may be flush with the upper surface of the concrete slab.

(b) Where a concrete haunch is used between the steel girder and the soffit of the slab, the sides of the haunch should lie outside a line drawn at 45" from the outside edge of the connector (Figure C6.9). Transverse reinforcing bars sufficient to satisfy the requirements of 7.6 should be provided in the haunch at least 40 mm clear below the surface of the connector that resists uplift.

(c) Where the shear connection is adjacent to a longitudinal edge of a concrete slab, transverse reinforcement provided in accordance with 7.6 should be fully anchored in the concrete between the edge of the slab and the adjacent row of connectors.

(d) The detailing of shear connectors should be such that concrete can be adequately compacted around the base of the connector.

(e) At the end of a cantilever, as for example in a cantilever and suspended span structure, sufficient transverse and longitudinal reinforcement should be positioned adjacent to the free edge of the concrete slab to transfer the longitudinal shear connector loads back into the slab.

77 ' .

COMMENTARY

Figure C6.9 Dimensions of haunches.

65.2

(g) Spirals need not be welded to the steel flange.

78

,

RECOMMENDATIONS

(0 The longitudinal spacing of the connectors should be no greater than 600 mm or four times the thickness of the slab, whichever is the lesser. Alternatively, connectors may be placed in groups, with the group spacing greater than that specified for individual connectors, provided consideration is given in design to the non-uniform flow of longitudinal shear and of the greater possibility of slip and vertical separation between the slab and the steel member.

(g) Where it is assumed in design that the stability of either the steel or the concrete member is ensured by the connection between the two, the spacing of the shear connectors should be sufficiently close for this assumption to be valid, and appropriate resistance to uplift should be provided.

(h) The distance between the edge of a connnector and the edge of the flange of the beam to which it is welded, should be not less than 20 mm.

(i) The material of the connector used should be of a good weldable quality. It has to be proved by proof-welding that the applied welding technique is suitable to guarantee reliable welds.

6.5.2 HEADED STUD CONNECTORS

(a) The overall length of the studs should be not less than 3 times the diameter. (b) The thickness of the steel plate to which a connector is welded should be

sufficient to allow proper welding and proper transfer of load from the connector to the plate without local failure or excessive deformation.

(c) Where the flange is subject to tensile stresses, the diameter of studs should not exceed 1.5 times the plate thickness. Where the flange plate is not subjected to tensile stresses, the diameter of the stud should not exceed 2.5 times the plate thickness.

(d) The dimension of the head should not be less than: - diameter of head = 1.5 x diameter of shaft, - height of load = 0.4 x diameter of shaft.

(e) The spacing of the connectors Qmin may not be less than: - in direction of shear force: Qmin = 5d; - transverse to direction of shear force: Qmin = 4d. If the transverse spacing is less than 4d the bearing capacity is to be proved.

( f ) Welding of stud connectors should be carried out in accordance with Clause 18.8.

(g) Where spirals round the studs are used, the dimensions should be as shown in Figure 6.2.

79

6.5.5

80

COMMENTARY

I I

! ! . . t

Recommended direction of thrust --b

Figure C6.10

RECOMMENDATIONS

7- E E 0 4

Figure 6.2

(h) To ensure proper filling and compacting of concrete between the spiral and the stud the space between the spiral and the edge of precast concrete slabs or other solid objects must be not less than 50 mm.

6.5.3 STUD CONNECTORS WITHOUT HEADS Suitable ties should be provided to resist separation forces (see 6.4.4.2).

6.5.4 HEADED STUDS USED WITH PROFILED STEEL SHEETING

General

(a) The concrete slab shall be connected to the steel beam with welded stud shear connectors with a diameter not greater than 19 mm.

Studs may be welded either through the deck or directly to the steel beam.

(b) Stud shear connectors, after installation, shall extend not less than 35 mm above the top of the steel deck.

Deck ribs oriented transverse to the supporting beams

(c) The maximum longitudinal spacing of connectors should be in accordance with 6.5.l(f).

(d) To resist uplift, the steel decking shall be anchored to all compositely designed steel beams at a spacing not exceeding 400 mm. Such anchorage may be provided by stud connectors, a combination of stud connectors and arc spot (puddle) welds, or other devices specified by the designer.

6.5.5 BLOCK-TYPE CONNECTORS

(a) The connectors should not be wedge shaped. Care should be taken for a suitable placing in relation to the direction of thrust (see Figure C6.10).

(b) Suitable ties should be provided to resist separation forces (see 6.4.5).

81

COMMENTARY

6.5.6 Anchorage length

1

------- -I Figure C6.11

Direction of thrust

F I

-----

Figure C6.12

82

RECOMMENDATIONS

6.5.6 ANCHORS AND HOOPS

(a) Anchors and hoops may be either butt welded or bent and fillet welded. When fillet welding is used the bend adjacent to the weld should be made in red hot condition. Special care should be taken that the material of the bar is of a good weldable quality.

(b) The anchorage length and the concrete cover should be in accordance with the relevant clauses in the CEB Recommendations. A hoop may be assumed tot be sufficiently anchored when the following conditions are met:

R > 7.5 4 I 0 g d > 4 R concrete cover 2 3 4.

Figure 6.3

c) The anchors and hoops should point in the direction of thrust. At midspan where the direction of thrust changes the connectors must be placed in both directions.

83

COMMENTARY

6.6.1

a, b) . LOAD

0 m

254 X146 X 43 UB or IPE 270

0 e

- - - 15 mm cover e e

L- Bedded in mortar or solid base . I

Figure

Note:

C6.13

LOAD

300 ~

Reinforcement mild steel

5: c

-

0 cn d

-

to be 10 mm diameter

d ) This requirement can simply be met by taking the specified concrete grade, but testing earlier than 28 days after manufacturing the specimens.

RECOMMENDATIONS

6.6 TESTS ON SHEAR CONNECTORS

6.6.1 STANDARD PUSH-OUT TEST The static strength of shear connectors that comply with 6.4.2(a) may be

determined by standard push-out tests made in accordance with the following requirements :

a) The dimensions of the test specimens must be as given in Figure C6.13. b) The steel section and the reinforcement must be as given in Figure C6.13.

I

c

c) Bond at the interface of the flanges of the steel beam and the concrete must be prevented by greasing the flange or by other suitable means.

d) The strength of the concrete at the time of testing must be 70% f 10% of the specified cylinder strength of the concrete in the beams for which the test is designed. Curing of the cylinders or cubes should be in accordance with CEB Recommendations. The push-out specimens should be air-cured.

e) The yield stress of the connector material must be determined. f ) The rate of application of load must be uniform and such that failure is

reached in not less than 15 minutes.

85

86

COMMENTARY.

RECOMMENDATIONS,

6.6.2 PUSH-OUT TEST FOR CHECKING UNUSUAL SITUATIONS

connector may be determined by push-out tests, in accordance with the following requirements :

a) The push-out tests should be carried out on test specimens generally as

If the conditions of 6.3.1 are not met, the characteristic static strength of a shear

shown in Figure 6.4.

IF in

I F

Steel section

Concrete slab

A d ~ypsum, mortar or solid base

Figure 6.4

b) The slab and reinforcement should be suitably dimensioned in comparison with the beams for which the test is designed. The following conditions should be met: - the length of the slab shall not exceed the minimum longitudinal spacing

- the width of the slab shall not exceed the effective width of the slab of the

-- the thickness of the slab shall not exceed the minimum thickness of the

- where a haunch in the beam does not comply with 6.3, the slab of the

of connectors on the beam;

the beam;

slab in the beam;

push-out specimen should have the same haunch and reinforcement as the beam.

c) Provisions of 6.6.1(c), (d), (e) and (f) apply.

6.6.3 EVALUATION OF TEST RESULTS Not less than three tests on nominally identical specimens must be carried out. When the deviation of any individual test result from the mean value obtained from all tests does not exceed lO%, the lowest test result must be taken as the ultimate load P,.

87 .

COMMENTARY ’

The reduction factor 0.8 in Formula (6.11) is in theory only required when strength of steel governs the failure. Sometimes the failure mode is so complicated that it is difficult to decide whether the steel or the concrete governs the failure condition. In that case Formula (6.1 1) should conservatively be adopted.

. . i

6.7.2 (a) Connection by friction may be relied upon only under service conditions. At

the ultimate limit state, for simplification it may be assumed that the bolts alone are carrying the shearing forces with a strength according to that of headed studs.

This simplification is based on the assumption that any clearance between the bolt and the surrounding concrete is small enough to ensure that sufficient redistribution of shear can take place due to slip without causing premature shear failure of the bolts. Where this cannot be assured, the calculated frictional resistance must be sufficient to resist the shear at the ultimate limit state. No check is then needed under service conditions. -

6.7.2.1

In the case of uniformly distributed loads, thehorizontal shear force in regions near supports may be assumed to be unifonnlj distributed over a length of Q = 3d,

88

RECOMMENDATIONS

If such deviation from the mean exceeds 10% at least three more tests of the same kind shall be made and the lowest result of these six tests must be taken as the ultimate load P,.

Alternatively, when at least 10 tests are carried out, the ultimate load may be determined as the load corresponding to a probability of 5% of results being less than P, . The value of the design strength can be calculated from the value so found of the ultimate load by:

Pd = 0.8 ue Pu (6.1 1) ue actud.

where ue actual = actual yield stress of the connector in the test specimen determined

ut2 = minimum specified yield strength of the connector material.

Where the connector is composed of two separate elements, one to resist longitudinal shear and the other to resist forces tending to separate the slab from the steel beam, the ties which resist the forces of separation may be assumed to be sufficiently stiff and strong if this separation in push-out tests, measured when the connectors are loaded to 80 per cent of their ultimate load, is less than half of the longitudinal movement of the slab relative to the beam.

according to 6.6.l(e),

6.7 FRICTION GRIP BOLTS

6.7.1 GENERAL High strength friction grip bolts may be used to provide the shear connection

between the steel member and the concrete slab forming the flange of the composite beam. Unless otherwise stated the Clauses on high strength preloaded bolts in the ECCS Recommendations apply.

.

6.7.2 DESIGN REQUIREMENTS: STATIC LOADING

6.7.2.1 Serviceability limit state

(a) The longitudinal shear resistance per unit length developed by friction alone between the concrete flange and steel beam should not be less than longitu-

89

~

COMMENTARY *

where d is the depth of the'steel beam.

8

Figure C6.14

Where the connection is subject to external tensile forces in addition to shear 2

(for example, where there is a tendency for uplift between the slab and the steel I

beam or where loads are suspended from the steelwork), it may also be necessary to take account of the reduction in effective clamping force in the bolt.

90

RECOMMENDATIONS

dinal shear force per. unit length at the serviceability limit state calculated in accordance with 8.1.

The mamimum load per connector should not exceed U x net tensile force in the bolt

Ym

ym = the materials safety factor for the shear resistance per bolt, which

p = the coefficient of friction at first slip, which may be taken as:

where

may be taken as 1.2, and

0.50 when t m e 2 10 mm, 0.55 when t m e 2 15 ~TUTI.

Where the slab is cast in situ, the friction coefficient may be increased by 10%.

(b) In determining the net tensile force in bolts account should be taken of the loss of tension due to shrinkage and creep of the concrete. Unless a more exact calculation according to CEB/FIP Recommendations is made loss of tensile force due to creep and shrinkage should be taken as not less than 30% of the preloading force. The loss of tensile force can be reduced by re-tightening after an interval of time. Loss of prestress in this case should be calculated in accordance with the CEB/FIP Recommendations.

6.7.2.2 Ultimate limit state

(a) Between any pair of adjacent critical cross-sections as defined in 7.1 the sum of the longitudinal forces built up by friction and/or shear must be not less than the change in the longitudinal force in the concrete flange over that length. The effects of slip between the concrete slab and the steel beam should be taken into account.

bolts alone in shear and bearing, the maximum load per bolt should not exceed the value obtained from Equation (6.1), but with:

d taken as the diameter of the shank of the bolt when there is no thread at

d taken as the diameter of the ‘stress area’ (dm) when the bolt is threaded at

Alternatively, where the shear resistance is assumed to be developed by fric-

(b) Where the shear resistance is assumed to be developed by the strength of the

the shear plane, or

the shear plane.

tion alone, the maximum load per bolt at the ultimate limit state should not exceed the value given in 6.7.2.1. No check need then be made at the serviceability limit state.

6.7.3 DESIGN REQUIREMENTS - FATIGUE For connections subject only to shear in the plane of the friction interface no

account need be taken of the effects of repeated loading. This applies also to connections subject to external tensile forces in addition to shear, provided 6.7.2.l(b) is considered.

91

92

COMMENTARY

& RECOMMENDATIONS

6.7.4 DETAILING OF FRICTION GRIP BOLTS

The method of tightening should comply with the requirements of the ECCS Recommendations. The design of the connection must ensure that there is a uniform bearing surface between the steel beam and the concrete flange. The interface should be free of paint or other applied finishes, oil, dirt, loose rust, loose mill scale, burrs and other defects which would prevent a uniform seating between the two elements or would interfere with the development of friction between them. Tight mill scale is not detrimental. The washer under the head of each bolt should be of sufficient stiffness to ensure that the bearing stress on the concrete is uniform.

Adequate reinforcement, in spiral or other form, should be provided to ensure that the load is transferred from the bolt to the interface without local splitting or crushing of the concrete, unless tests show it to be unnecessary. Con- sideration shall be given to local splitting particularly where the slab is deeply recessed around a bolt.

COMMENTARY

7. I Due to the problem of uniquely defining points of contrajlexure in continuous

beams, these are not considered to be critical cross-sections. It is therefore convenient to use a design strength for shear connectors (given in 7.2) that is applicable for the entire length of a continuous beam, rather than to use different values in regions of positive and negative bending moment.

a support (critical section of type (e)), the effective breadth of this flange may be assumed to vary linearly from zero at its end to the full effective breadth over a length of a t least the total effective breadth. Within this length, connectors may be spaced uniformly.

I

Where the concrete fzange of a composite beam ends at a cross-section other than

Elevation m Figure C7.1 Effective breadth of composite slab ending within region of sagging

moments.

7.2 Maximum loads for connectors in continuous beams and cantilevers are reduced

below Pd to compensate for the reduction in the stiffness and strength of shear connectors due to flexural cracking where the concrete slab is in tension.

94

RECOMMENDATIONS

Section 7. ‘Design of the shear connection - ultimate limit state

7.1 CRITICAL CROSSSECTIONS

ance with 5.4.2, shear connectors provided to resist static loading may be spaced uniformly between adjacent critical cross-sections.

In beams of compact cross-section and in beams designed for flexure in accord-

Critical cross-sections of composite beams are:

a) allsupports; b) all cross-sections of maximum sagging (positive) moment; c) free ends of cantilevers; d) points of application of heavy concentrated loads, for example, from

columns; e) points where there is a sudden change in the cross-section of the member;

and f ) in tapering members, points so chosen that the ratio of the greater to the

lesser second moment of area at any pair of adjacent points does not exceed two.

7.2’ MAXIMUM LOADS PER CONNECTOR

ctoss-sections may be assumed to resist the same proportion of their design static strengths, Pd. The maximum load per connector should not exceed:

At the ultimate limit state, all the connectors between an adjacent pair of critical

95

COMMENTARY

7.4

composite member (for example, when the steel flange which is connected to the concrete is much smaller than the other steel flange), shear-flexural failuve on a surface such as ABC iri Figure C7.2 becomes possible. A check should then be made that the shear connection is sufficient to develop the forces in the slab required at all cross-sections, not just at critical sections.

When the bending strength of the steel beam alone is much less than that of the

A

Centre line

I_ B - !

1 I I

I C

Steel beam 1 Figure C7.2

7.5.1

When partial shear connection is used, the deformation capacity required of a connector, before it begins to lose strength, increases with increase in the span of the beam. There is at present no experimental evidence to validate the use of partial shear connection in long-span beams. The limiting span of 20 m is given for this reason.

I t is thought that the use of partial shear connection is not common enough to justify the inclusion of design methods for members carrying concentrated loads or of non-uniform section.

96

RECOMMENDATIONS

Pd

0.93 Pd in other continuous beams; 0.80 Pd in other cantilevers;

in simply supported beams and Class I or I1 prestressed continuous beams and cantilevers;

where Pd is the design strength determined in accordance with 6.4.

7.3 LONGITUDINAL SHEAR

tudinal force in the concrete slab when the section resists a bending moment M’, , as appropriate, calculated in accordahce with 5.3.1.

span. I

The ultimate longitudinal force Fu at each critical cross-section is the total longi- or

The length of beam between any pair of adjacent critical cross-sections is a shear

7.4 COMPLETE SHEAR CONNECTION

For complete shear connection, the product of the maximum load per connector (as given in 7.2) and the number of connectors provided in each shear span must be not less than the change in longitudinal force F, (as given in 7.3) over the length of that span.

7.5 PARTIAL SHEAR CONNECTION

7.5.1 SCOPE

I The design methods of 7.5 are applicable only to simply-supported and con-

tinuous beams that are subjected to predominantly static loading and not to heavy concentrated loads, such as loads from columns. Also, the methods of 7.5.2.1 and 7.5.2.2 are applicable only to beams of span not exceeding 20 m, that have steel members of uniform crosssection.

97

COMMENTARY

7.5.2

Further research is needed before the methods of 7.5.2.1 and 7.5.2.2 can be used for types of shear connector other than headed studs in accordance with 6.3.2.

7.5.2. I The limit N Q: 0.5 Nf in this and the following methods is arbitrary. Results of

short-term labomtory tests suggest that the limit could be reduced to about 0.4 N f , but there is no evidence of satisfactory long-term behaviour of beams with so low a degree of shear connection (Johnson, R P, and May, I M, ‘Fartial-interaction design of composite beams ’; Structural Engineer, 53,305-11, August 1975).

the results of partial-interaction analyses for the ultimate strength of beams with partial shear connection.

L,

Figure C7.3 shows that the method of 7.5.2.1 is a conservative approximation to

M/M,

1.0-. - Partial-interaction analysis /

I

1

0 1 .o N/Nf

Figure C7.3

7.5.2.2

values of N/Nf corresponding to points A , B, and Con Figure C7.3.

behaviour of the shear connectors. Where the degree of shear connection or the deformation capacity of the connector is low, or the span of the beam is long, premature failure of the shear connectors may occur before M, is reached.

Figure C7.4 shows the stress distributions given by the method of 7.5.2.2 for

The moment of resistance Mr calculated by this method relies on ideal plastic

98

RECOMMENDATIONS

7.5.2 SIMPLYSUPPORTED BEAMS Three methods are available for beams where the design bending moment at a

critical crosssection in the midspan region, M,, is less than the ultimate moment of resistance M,, given by the method of 5.3.1. The methods of 7.5.2.1 and 7.5.2.2 are applicable only to beams with shear connectors that are flexible in accordance with 2.1 and 6.3.2. The method of 7.5.2.3 is applicable to beams with either flexible or stiff connectors.

7.5.2.1 Simple method

the number of shear connectors N must be such that: In each shear span (as defined in 7.3) adjacent to the cross-section considered,

NQNf(M,-Mp)/ (MU-Mp) and N Q 0 . 5 N f where M, and Mu are as defined in 7.5.2,

M, is the plastic moment of resistance of the steel section alone, and Nf is the number of connectors required for complete shear connection,

calculated in accordance with 7.4.

7.5.2.2 Partial-interaction analysis

by simple plastic theory in accordance with 1) to 4) of 5.3.1 and the following assumptions :

The ultimate moment of resistance Mu,, not exceeding MU, may be calculated

1 . The compressive force in the reinforced concrete slab is equal to the lesser of the sums of the design strengths of the shear connectors in the two shear spans adjacent to the critical section considered.

COMMENTARY

7.5.3 It is usually possible to so choose the quantity of top longitudinal reinforcement

in the slab above an internal support, that M u does not greatly exceed M;. The longitudinal shear in a hogging (negative) moment region may exceed the

value calculated from the yield strength of the reinforcement, due both to the ten-

I

P 'Vd

fyd I

Point A Point B Point C

Figure C7.4

7.5.2.3 This is the only method of design with partial shear connection that is given for

use when the shear connectors are not 'flexible'in accordance with 2.1 and 6.3.2. I t corresponds to the straight lines OD and DC in Figure C7.5. The curve ABC is copied from Figure 7.3, to show that this method requires the use of more connectors than the method of 7.5.2.1. MDL is the bending moment in the steel section alone (Stark, J W B, "Simply supported steel and concrete composite beams", Netherlands Committee for Concrete Research, and Steel Constructional Associa- tion, Waltman, Devt, I 9 74).

I : - N/Nf NJN" 1 .o

(a) Unpropped construction

Figure C7.5

I ,N/Nf NJNU 1 .o

(b) Propped construction

100

RECOMMENDATIONS

2. The shear connectors behave in an ideal plastic manner. 3. Slip occurs at the steelconcrete interface, so that the steel beam and the con- '

Crete slab have different neutral axes.

7.5.2.3 General method

7.5.2.1) for the critical section considered, and also Me and Ne, where

This method requires the calculation of Mu, Mr, and Nf (as defined in 7.5.2 and

Me is the sum of the bending moment in the steel section and the bending moment given by full-interaction elastic analysis of the composite section, at which the design yield stress in an extreme fibre of the steel beam or a flexural compressive stress of 0.6 f d in the concrete slab is first reached, f d being the design cylinder strength; and

Ne is the number of shear connectors required in the shear span considered (in accordance with 7.4) when the compressive force in the slab at the critical cross-section is that corresponding to Me.

The minimum numbers of shear connectors N that must be provided in each shear span adjacent to the cross-section considered are as follows:

When Mr < Me,

N

When Me < Mr 4 M U ,

NQ:Ne+

Q. Ne Mr/Me and N 4 0.5 Nf.

Mr- Me (Nf- Ne) and N 4 0 . 5 Nf. Mu - Me

7.5.3 CONTINUOUS BEAMS This design method is applicable only to a shear span (as defined in 7.3) that

extends from an internal support to a critical cross-section in a sagging (positive) moment region, for which the design ultimate bending moments are M'r at the internal support and M, in the midspan region.

101

COMMENTARY

sile strength of concrete and to strain hardening of the reinforcement, which is likely to occur before jlexural failure of the beam.

hogging moment regions. The spacing of shear connectors in such a region should not exceed that in the adjacent sagging moment region.

For these reasons no provision is made for the use of partial shear connection in

7.6

These rules are based on the results of research on shear transfer in reinforced concrete (Mattock, A H, and Hawkins, N M, 'Shear transfer in reinforced concrete - recent research': Journal of Prestressed Concrete Institute, MarchlAprill9 72).

The method enables the interaction between transverse slab bending (negative or positive) and longitudinal shear to be taken into account.

It should be noted that Equations (7.1) and (7.2) are independent of longitudinal stress and consequently independent of a favourable state of longitudinal compression due to prestressing if there is such a state at all.

account in Equations (7.1) and (7.2) or by some other method requires consideration.

concrete tee-beam, but in regions remote from the connectors, the CEB/FIP Recommendations on this subject may be appropriate.

The simple method of providing sufficient transverse rein forcement to develop the design strength of the connectors is conservative in beams subject to repeated loading where the connector spacing is determined by fatigue rather than static strength, but it avoids the problems of defining critical cross-sections in beams where bending moment envelopes have to be considered. Alternatively, the shear force per unit length may be determined by elastic analysis.

The question of whether the state of longitudinal compression can be taken into

The transfer of longitudinal shear in a composite beam is more severe than in a

102

RECOMMENDATIONS

The ultimate longitudinal force in the slab at the internal support, F'", due to the moment of resistance M'u (which may exceed M'r) given by 5.3 .l, is calculated in accordance with 73. Let N'f be the number of shear connectors required for this force, in accordance with 7.4.

The number of connectors N corresponding to the bending moment M, is calculated in accordance with 7.5.2, as if the sagging moment region of the shear span considered were part of a simply-supported beam.

uniformly spaced over the length of the shear span. The number of connectors provided must be not less than N'f t N. They may be

I

7.5.3.1 Shear span adjacent to a simple end support

When the method of 7.5.3 is used for internal shear spans, end shear spans should be designed in accordance with 7.5.2.

7.6 TRANSVERSE REINFORCEMENT

7.6.1 LONGITUDINAL SHEAR

The total longitudinal shear force per unit length of beam should be taken as the design maximum load per connector at the ultimate limit state (Clause 7.2)multiplied by the number of connectors per unit length. The shear force per unit length acting on any plane through the concrete, vg, should satisfy the following:

VQ > kl sLS t 0.7Ae fyf (7.1) k2 Lsfck (7.2)

where kl is a constant equal to 0.9 for normal density concrete and 0.7 for light-

weight aggregate concrete. k2 is a constant equal to 0.19 for normal density concrete and 0.1 5 for light-

weight aggregate concrete. Ls is the length of the shear plane under consideration. Typical shear planes are

shown in Figure 7.1. s is a constant stress of 1 N/mm2, reexpressed where necessary in units con-

sistent with those used for other quantities. Ae is the sum of the cross-sectional areas of transverse reinforcement per unit

length of beam'crossing a shear plane that can be assumed to be effective in resisting shear failure on that plane.

fyr is the characteristic yield strength of the reinforcement. fk is the characteristic 28day cylinder strength of the concrete.

If f k is less than 16 N/mm2, the term kl sLs in Equation (7.1) should be replaced by k3fckLs, where k3 is a constant equal to 0.05 for normal density concrete and 0.04 for lightweight aggregate concrete.

103

COMMENTARY ’

1 04

RECOMMENDATIONS

I

a I

a I r A t

I I

I I I 1-m-- Ell I' I I I 1 I I I I

1 I * I t 1 I

a 1 b ' a

a

Fl c - c

Figure 7.1 Shear planes.

105

COMMENTARY

7.6.2 The values of the constant terms and coefficients used in 7.6.1 and 7.6.2 are

subject to revision when the partial factors of safety on loads and material strengths have been established.

106

RECOMMENDATIONS

7.6.2 INTERACTION. BETWEEN LONGITUDINAL SHEAR, TRANSVERSE PRESTRESSING AND TRANSVERSE BENDING

The effect of interaction between longitudinal shear and transverse bending or transverse prestressing of the slab may be taken into account by replacing Equation (7.1) by the following:

VQ P k I sL, t 0.7Aefyr - 1.6F where F is the nett force per unit length of beam acting normal to the shear plane under consideration due to transverse bending of the slab and/or transverse prestressing; taken as negative (minimum compression) or positive (maximum tension), as appropriate. In assessing F no account shall be taken of transverse bending of the slab due to loads of a non-permanent nature, such as imposed loads or partitions that may be subsequently removed, if their effect is to increase the ultimate shear strength . 7.6.3 MINIMUM TRANSVERSE REINFORCEMENT

The cross-sectional area of transverse reinforcement per unit length of beam that crosses a possible plane of shear failure and which can be considered as effective in resisting shear should be not less than the amount given by the following equation:

A, fyr 4 O.75sLs t 1 .lF where the terms are as defined in 7.6.1 and 7.6.2.

7.6.4 SPACING OF TRANSVERSE REINFORCEMENT Transverse reinforcement may be uniformly distributed over any length where

the shear connectors are uniformly spaced. The spacing of bottom transverse bars, if provided to satisfy the requirements of 7.6, should not exceed four times the projection of the connectors (including any hoop which forms an integral part of the connector) above the bars, nor 600 mm, whichever is the lesser.

107

COMMENTARY

For compact beams subject to predominantly static loading the requirements of Section 8 need not be considered. (See Table 3.3 in Section 3.)

I 8.2 I

I The load is limited to 0.6 Pd to ensure that slip at the steel-coricrete interface is not high enough to invalidate calculations for stresses and deflections based on full- interaction theory.

8.4 I n composite 'beams where the shear connection has sufficient static strength to

satisfy the requirements of 8.3 the influence of maximum stress on fatigue behaviour may be neglected. The shear connection required a t any cross-section is governed by the range of longitudinal shear per unit length, AV, where AV = U l n a - Vlnin and vmax and vmi, are calculated from the maximum and minimum vertical shears at that section due to the loading cycle considered.

For the.fatigue limit state, the design ranges of loads and stress for shear connectors may be assumed to be equal to the characteristic ranges AV as given in Section 6 (ie, T~ = 1.0).

trum specified in National Codes. For fatigue investigations, the design loading should be taken as the load spec-

108

RECOMMENDATIONS

Section 8. Design of the shear connection - serviceability limit state

8.1 LONGITUDINAL SHEAR Longitudinal shear per unit length of beam, whether simply supported or con-

tinuous, should be calculated on the basis of elastic theory, using the properties of the cross-section determined in accordance with 4.4, and assuming the concrete to be uncracked.

8.2 MAXIMUM LOADS PER CONNECTOR - STATIC LOADING The maximum load per connector at the serviceability limit state should not

exceed 0.6 Pd, where Pd is the design strength ,determined in accordance with Section 6.

8.3 DESIGN REQUIREMENTS - STATIC LOADING The size and spacing of the connectors at each end of each span should be not

less than that required for the maximum loading considered. This size and spacing should be maintained for at least 10 per cent of the length of that span. Elsewhere, the size and spacing of connectors may be kept constant over any length where, under the maximum loading considered , the maximum shear force per unit length does not exceed the design shear flow by more than 10 per cent. Over every such length the total longitudinal shear force must not exceed the product of the number of connectors and the design static strength per connector.

8.4 DESIGN FOR FATIGUE

109

COMMENTARY

The size and spacing of the connectors at each end of each span should be not less than that required for the calculated shear range; This size and spacing-should be maintained for at least 10 per cent of each span. Elsewhere, the size and spacing of connectors may be kept constant over any length where the calculated shear range does not vary by more than 10 per cent from the design stress range.

Over every such length the spacing should be such that the longitudinal shear ranges per unit length multiplied by the connector spacing, ie, the shear ranges per connector, satisfv the following requirements for the design spectrum of loading:

1. For stud connectors, the allowable number of occurrences N for each cycle of service loading giving a shear range AV expressed as a percentage of the Characteristic static strength should be derived in accordance with Section 6 and using graphical interpolation where necessary. The summation of the ratios of the design number of cycles n for each service loading to the corresponding allowable number of cycles N should not exceed unity.

2. For other types of connector, the allowable number of occurrences N for each cycle of service loading giving a nominal shear-stress range on the weld throat.should be derived in accordance with Section 6 and using graphical interpolation where necessary. The summation of the ratios of the design number of cycles n to the corresponding allowable number of cycles N should not exceed unity.

RECOMMENDATIONS

111

, ~~

COMMENTARY, ~I . " 3

I

9.1.1

For the purpose of calculating the restraint force in the concrete slab due to temperature effects, the strain should be assumed to act over the full breadth of flange, but in stress calculations, the effective breadth of flange should be used.

of temperature, the calculation o f moments and reactions should take account of the secondary (parasitic) effects that occur in statically indeterminate structures subject to internal strains.

In continuous beams, in addition to the primary (statically determinate) effects

I 9.1.2

nectors due to the longitudinal force VQ: The following assumptions may be made when calculating the loads of con-

a ) for flexible connectors, that the load is transferred uniformly at a rate V@, over a length Qs measured from each end of the beam, where Qs is taken as the effective breadth of the concrete flange, or is as defined in national codes; and

b) for rigid connectors, that the rate of transfer of load varies linearly from a maximum o f 2 VdQ, at each end of the beam to zero at a distance Qs from each end of the beam, where Qs is as defined in a) above.

'I12

RECOMMENDATIONS

Section 9. .Temperature, shrinkage and creep

9.1 TEMPERATURE EFFECTS

9.1.1 GENERAL

beams in buildings. Elsewhere the effects of temperature should be considered as follows.

1. Longitudinal shear forces due to temperature effects should be considered during construction and at the serviceability limit state.

2. In stress calculations for beams where the cross-section is slender, temperature effects need only be considered at the ultimate limit state and during construction.

3. In stress calculations in beams where the cross-section is compact (see 5.2), the effects of temperature need only be considered during construction and at the serviceability limit state.

It is normally not necessary to consider the effects of temperature on composite


perature of the concrete slab being different from that of the steel beam or due to a temperature gradient through the cross-section. They may also occur where the concrete has a coefficient of linear thermal expansion significantly different from that of the structural steel section, in which case, differential expansion will occur under a uniform change of temperature.

The longitudinal shear force Vg due to primary effects of temperature should be assumed to be transmitted from the concrete to the steel beam by shear connectors at the ends of the beam, ignoring the effect of bond.

In the absence of more precise information, these concentrated shear forces may be calculated by an elastic analysis assuming full interaction, using the properties of the crosssection as defined in 4.4, and assuming the concrete to be uncracked. The modulus of elasticity of concrete, E,, should be that value appropriate to short-term loading.

Longitudinal shear forces may occur in a composite beam either due to the tem-

113

I 9.2. I

In continuous composite beams in addition to the redistribution of stresses that occurs in simply supported beams (primary effects), shrinkage and creep will also cause a redistribution of the bending moments and support reactions, which should be taken into account.

9.2.2 and 9.2.3 The conditions for maximum shrinkage (very dry environments) also correspond

to the conditions for mdximum creep.

* I

114

RECOMMENDATIONS

9.1 3 LONGITUDINAL'STRESSES AND STRAINS

of the effects described in 9.1.2 may be calculated from simple elastic theory, assuming full interaction between the concrete slab and the steel beam.

Longitudinal stresses and strains in the concrete slab and steel beam due to any

The concrete slab should be assumed to be uncracked with the effective breadth of concrete flange determined in accordance with the CEB/FIP Recommendations for concrete structures. The modulus of elasticity of concrete, E,, should be that value appropriate to short-term loading.

9.2 SHRINKAGE AND CREEP I

9.2.1 GENERAL Shrinkage and creep should be determined in ,ccordance with the CEB/FIP '

Principles and Recommendations, except that where the flange of the steel beam is completely encased in concrete, reference should be made to specialist literature.


to be transmitted from the concrete slab to the steel beam in the manner described in 9.1.2 and may be calculated using the assumptions given in 9.1.2 but using the free shrinkage strain and a modulus of elasticity for concrete appropriate to long- term loading determined in accordance with CEB/FIP Recommendations.

The longitudinal shear force due to shrinkage modified by creep may be assumed

9.2 3 LONGITUDINAL STRESSES AND STRAINS The longitudinal stresses and strains due to shrinkage modified by creep may be

calculated using the assumptions given in 9.1.3, but using the free shrinkage strain and a modulus of elasticity for concrete appropriate to long-term loading determined in accordance with the CEB/FIP Recommendations.

115

COMMENTARY

I . s . .

. v

10.1

Methods for calculating crack width differ from country to country, but most are based on the results of research on reinforced concrete rectangular beams and slabs.' Until the application of such methods to composite beams has been thoroughly assessed, no detailed method of calculating crack width is given in this Section.

. - ..

116'

RECOMMENDATIONS

Section 10. Control of cracking

10.1 GENERAL (1) In concrete elements designed for Class N verification (reinforced concrete)

adequate reinforcement should be provided to prevent cracking from adversely affecting the appearance or durability of the structure. b i t i n g crack widths and the principles on which methods for calculating crack widths are based should be in accordance with the CEBIFIP Recommendations.

(2) For prestressed beams see Section 12.

I 117

COMMENTARY I

11.1 i

ECCS Advisory Committee I I is preparing Recommendations on deformations of structures.

11.2

( I ) Provided the stresses in the steel or concrete due to any combination of design loads at the serviceability limit state or during construction do not exceed the limits given in 5.5.4 deflections may be calculated by elastic theory using the elastic properties given in 3.4 assuming fi l l interaction between the steel beam and the concrete slab, and neglecting concrete in tension in hogging (negative) moment regions, in accordance with 4.4.

For prestressed composite beams see Section 12. In the absence of a rigorous analysis, allowance for in-plane shear flexibility

(shear lag effects) may be made in calculations based on elementary theory of bending, by using an appropriate effective breadth of jlunge.

Consideration should be given to the effects of shrinkage and creep. Where appropriate the CEBIFIP Recommendations for Structural Concrete may be used.

Alternatively, the deflections due to permanent loading may be calculated by using a modulus of elasticity of concrete appropriate to long-term loading, determined in accordance with 3.4. Where appropriate, proper account should be taken of the deflections of the steel section due to loads applied to it prior to the development of composite action.

(2) Where it is necessary to determine the deflections of a composite beam due to loadings which cause the stresses in the steel or concrete to exceed the limits given in 5.5.4 the deflection should be calculated using non-linear elastic plastic theory. The stress-strain relationship for structural steel should be that specified in the ECCS Recommendations. The stress-strain relationships for concrete and reinforcement should be those specified in the CEBJFIP Recom- mendations.

118

11.1 GENERAL

Where it is necessary to check deflections, the distribution of bending moments shall be determined in accordance with 4.4, with rf taken as 1 .O, as proposed in the Commentary to Clause 3.7.2.

11.2 CALCULATION OF DEFLECTIONS Criteria for limiting deflections depend on the functional criteria for the structure

considered, to such an extent that no specific recommendations can be given.

RECOMMENDATIONS

Section I 18 Deflections

/ I I

119

I

,

COMMENTARY

11.3 When according to 7.5.2 or 7.5.3 it is required that N 2 0.5 Nf(or even N > 0.4

Nf) then the deflection generally may 'be calculated according to 11.2 (1) assuming f i l l interaction. Supporting evidence from results of tests by Baldwin (USA) and Stark (Holland) is shown below.

Beams with stud connectors

0.3 c 0.2 -

0.1 -

0.1 L 0 ! 0.4 0.6 0.8 1 .o

120

' Not permitted ' 6, - 6f " 0 , so 6, = 6f .

Figure C11.1 Short-term deflection of beams with partial shear connection ' (Johnson, R P, and May, T M, "Partial interaction design of

composite beams", Structural Engineer, Volume 53,305-31 1 , August 1975).

RECOMMENDATIONS

11.3 DEFLECTIONS OF SIMPLY-SUPPORTED BEAMS WITH INCOMPLETE CONNECTION

The deflection 6 of simply supported composite beams with flexible connectors designed in accordance with 7.5.2 should be determined taking into account the effect of slip at the serviceability limit state.

11.4 LIMITATIONS ON DEFLECTIONS

(1) In bridges, no limitations are normally imposed other than to require that the deflection of the superstructure or any part of it should not adversely affect the appearance or efficiency of the structure. The calculation of deflections will normally only be necessary where:

a) specified minimum clearances may be exceeded, b) the surface water drainage would be impaired, c) the method of construction requires careful control of profde.

In buildings in addition to the reasons given in (1) limitations on the deflections may be required to enable other elements of the structure, eg, claddings, glazing, to be positioned and to function correctly. In the absence of further information, deflections should not exceed the limits specified in national codes.

COMMENTARY .

. ..

12.1

is reduced by high inelastic deformations, cracking of concrete, or plastic deformations up to failure. Compact composite beams are thereforegenerally not prestressed.

The instantaneous losses of tension or prestressing (losses in jacks or at anchorages, frictional forces in the ducts or between the concrete part and the steel beam, reductions in the lengths'of members at tensioning) should be taken into account as appropriate, according to the CEBIFIP Recommendations. The same applies to the deferred losses of tension due to relaxation of the prestressing steel and due to creep and shrinkage of the concrete.

I I The effect of prestressing isgenerally important in the elastic range. This effect

12.2. The stress state due to the weight of wet concrete in the steelwork of a com-

posite beam which is unpropped during construction, should not be regarded as a stage of prestressing.

12.3 I Regions near end supports may be in a different class from the rest of the beam,

due to the location of anchorages for tendons. Temperature differences and shrinkage cause concentrated anchorage forces and tensile stresses a t the ends of composite beams. These tensile stresses cannot in general be eliminated by the addition of compressive stresses due to prestressing. Therefore the end regions may belong to class IV members even when the Composite beam should meet the requirements of class I or 11 concrete members.

RECOMMENDATIONS

Section 12. Prestressing in composite construction

Prestressing can reduce, or in some circumstances prevent the cracking of concrete under service conditions, so increasing stiffness and improving the protection of steel and reinforcement from corrosion.

1 2.2 METHODS OF PRESTRESSING Prestressing may be achieved mainly: 1) by means of imposed deformations, for example by jacking of the supports; 2) by means of prestressing tendons, tensioned before or after connecting the

concrete flange to the steel beam, and bonded to the structure; 3) by means of a combination of both above-mentioned methods. Special consideration should be given to composite beams which are prestressed

by an external system or by tendons not directly bonded to the concrete (tendons in ungrouted internal ducts, or external tendons even when grouted in ducts). In these circumstances, the calculation of the prestressing forces must take account of the deformation of the whole structure.

12.3 DEGREE OF PRESTRESSING

The degree of prestressing in composite beams shall be classified in accordance with the CEB/FIP Recommendations for concrete structures, with the exception of the end regions of beams.

123

COMMENTARY

12.5

ing conditions, for example for dead load and jacking.

tions based on elastic theory, by using a modulus of elasticity of concrete, E*c, given by: I

Care must be taken of shear lag effects, which may be different for certain load-

For long-term loading, the effects of creep may be taken into account in calcula-

E C E'c = I + $ &

where E, is the short-term modulus of elasticity and & is the creep coefficient obtained from the CEBfFIP Principes and Recommendations and J/ takes account of the properties of the composite cross-section and the type of loading.

12.6

The mc thod of all( wing for creep given in Commer the ultimate limit state.

tary 12.5 is also applicable a t

12.6.1 For example, yp should be taken as 1.2 where local effects of prestressing are

unfavourable, as where prestressing tendons are anchored and concentrated forces have to be transmitted from the concrete part to the steel beam.

12.7 .

Delayed strains in the concrete lead to stress redistributions from the concrete flange to the steel beam, thereby causing earlier crack formation under increased loading which results in reduced stiffnesses.

124

RECOMMENDATIONS

12.4 LIMIT STATE REQUIREMENTS Composite members, which are prestressed, should be designed for the service-

ability and ultimate limit states in accordance with the general requirements of this Code.

1 2.5 SERVICEABILITY

the appropriate CEB/FIP Recommendations for concrete members.

linear elastic anlysis assuming an uncracked slab should be carried out.

Stress limitations in the tension zone of the concrete part should comply with

In order to determine the distribution of bending moments and shearing forces a

The compressive stresses in the concrete part due to flexure, including stresses due to imposed deformations and prestressing forces, should not exceed 0.6 fck.

The stresses in prestressing tendons are limited as in the appropriate CEB/FIP Recommendations.

The stresses in the steel beam are limited in accordance with 5.5.4, except that before creep and shrinkage have taken place, the maximum tensile stress in the structural steel should not exceed 0.95 times the characteristic yield strength divided by rm.

12.6 ULTIMATE LIMIT STATE

1 2.6.1 PARTIAL SAFETY FACTORS The partial safety factors for prestressing forces and imposed deformations

should be in accordance with Section 3.9, taking account of the various combinations with other actions.

12.6.2 SLENDER COMPOSITE BEAMS The appropriate clauses of this Code on slender beams apply.

12.6 3 COMPACT COMPOSITE BEAMS The definition of compact beams in 5.2.1 refers to the depth of the web in com-

pression. In calculating th is depth, account should be taken of prestressing tendons, at the appropriate yield stress, provided that the tendons are prestressed as appropriate and directly bonded to the concrete part.

12.7 . CONTROL OF CRACKING Requirements with regard to the durability of composite structures, particularly

the choice of appropriate limit states of crack width, should comply with the CEB/ FIP Recommendations. Special consideration should be given to the partial coefficient rc as given in Section 3.7 3.

125

13.2 Methods of checking for vibration serviceability are available: Mason, D, “Testing and design for vibrations of office floors with composite

construction”. Proceedings of Conference on Steel Structures, p p 140- 148, Monash University, Australia, May 1977.

96- 100, Oosby Lockwood Staples, London, I9 75. Johnson, R P, ‘X‘omposite structures of steel and concrete’: Volume 1, p p

13.3

Appendix D of British Standard 5400, “Steel, concrete, and composite bridges”, Part 2, ‘Zoads’: British Standards Institution, 1978.

bridges’; Supplementary Report 275, I I6 pp, Transport and Road ,Research Laboratoty, Crowthome, England, May 1977.

Vibration serviceability requirements for footways and cycle tracks are given in

More detailed in formation is available: “Symposium on dynamic behaviour of

I 126

RECOMMENDATIONS

Section 13. Vibration

13.1 GENERAL Consideration should be given to the possibility that in structures subjected to

fluctuatingloads in service, the frequency or amplitude of vibration may be sufficient to cause distress to users or local damage to the structure.

13.2 BEAMS FOR BUILDINGS

Vibration should be considered in the design of composite floor structures of high span-to-depth ratio that may be subjected to dynamic or impact loading, for example from machinery or fork-lift trucks.

13.3 BEAMS FOR BRIDGES In highway bridges, the effects of vibration due to traffic need not be considered. In footbridges and cycle track bridges, consideration should be given to the

vibration which can be induced by resonance with the movement of users.

127

COMMENTARY

I

14.2

bed: In geneml the following tolerances are realistic for construction without a mortar

128

Shear connection Tolerance Length of surface considered

Studs

Friction grip bolting

1.0 0.3

1.0 0.3

RECOMMENDATIONS

Section 14. Composite beam with precast slab

14.1 GENERAL For the design of the concrete slab, its reinforcement, the beams, and the shear

connectors the same rules have to be applied as for concreteaslabs cast in situ, unless stated otherwise in this Section.

14.2 JOINT BETWEEN STEEL BEAM AND CONCRETE SLAB

300 mm wide.

the bearing surface between the concrete slab and the steel flange must be small enough to avoid excessive local stress in the concrete slab, especially in case of friction grip-bolting according to 14.4.3.

rosion.

The application of a mortar bed generally requires a steel flange not less than

If the slab is laid without a mortar bed, the vertical tolerances of the flatness of

Care has to be taken to protect the upper flange of the steel beam against cor-

14.3 SHEAR CONNECTION

concrete slabs which are filled with concrete after erection, the size and shape of recess and quality and method of compacting of concrete in-fill should be checked by tests in accordance with 6.5.

the steel beam after erection of the slab, care has to be taken to avoid damage to the concrete by excessive heat, if welding is used.

If shear connectors welded to the steel beam fit into joints and/or recesses of the

'If shear connectors are embedded in the concrete of the slab and connected with

14.4 TRANSVERSE REINFORCEMENT Where a joint between the concrete slabs is parallel to the steel beam and above

the steel beam, continuous transverse reinforcement is not required, but the requirements of 7.2 have to be fulfilled for each of the two slabs independently.

14.5

Columns

\r

1 Tension member required --

Joints between deck slabs, above steel beams

I +

+

I

i

COMMENTARY

Ii + Region of increased pressure

- Region of decreased pressure of stud against slab

of stud against slab

Figure C14.1

130

RECOMMENDATIONS

14.5 CONCRETE DECK AS DIAPHRAGM If the concrete deck of a composite structure is designed to resist lateral forces,

account should be taken of the interaction between the resulting horizontal shear forces and those due to the effects of the composite action, as these may add up in the joints between the concrete slabs parallel to the steel beam.

reinforcement in the slabs or tension members connecting the steel beams. Tension forces in the edges of the concrete deck may require additional transverse

. .

14.6 SHRINKAGE AND CREEP

by considering the age of the precast slabs at the time of erection. Appropriate values should be used for a slab composed of a precast slab and a layer of in-situ concrete.

Reduced values for shrinkage and creep of the concrete slab may be introduced

131

COMMENTARY

I '

15.1.1 I For designs involving the composite action between a profired steel sheetlcon-

Crete slab with the supporting beams, see Section 6 for shear connector design and 5.1.2 for effective thickness of flange.

Figure C15.1 Typical floor with profiled steel sheets: (1) floor finish; (2) profded sheet; (3) structural concrete, (4) mesh reinforcement; (5) topping.

( 6 ) - deformations

Em bossments Indentations

Figure C 1 5.2

RECOMMENDATIONS

Section 15. Composite floors with profiled steel sheet'

15.1 SCOPE

1 5.1 .l COMPOSITE FLOORS The design of these floors is based on the composite action which occurs between

profiled steel sheeting and concrete when spanning in the direction of the ribs.

Designs may be carried out on slabs for which the composite behaviour has been established by test referred to in 15.4.2. Pure bond between steel sheet and concrete is not considered effective for composite action which must be achieved by positive mechanical interlock. This interlock may be ensured by one or more of the following means:

a) the profde shape (with reentrant form), b) deformation of the profile (indentation or embossment), c) end anchorage.

133

15.1.2 For structures where the imposed load is largely repetitive or applied abruptly

in such a manner as to produce dynamic effects profiled steel sheet is permitted provided the engineer gives careful consideration to its detailed design and use with especial regard to maintaining the structural integn'ty of the composite action. Treatment of vibration is covered in Section 13.

15.2.2

g/m2 has been found satisfactory. Depending on service conditions a coating class ranging from 100 g/m2 to 2 7.5

15.2.3 For design calculations based on the guaranteed minimum yield stress the bare

metal thickness of the sheet shall be used. Where the yield stress is based on coupon tests carried out on the galvanised material then it shall be made clear in calculations that this is the effective yield stress of the material based on the total sheet thickness.

15.2.4 The requirements for slab thickness and cover derive from consideration of

aggregate size and the structural integrity of composite slabs using certain profiled steel sheets in a transverse direction.

Figure C 1 5.3

Minim urn thickness of slab

d - e 2 50mm

Maximum diameter of aggregate d - e D G 3

d 3 90mm D < 30mm D < w'/3

--

134

RECOMMENDATIONS

15.1.2 CLASSES OF STRUCTURE

This section applies to designs for building structures where the imposed loads are predominantly static. Other applications are not excluded provided an appropriate design method is presented.

1 5.2 MATERIALS

15.2.1 STEEL

and 32 - 1966, with a minimum grade of Fe 360. The basic material should be mild steel in accordance with Euronorms 25 - 1972

1 5.2.2 COATING Galvanising should be in accordance with the IS0 standard on galvanising:

“Continuous hot-dip coated carbon steel sheet of structural quality, IS0 4998 - 1977”, (IS0 TC 17), or to National Standards.

1 5.2.3 MINIMUM SHEET THICKNESS

reinforcement the bare nominal metal thickness of sheet should not be less than 0.70 mm.

For floors where the profiled sheet constitutes the loadcarrying element as the

1 5.2.4 MINIMUM SLAB THICKNESS

ribs not less than 50 mm. The overall thickness of the slab shall not be less than 90 mm and cover over the

135

COMMENTARY

15.3.3

the yonding effect is covered by the uniformly distributed construction load of 1.0 kN/m’. For deflections greater than 20 mm the ponding effect may be

In cases where the deflection is not greater than 20 mm it may be assumed that

136

RECOMMENDATIONS

1 5.2.5 TOLERANCE

differ from the specified dimensions by more than the tolerance listed below. Dimensions of a profiled steel sheet which is to be used compositely shall not

Overall depth of deck

Sheet thickness

Dimension of shear transfer devices

t 4% - 1% + 10% - 5%

4- 20% - 10%

1 Spacing of shear transfer devices + 10%

Due to the variations in types of embossment and indentation the manufacturer shall specify each form in an acceptable manner referring to depth, length, radii and slopes and shall further maintain this form during manufacture within the above tolerances.

I 15.3 DESIGN METHODS - SHUTTERING

15.3.1 LOADS

deck as shuttering: I The following loads shall be taken into consideration in calculations for the steel

dead load of the deck, dead load of the wet concrete, construction loads. The construction loads represent the weight of tradesmen and concreting plant

and take into account any impact or vibration which may occur during construction. The construction loads shall be taken as the worst case of either a) a uniformly distributed design service load of 1 .O kN/m2 or b) a knife edge service load of 1.5 kN/m parallel to the supporting beam, placed in the most unfavourable position.

Wherever possible unsupported edges to the steel deck should be avoided otherwise particular care should be taken during design and construction as unsupported edges may suffer gross deformations when subjected to concentrated loads.

15.3.2 CALCULATIONS FOR STEEL DECKING

mendations for steel structures. Decking may also be designed on an experiemental basis in appropriate cases.

Design calculations should generally be in accordance with the European Recom-

15.3 3 DEFLECTION

greater than 20 mm the ‘ponding’ effect (increased concrete depths due to the deflection of the sheeting) should be considered.

The requirements of IS0 proposal TC 98 SE 4 shall be observed. For deflections

137

COMMENTARY

allowed for by considering'an extra uniformly distributed load of 0.7 x specific weight of concrete (kN/m3) x central deflection (m) klV/m2 in addition to the construction load.

0.7 x 6 x weight of concrete r Concrete to allow for ponding effect

I

Deflection neglecting ponding effect

Figure C15.4 Ponding effect.

15.4.1

(1) Effective bond ensured by profile shape. The interaction between profiled steel decking and concrete in surface bond is complex. The bond developed between a hardened concrete slab and a completely f lat steel sheet is random and may easily be destroyed by slight impact or by shrinkage of the concrete. For this reason composite designs for plain open profiled sheets are not recommended unless some form of positive interlock is employed. Plain profiled sheets with some form of re-entrant angle develop a more reliable effective bond with the concrete due to the interlocking shape.

(2) Deformations, anchor straps or other mechanical connections. Composite slabs in this category rely on some form of discrete mechanical connection between sheet and concrete slab.

For profiled sheets with deformations the composite action is dependent on the type feg, inclined ribs or lugs, cusps, dimples, etc), depth and number of deformations, and the span of the slab. Failure of the composite slab using this type of sheet is generally by longitudinal shear. Design based on longitudinal shear capacity is semi-empirical as referred to in Section 15.4.

Slabs with profiled sheets with anchor straps at discrete spacings throughout the slab span may be designed on the same basis as composite beams with shear connectors. With a known value of anchor strap shear strength the design load m y be calculated.

The use of other types of mechanical connection will generally require tests to establish the shear capacity of the connection, and to show an adequate deformation capacity.

138

RECOMMENDATIONS

15.4 DESIGN AND TESTING OF COMPOSITE SLABS

1 5.4.1 COMPOSITE ACTION

The composite action which can occur between a profiled steel sheet and a concrete slab depends on the degree of connection present at the interface between the concrete and steel. Composite action may be achieved in at least one of the following ways:

1) by effective bond ensured by the profile shape (with a re-entrant form);

2) by deformations in the profiled sheet, anchor straps or other mechanical connections positioned throughout the span;

3) by anchorages at the ends of each span of the slab preventing slip between

The ultimate load at the limit state of collapse is identified by two main failure

a) Longitudinal shear in slabs with incomplete shear connection. The behaviour of such slabs under test is characterised by slip failure.

b) Flexure in slabs with complete shear connection. The slab is able to develop full flexural strength.

Either failure mode is possible for a composite slab using a particular profiled steel sheet. The mode is primarily dependent on the span and depth of the slab.

Design for longitudinal shear shall be based on test information for each type of profiled steel sheet. The tests should provide data for the ultimate strength design equations and are referred to in 1 5.4.2 and 1 5.4.3.

Design for flexure may be based on conventional reinforced concrete theory for the ultimate state provided tests carried out initially for each type of profiled steel sheet have confirmed that failure is by flexure. The tests are referred to in 15.4.2 and 15.4.3.

the concrete and the profiled sheet.

modes (see also 2.1), which are as follows.

139

COMMENTARY

(3) Anchorages. Composite action between the steel decking and concrete slab can be ensured by the use of anchorages at the ends of the span which prevent slip between the concrete slab and the profiled sheet. Anchorages of this type m y be provided to produce composite action with plain profiled sheets or to enhance the load-caving capacity of a composite floor slab. The anchorages, which may take the form illustrated in Figure C15.5, are required at end supports for simply supported slabs and at the ends of each span for continuous slabs.

Tests must be carried out to indicate that:

I ) the anchorage is effective, and 2) the proposed design method for the slab is satisfactoty. I

If it is proposed to omit anchorage at internal supports this must be justified by test.

d Figure C15.S Anchorage at end support.

Design for flexure. Propping during the construction stage has no effect on the ultimate strength of the composite slab.

15.4.2

I f type I tests are made over a sufficient range of parameters, information may be obtained on the effective bond or anchorage force which may then be inserted into appropriate design expressions to give an approximate general method of calculation .

I f type 2 tests are made, the test must be full gcale and must simulate the actual site conditions and the results obtained may not be interpolated or extrapolated for other cases.

Type 3 tests are for composite slabs with an abnormal structural form (eg, skew- slabs) or for special loadings (eg, fork-lift trucks).

140

RECOMMENDATIONS

1 5.4.2 THE USE OF TEST RESULTS

types of test may be carried out, as follows. For each of the three forms of composite action considered in 15.4.1, three

1. Tests carried out to establish a semiempirical basis for design (15.4.3).

2. Tests carried out on a particular structural application in accordance with standard test procedures (1 5.4.4).

3. Tests carried out to establish a design for a specific structural application using non-standard test procedure.

In general, every application of test results to a structure should take account of the current building and safety regulations.

141

COMMENTARY

15.4.3

A typical graph of longitudinal shear failure is shown in Figure C1.5.6.

V € =I

s =

b = fc =

m = d = k =

i

Lv =

P =

maximum experimental shear (NI spacing of shear transfer device (mm) slab width (mm) concrete cylinder strength (N/mm* 1 slope of 15% reduction line effective depth (mm) intercept on 15% reduction line distance from support to nearer point load in a symmetrical two point loading system (shear length). reinforcement ratio.

1

Figure C15.6

To establish the design relationship for longitudinal shear capacity, tests are required on specimens in regions A and B indicated in Figure C15.6. Region A includes specimens with small slab depths and region B with larger depths and small shear lengths. A minimum number of three tests in each region is sufficient provided the variation from the mean of the three results is not greater than 2795%. When the variation is greater than +7%% three further tests should be carried out and the six test results may be used to obtain the regression line. The regression line is reduced by up to 15% to ensure that the experimental value willgenerally fall above the reduced line and to account for some minor variations in the test results, profile thickness and dimensions. When eight or more tests are carried out the reduction line may be taken as 10% below the regression line.

15.4.4

A minimum of three full-scale tests shall be carried out on the proposed floor construction using actual loading, or, in the case of uniformly distributed loads, a close simulation of the loadingas shown in Figure C15.7. In the case of continuous spans, either multiple spans shall be tested or the support moments simulated on a single span.

The width of the slab should have a value not less than each ofi

i) three times the overall depth ii) 600 mm, and iii) width of the profiled sheet. Thin steel-plate crack inducers extending to the full depth of the slab and coated

with a debonding agent shall be placed across the full width of the test slab. Alter natively, the crack inducer may be limited to the tensile zone of the concrete. In

142

RECOMMENDATIONS

1 5.4.3 TESTING. SEMI-EMPIRICAL BASIS OF DESIGN (TYPE 1 , 1 5.4.2)

15.4.3.1 General The variables to be investigated include the type of steel decking, steel deck

properties, loading arrangement, concrete properties and shear lengths. From these tests the ultimate applied load, the mode of failure and the load/deflection and load/slip performance is obtained.

1 5.4.3.2 Design load - longitudinal shear mode

To derive a representative linear relationship for the ultimate longitudinal shear capacity the full practical range of values of slab depth and shear lengths should be covered in the tests.

derived from a reduction of the regression line (see Figure C15.6). Typically, the calculated ultimate shear capacity is:

~

The design load is determined from the test information using the coefficients

(15.1)

where 9 is a capacity reduction factor (= 0.8) based on the mode of failure and the behaviour prior to,failure. Some adjustment to Vu may be necessary to allow for the dead load resulting from propping.

15.4.3.3 Design load - flexural mode

out. Generally three tests shall be performed at the minimum span and three tests at the maximum span of the range, for different slab thicknesses.

Provided the ultimate experimental failure load is greater than the calculated ultimate flexural design load then conventional reinforced concrete theory may be used.

To establish that design can be based on flexural capacity, tests should be carried

15.4.4 TESTING. FULL SCALE SLAB (TYPE 2,15.4.2)

1 5.4.4.1 General

This type of test is carried out for a particular structural application where the test arrangement simulates the actual site conditions. The result obtained shall not be extrapolated for other cases and careful consideration shall be given to any interpolation of these results. The test procedure is intended to represent loading over a period of time. Crack inducers are used to ensure that cracks form in the tensile zone of the slab.

1 5.4.4.2 Design load For a proposed design imposed load P the slab should be subjected to 10 000

cycles of load between approximately 0.3P and 1.5P followed by loading to failure under static load P,. The design load is the minimum value from three tests of:

143

COMMENTARY

the case of four-point loading these shall be positioned under the central point loads as shown in Figure Cl 5.7.

Concrete and outline of crack inducers

Figure C 15.7

The surface of the steel decking shall be used in the ‘as rolled’ condition, no attempt being made to improve adherence of the concrete by degreashg the surface of the sheets.

I

144 I

RECOMMENDATIONS

a) from dynamic loading - P b) from static loading with complete shear connection, ie, no excessive slip -

PU/2. c) from static loading with incomplete shear connection, ie, sudden excessive

d) from deflection - one half of the load at a deflection of span/50. slip - Pu/3.

1 5.4.5 LOADING

been given in 15.3.1.

posite slab:

The loads to be taken into consideration for the design of the steel deck have

The following loads shall be taken into consideration in the design of the com-

prop reactions, dead loads (topping, insulation, false ceilings, services, etc), live loads. The structural system is in general that of a beam continuous over several spans.

15.4.6 NEGATIVE REINFORCEMENT The negative support moments should be taken by top reinforcement designed

in accordance with normal reinforced concrete practice. When the slab is designed as a series of simply supported beams there is a risk

that cracks will develop over the supports under working loads. In order to reduce this form of cracking, the following minimum percentage of top reinforcement should be used:

145

COMMENTARY

15.4.7

I I

I' - 1

Figure C15.8 Distribution of concentrated loads: (1) transverse reinforcement; (2) durable topping.

Effective load width (measured in the concrete immediately above the ribs):

b m = b , + 2 a + 2 ( d - e ) Effective breadth of the slab:

Bending: simple beam:

4 continuous beam: bt = bm + (I - 5) x'

Shear: bt'= bm + (I -%)Y

where x' = the distance from the support to the load, Q = the span of the deck.

146

RECOMMENDATIONS

1 5.4.7 DISTRIBUTION OF CONCENTRATED LOADS

When point or line loads are considered in the calculations, an effective breadth of the slab may be assumed.

If point or line loads are the design criterion, the distribution of these loads should be ensured by the use of transverse reinforcement in this region. This shoulc be placed on the deck and its section should be at least 0.2% of the gross concrete section.

147

-__ ___ ~-

COMMENTARY

16.1

Composite columns may be used not on& as columns for buildings and bridges, but they may be used also for load-bearing stmctures exposed to fire, transmission towers, temporary structures, etc. I

I

16.2.1 The mild and high yield structural steel should conform with accepted European

grades or corresponding national grades with a yield strain not exceeding 0.2% unless test results show that methods contained herein can be applied to higher yield steels. The structural steel components may be either rolled or fabricated sections.

16.2.2

The concrete should be a normal density or lightweight concrete with a characteristic cylinder strength of not less than 20 Nlmm’ and with a maximum aggregate size not exceeding u/3 for concrete encased steel sections, d/6 for concrete filled steel tubes, and in no case greater than specified in the Model Code for Concrete Structures. The concrete of encased sections must have longitudinal and transverse rein forcement. No reinforcement is needed for concrete filled steel hollow sections, however additional rein forcement can be used for fire protection requirements. Note: U and d are as shown in Figure 16. I .

16.3.1 The structural steel members should be encased in, or filled with concrete over

the whole column length if the simplified methods given in this commentary are to be used. The major and minor axes of composite column sections should be taken as the major and minor axes of the structural steel section.

Figure C 16.1 Encased composite columns, additional cross-sections.

148

RECOMMENDATIONS

Section 16. Composite columns

16.1 SCOPE This section applies to composite columns of buildings and bridges, which may

be either concrete encased steel sections or concrete filed steel tubes in which concrete and steel interact completely to resist the load. The columns may be either statically determinate, or rigidly connected to other members at one or both ends.

16.2 MATERIALS

1 6.2.1 STRUCTURAL STEEL The characteristic values of properties are given in Recommendations for Steel

Construction - European Convention for Constructional Steelwork - Sections 2 and 3.

16.2.2 CONCRETE AND REINFORCEMENT STEEL The characteristic values of properties are given in the CEB/FIP Model Code for

Concrete Structures, Sections 2 , 3 , 4 , 1 0 and Appendix C.

16.3 COMPOSITE COLUMN CROSS-SECTIONS

16.3.1 GENERAL The design of composite columns with cross-sections of the following type

(Figure 16.l)is based on fully composite action up to failure between the structural steel elements and the concrete elements including reinforcement.

M

I

I Figure 16.1 Composite column cross-sections.

149

COMMENTARY

16.3.2 The symmetrical built-up sections with open webs of the types shown in Figure

C16.1, eg, double-channels or four angles, with intermediate and end battens may also be used in encased columns (see R Q Bridge and J W Roderick, ‘Behaviour of built-up composite columns ”, ASCE, St 7, July I9 78).

In the case of unsymmetdcal structural steel sections encased in concrete, or steel columns which are partially encased in concrete insufficient test results are available, so that further considerations will be needed before their use in design.

16.3.3 b is the external dimension at the wall of a rectangular hollow section. d is the outside diameter ofa circular hollow section. Es is Young’s modulus of elasticity of steel. f r is the characteristic yield strength of the steel. The limitations on wall thickness are needed to control local buckling. Where

these limitations are not met it is possible to allow for the reduced effectiveness of the steel in column strength calculations (see J P Grimault and J Janss, ‘Xeduction of the bearing capacity of concrete filled hollow sections due to local buckling”, Prelim Report, Stability of Steel Structures, Liege, I9 77).

16.3.4 In Equation (16.3) h, = the height of the structural steel section. Nearly all test

specimens had a concrete cover of 2 40 mm. For larger cross-sections the coverlh, ratio must be geometrically similar to that ratio adopted in the test specimens.

The limits for encased I-sections may be the same as for concrete filled sections: 0.1 < a C 0.8 and are based on parameter ranges studied at Imperial College. (See A K Basu and W Sommerville, “Derivation of formulae for the design of rectangular composite columns’: Proceedings of the Institution of Civil Engineers Supplemen- tary Volume, Paper 7206S, 1969.)

In the formulae from (1 6.6) to ( I 6.13) the symbols have the following meaning:

Ac, As, Ar

fck , f sk , frk

: ,The area of the concrete cover or core, area of the structural steel section and the area of reinforcement respectively.

: The characteristic strengths of concrete, structural steel and rein forcement respectively. The values must be taken according to the recommendations of Clause 16.2.

Tmc, YmS, Tmr : Material partial safety factors of concrete, structural steel and rein forcement respectively. The values must be taken according to the recommendations of Clause 16.2.

0 For the purpose of comparisons between short-term test results and calculated load-canying capacities, fck should be taken as fck = 0.83 fa.

150

RECOMMENDATIONS

16.3.2 STRUCTURAL STEEL SECTIONS

or any symmetrical fabricated sections of solid web construction. The minimum height of section h, must be not less than 100 mm.

The steel sections used in encased columns can be standard rolled I or H sections

16.3.3 STEEL HOLLOW SECTIONS

bricated rectangular or circular sections. The steel hollow sections should have a wall thickness of not less than:

Steel tubes used in concrete filled tubular construction may be hot-rolled or fab-

w m -. for rectangular hollow sections (16.1)

d d m s - for circular hollow sections (16.2)

The minimum outside diameter of circular hollow sections must be not less than 100 mm. The minimum dimensions of rectangular hollow sections must be not less than 100 x 80 mm and square hollow sections not less than 100 x 100 mm.

or

16.3.4 CONCRETE COVERING, FILLING AND REINFORCEMENT

(Figure 16.1) of: The concrete cover which may be taken into account, should have a depth U

40 mm < U < 0.3hS.

The cross-section property, given by the concrete contribution parameter a, as

(16.3)

defined below, should lie between the following limits:

0.2 < a < 0.8 - for concrete encased sections, and 0.1 < a < 0.8 - for concrete filled steel hollow sections

(16.4)

(16.5)

The concrete contribution parameter a is given by:

where and where and where

and

N", a = F q (16.6)

(16.7)

(16.8)

(16.9)

(16.10)

(16.1 1)

(16.12) (1 6.1 3)

151

COMMENTARY

A suggested minimum diameter for the four longitudinal bars is 8 mm. It is also

A limit of 3% is placed on ArlAc as this corresponds to the maximum percentage possible to provide suitable steel mesh to prevent concrete spalling.

used in tests on composite columns to date. This figure may be increhsed if test results show the methods described herein to be applicable to design.

!

16.3.5 It is assumed that composite action between structural steel section and concrete

including reinforcement takes place up to failure. This provides uniform loading of the whole cross-section; I t must be ensured by adequate detailing of connections and joints. ,

16.3.6

five times the least laterd dimension of the column cross-section. Converted into the slenderness ratio x this Q/d ratio gives about h = 2.0. In cases where Q/d > 30 or Q > 12 m it will be necessary to ensure that special instructions are given so that the concrete filling operation can be carried out adequately.

The lengths used in slender column tests reported to date did not exceed forty-

16.4. I A non-linear stress-strain curve for concrete is given in Figure 5. I , Clause 5.4.1.3. Design tables and charts which have been obtained by an analytical method

using these basic principles may be safely used for the design of composite columns: for example, Monographie No 5, fascicule I et 2, Talcul des poteaux en profils crew remplis de beton. Methode de calcul et technologie de mise en oeuvre” (French, German and English version) edited by CIDECT, I9 79. The background is explained by R Anslijn, J Janss, in Tomputation of the ultimate loads of steel columns encased with concrete ” (in French), CRIF Report MT89, April 1974. Comments on the theoretical and experimental background to Clause 16.4 are contained in the Manual on Stability of Steel Structures, Appendix A (ECCS, 1976).

152

RECOMMENDATIONS

where f d , f d , frd = design strength of concrete, structural steel and reinforcement respectively.

ment should be provided in concrete encased sections. Stirrups of an appropriate diameter should be provided throughout the length of the column at a spacing not exceeding 20 mm. At least four longitudinal bars should be provided, capable of supporting the reinforcing cage during concreting.

Longitudinal bars included in the calculation of column strength should have cross-sectional area given by the limit:

To prevent local spalling of the concrete in a concrete encased section, reinforce-

(1 6.14)

The concrete cover to the reinforcement should not be less than the minimum permitted in the CEB/FIP Model Code for Concrete Structures, nor less than 25 mm for longitudinal bars.

1 6.3.5 COMPOSITE ACTION

shear connectors are needed. They should be designed in accordance with the recommendations of Sections 6 , 7 and 8 of this Code.

If the shear stresses at the steel-concrete interface are excessive, then mechanical

16.3.6 SLENDERNESS LIMITATION

Clause 16.5 should not exceed 2.0. The column length should be such that the slenderness ratio h as defined in

16.4. LOAD-CARRYING CAPACITY ANALYSIS

1 6.4.1 GENERAL PRINCIPLES Any inelastic column analysis which complies with the principles listed below

may be applied: . - There exists complete interaction between steel and concrete up to collapse. - Sections which are plane before bending remain plane after bending. - A non-linear stress-strain curve for concrete according to the CEB/FIP Model

Code for Concrete Structures should be used. Concrete in tension should be assumed to have zero strength.

- Ideal elastic - ideal plastic stress-strain curves for structural steel and reinforcing bars may be used, which comply with the ECCS European Recommen- dations for Steel Construction and the CEB/FIP Model Code for Structures respectively.

153

COMMENTARY

Generally it is more convenient to assume only representative geometrical imperfections, which may be chosen according to the ECCS European Recommen- dations for Steel Construction. They should be consistent with those adopted for assessing the strength of axially loaded bare steel columns.

16.4.2

may be used. One &ch method is the equivalent pin-ended column approach in which the restrained column is replaced by a pin-ended column of length equal to the effective length of the restrained column and having end eccentricities proportional to its stiffitess as determined by a linear elastic analysis of the restrained column as a frame with joint moments.

Alternatively, empirical methods which have been shown to lead to safe designs 1

16.5. I In the Equations (16.35) - ( I 6.19) the symbols have the following meanings: N = normal force acting in the column; Nu = as is given in Equation ( I 6.8) of Clause 16.3.4; Qk = effective column length which should be determined as given for bare

steel columns in ECCS European Recommendations for Steel Construc- tion;

I,, Is, I, = second moment of the total area of the concrete part assumed to be

E, = as is defined in Clause C16.3.3; E, = elastic modulus of longitudinal reinforcement.

The application of the ECCS column curves for bare steel columns to the predic- tion of composite column strength is discussed in the Manual of Stability of Steel Structures and by K S Virdi and P J Do wling in '2 unified design method for composite columns 7 ISBSE Publications, Volume 3644 Zurich, 1976.

In the case of concrete filled steel hollow sections, the tubes retard the concrete curing. As a result the influence of long-term loading on the time-dependent concrete strains is smaller than in the core concrete casing.

uncracked, the steel section, and the reinforcement respectively;

154

RECOMMENDATIONS

The applied forces, bending moments, resulting stresses etc should be calculated using second order theory (p - A effects). Proper account has to be taken of the decrease in stiffness due to the spread of plastified and cracked zones.

Geometrical and structural imperfections of materials, including residual stresses in rolled or welded sections have to be assumed as appropriate. Creep and shrinkage parameters for the concrete part have to be chosen in accordance with the CEB/FIP Model Code for Concrete Structures.

16.4.2 MOMENTS AND FORCES IN COLUMNS

ultimate limit state should be determined by an appropriate inelastic analysis in which due account has been taken of the end restraints afforded by members framing into the ends of columns.

The moments and forces acting in the two principal planes of the column at the

16.5 DESIGN METHOD

16.5.1 GENERAL

The composite column should be designed so that:

i) the maximum factored moment on each axis is not greater than the design ultimate moment of resistance of the section about the corresponding axis

M < Mu (16.15) where MU is defined as appropriate in the two hand methods given in the commentary ;

the factored load on the column is not greater than the ultimate loadcarrying capacity of the composite column about the appropriate axis

N < N k

ii)

(1 6.1 6)

The design ultimate moment of resistance of the section Mu depends on the type

The ultimate load-carrying capacity of the composite column is given by:

and properties of the composite cross-section and the location of its principal axis.

Nk = KN, (1 6.1 7) K = reduction factor dependent on the equivalent slenderness ratio x, the

effective buckling curves for bare steel columns according to the ECCS European Recommendations for Steel Construction, materials and cross- sectional properties, shape and ratios of bending moment distribution in column.

= equivalent slenderness ratio as given by:

N U

x = J N a (1 6.1 8)

155

COMMENTARY

V

.I and H sections stress rdieved by heat treatment ir{ - buckling about x x axis - bxkl ing about y y axis

V

K1l

a b

Squash load

Euler critical load

0.5

I I I I w

0.5 1 .o 1.5 2.0 x Figure C16.2 European buckling curves for bare steel columns.

Shape of steel section Curve

Rolled tubes Welded tubes a

Nelded box sections

X - x=h, y - y = b y

['and H rolled sections - buck,ing about

the x x axis h/b> 1.2 h/b< 1.2 .!. 1' - buckling about

the y y axis h/b 1.2

Y hlb 1.2 I and H welded sections '

- buckling about x x axis e) flame cut flanges b) rolled flanges

- buckling about

a) flame cut flanges I b y y axis

K 1 for column curves

0 1.000 1,000 1.000 0.1 1.000 1.000 1.000 0.2 1.000 1.000 1.000 0.3 0.978 0.965 0.951 0.4 0.954 0.925 0.900 0.5 0.923 0.885 0.844 0.6 0.884 0.838 0.783 0.7 0.845 0.785 0.719 0.8 0.796 0.727 0.654 0.9 0.739 0.663 0.593 1.0 0.675 0.599 0.537 1.1 0.606 0.537 0.486 1.2 0.542 0.480 0.438 1.3 0.480 0.429 0.395 1.4 0.427 0.383 0.357 1.5 0.381 0.343 0.323 1.6 0.341 0.307 0.293 1.7 0.306 0.277 0.265 1.8 0.277 0.250 0.241 1.9 0.251 0.227' 0.220 2.0 0.228 0.207 0.202

x a b c

I

Table C16.1 Strut curve selection chart and values of coefficient KI for column curves.

156

RECOMMENDATIONS

N, = Euler critical load

(16.19) lr2 N, = 2 (E,Ic t EsIs t ErIr) l k

E, = effective concrete elastic modulus E, = 6 0 0 f d (16.20)

A discussion of the value of Ec to be adopted for strength predictions is contained in the previously quoted reference. Subsequent numerical studies showed that the value given by (16.20) was satisfactory for design purposes.

In the absence of more exact information it is permissible to account for the effects of the timedependent strains of concrete on the carrying capacity of encased structural steel sections by reducing the elastic modulus of concrete to:

E, = 3 0 0 f k (16.2 1)

If only part of the load acting on the column is permanent, then linear interpolation is permitted between the values of Equations (16.20) and (16.21):

N permanent N E,, = E, ,1 - 0.5 (16.22)

A reduction of concrete elastic modulus is not required for concrete filled steel hollow sections except in the case of very slender columns with large values of the concrete contribution parameter.

In the case of short axially loaded composite columns consisting of concrete filled circular steel tubes, provided d/t exceeds twenty, a higher concrete strength due to the beneficial effects to the confinement and a lesser steel strength may be taken into account. Then the squash load Nu is given by:

NU = NUC + NUS, where Nu, = Ac fcd; (1 6.23)

NUS = As fsd; (16.24)

fcd = [fck + 771 fsyI/rmc; (1 6.25)

fsd = r)2fsy/rms. (16.26)

r), and q2 are constants where values are listed in the table below for different values of Q/d.

Q/d r)l r)2

0 9.78 0.76 5 6.60 0.80

10 3.94 0 8 5 15 1.86 0.90 20 0.49 ' ' 0.95 25 0.00- 1 .oo

~

Table 16.1 The values of factors q l and 772.

157

COMMENTARY

16.5.2 The value of coefficient K l may be taken either from Figure Cl 6.2 or Table

C16.1 taking due account of the steel section involved and the value of slenderness ratio given by Equation (16.18). I t may also be calculated by the formula

i where

6 = 0.158 for curve a, = 0.281 for curve b, = 0.384 for curve c.

16.5.3 and 16.5.4 Two methods are given here. that comp& with these Clauses.

Method A

(Cl 6.2)

where K 1 is derived as in Clause 16.5.2, K 2 and K 3 are determined as below, M = maximum factored moment acting about the appropriate axis, calculated

Mu = design ultimate moment of resistance calculated about the appropriate

Equation (C16.2) governs for columns subjected to uniaxial bending about the minor axis. Columns subjected to major axis moments, but unrestrained from failing about the minor axis are likely to fail in biaxial bending and the requirements of Clause 16.5.4 should be met.

in accordance with clause 16.4.2,

axis as given later in the Commentary.

Coefficient K 2 : Values of the coefficient K 2 about each axis may be calculated as follows between the limits:

OGK2 GK2 ( ~ ~ 0 )

and

except that if K2 is calculated to be negative it should be taken as zero. K2 (Q = 0) Q 0.75

K 2 (Q = o)= 0.9a2 + 0.2

158

RECOMMENDATIONS I 1

In cases where a column carries loads before the composite action has taken place, this column has to meet the requirements for bare steel columns. In calculating the loadcarrying capacity of the composite column the preloading of the steel section should be considered as appropriate.

16.5.2 AXIALLY LOADED COLUMNS For axially loaded columns expression (1 6.16) should be satisfied by using the

value of K1 which depends on the equivalent slenderness X about the appropriate axis and is based on the buckling curves for bare steel columns given in the ECCS Recommendations for Steel Construction.

16.5.3

set out in Clause 16i4.1, conforms to the preceding Clauses 16.2 and 16.3, and which can be shown to produce safe results (for example, by testing) may be used to design composite columns subjected to combined axial compression and bending. Two methods which are acceptable are given in the Commentary. In each case the interaction curves generated by validated beamcolumn analyses have been used as bases for design.

COMBINED AXIAL COMPRESSION AND UNIAXIAL BENDING Any inelastic beam-column design method which complies with the principles

16.5.4 COMBINED AXIAL COMPRESSION AND BIAXIAL BENDING Design methods derived from inelastic analyses based on the same principles are

referred to in the preceding Clause may be used to determine the strength of columns failing in a biaxial mode. Approximations of biaxial interaction surfaces are used in the two methods presented in the Commentary. A more accurate design approach is available and referred to in the Commentary.

159

COMMENTARY

For concrete encased steel sections and concrete filled rectangular tubes Kz = K ~ ( Q = 0) {I90 - 25 (2P - 1) (1.8 - a) - ijx] 1/30 (2.5 - P)J/

K z = K z + o ) { [ l l s - 30(2fl- 1)(1.8- a ) - iix]l[SO(2.1- fl)]]

= the ratio of the smaller to the larger of the two end moments acting about each axis, used with the additional subscripts x and y to denote the plane of bending under consideration, the sign convention being such that (3 is

while for concrete filled circular tubes:

where 6

positive for single curvature bending; I Q = 100 for columns designed to curve a,

= 120 for columns designed to curve b, = 135 for columns designed to curve c.

Coefficient K3: Values of the coefficient K 3 m y be calculated as follows for concrete encased steel sections and concrete filled rectangular steel tubes:

For major axis bending, K3X = 0 For minor axis bending,

= 0.425 - 0.07Sfly - 0.005 $, but should be within the limits, (0.2 - 0.25a) 2 2 0.03 ( 1 +fly) .

In the case of concrete filled circular steel tubes,

K J = K J ( g = 0) + { [ ( O S 6 + 0.4) (a' - 0.5) + O.l5]X/(l +I3)/ where K 3 (g = 0) = 0.04 - ails and should never be taken as less than zero.

Calculation of Mu for composite sections The design ultimate moment of resistance of the concrete encased stmctural steel and concrete filled hollow sections about the appropriate axis may be given as

Mu = N U S e, where NUS is determined from Equation ( I 6.9) or ( 1 6.24).

for concrete encased steel section whose plastic axis is outside the steel section - major and minor axis bending (Figures C16.3 and Cl 6.4) - as

eu is the design ultimate eccentncity of the composite section and may be taken:

- - 1 (Ash-- + - frdAr dr),. -2As bP fsd

for concrete encased steel sections whose plastic neutral axis is within the top flange - .major axis bending (Figure Cl 6.5) - as

160

RECOMMENDATIONS

161

COMMENTARY

r C16.3 *

C16.6

C16.9

V I

I- d

C16.4

t b

I 1

C16.5 b I

C16.7 C16.8

I

t b

Figures C16.3 - C16.9 The dimensions of concreteencased and concrete-filled steel sections and location of plastic neutral axis in bending.

162

RECOMMENDATIONS

163

COMMENTARY

for concrete-encased steel section whose plastic neutral axis is in web - major axis bending (Figure Cl 6.6) - as:

(As + 2twdw) jfrd ht,

pb + 2t, fsd - - 2As [Ash + 2t,dL - 2bftf(dS - d,) -

for concrete-encased steel section whose plastic neutral axis is in flanges - minor axis bending (Figure C16.7) - as:

1 e, = -

2AS u2 +‘4t ) +f*d A d d , . IASh + 4tfd3 - pb + 4tf f s d

for concrete filled rectangular hollow section (Figure Cl 6.8) - as:

pb + 4tf for concrete filled circular hollow section (Figure C16.9) - as:

29 e, = ec +- (es - e,); n where

2(t+tX) AS e = [ I - d 1 ‘OStx(d- 2 t - t x )+2 t (d - t )

tx = % (d - 2t) (1 - J T p ) 2 (d - 2t) sin3 9 3(2e - sin’ 9 ) e, =

sin 9 [d3 - (d - 2t)3/ e, =

p = the concrete to structural steel strength ratio as given by

39 [d2 - (d - t )2]

0.48 fad

= rmcfsd

fed, fd , f d = design values of concrete, structural steel and reinforcement strengths respectively as defined in Equations (1 6.1 I ) , (1 6.12) and (1 6.13) of Clause 16.3.4.

As, Ar = area of structural steel section and reinforcement respectively.

16.5.4

For columns failing in biaxial bending conditions ( I 6.15) and ( I 6.16) should be satisfied by taking the coefficient K as Kxr as given by

1 - Kxy - 1 1 1 - + - + -

Kx K y KlX

164

RECOMMENDATIONS

165

COMMENTARY

I I where

K 1x

K x , Ky

= the K1 coefficient for the column with no end moments constrained to bend about the major axis only as determined from clause 16.5.2.

coefficient for the column bending about the major and minor axes respectively as determined from Equation (Cl 6.2).

Method B

16.5.3

To design columns subjected to axial compression and uniaxial bending, the forces have to be calculated using second oder theory ( p - A effects). These fac- toted moments should not exceed the ultimate cross-sectional strength at any position along the column length. The design procedure is illustrated graphically below:

I

-. N NU

Figure C16.10

The limiting axial load capacity, Nk, can be determined from Clause 16.5.2 as point B in the above figure. A t this load level no additional bending moment can be applied to the composite column. The accompanying bending moment, denoted by Point A on the interaction curve is due solely to geometrikal imperfections and residual stresses.

The distance between the straight line 0 - A and the interaction curve denotes the moment caving capacity:

Mk < 0.9sMu.

The following diagrams are presented to facilitate the use of this method:

166

RECOMMENDATIONS

167

I .,-

I

A

COMMENTARY

1 .o

ae

0.6 Parameter -

a4 0.8

0.2

0 d

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 M - M"

Figure C16.12

Nu 1.0

0.8

0.6

0.4

0.2

n 0.2 014 0.6 0.8 l'.O 1:2 1:s 1.6 1.8 E

M"

Figure C16.13

168

RECOMMENDATIONS

169

~

COMMENTARY

" t N" 1.0

0.8

0.6

0.4

0.2

U --- 0.2 0:s 0.6 018 1.0 1.2 1.4 1.6 .- M M"

Figure C16.14

16.5.4

face instead of the above simple interaction curves should be used.

capacities separately for each axis

For composite columns subjected to biaxial bending a three dimensional sur-

Onesimpleapproach which may be used is to calculate both moment carrying

Mk,x = 0.9& Mu,x

Mk,r = 0.9Sy Mu,y

Linear interpolation between the two uniaxiul moment carrying capacities gives an interaction function for biaxial bending as shown graphically below.

I

Figure C16.15

170

i RECOMMENDATIONS

171

COMMENTARY .

166.1

Composite columns subjected to transverse shear forces, V, may be designed by assuming that the shear is resisted by the steel web alone for strong axis bending and the steel flanges alone for weak axis bending, Shear and noma1 stresses should be combined and must not exceed the equivalent yield as given by the von Mises yield criterion.

172

RECOMMENDATIONS

16.6 THE NEED TO PROVIDE MECHANICAL SHEAR CONNECTION

16.6.1 PROVISION OF SHEAR CONNECTORS ALONG COLUMN LENGTH No mechanical shear connectors need be provided to either type of composite

column provided that at the factored ultimate load the shear at the interface between steel and concrete complies with the following limitations:

T < 0.4 N/mm2 for concrete filled tubes, T < 0.6 N/mm2 for concrete encased sections. Where these limits are exceeded adequate shear connectors must be provided

unless it can be demonstrated by tests that no such connectors are needed to achieve full interaction up to collapse.

1 6.6.2 SHEAR CONNECTORS AT BEAM-COLUMN INTERSECTIONS Special attention should be paid to the way in which forces are transferred from

beams to columns to ensure that the basic principles outlined in Clause 16.4.1 are applicable. A clearly defined load path which does not involve significant interface slip between concrete and steel should be identified. In certain cases it may be necessary to provide some form of mechanical shear connector in such regions to ensure that the concrete and steel are equally strained to comply with the basic assumptions of the design approach outlined within Clause 16.5.

16.7 SERVICEABILITY LIMIT STATE

1 6.7.1 GENERAL As well as conforming with the ultimate limit state columns must also be designed

to behave satisfactorily at the serviceability limit state. Two readily identifiable serviceability limit states are excessive cracking and excessive deflections.

16.7.2 TENSILE CRACKMG OF CONCRETE No check for crack control need be made for: - concrete filled hollow steel sections, - concrete encased steel sections in which the design axial load is greater than

Where the design axial load in concrete encased steel sections is less than the value of N, given above the column should be considered as a beam for the purpose of checking crack widths. Reinforcement should be provided in accordance with the CEB/FIP Recommendations.

N, = 0.23 uu A,.

173

COMMENTARY

The number of possible combinations of different types of beam, column and joint is so great that it is considered to be impracticable to give specific recommendations for composite frames for buildings. It is envisaged that appropriate Clauses fiom the CEBIFIP and ECCS Recommendations will be used.

I 6

174

RECOMMENDATIONS.. .

Section 17. Framed structures for buildings

175

COMMENTARY

I

If experimental evidence is not available on the cement which it is proposed to use, it can be assumed for normal concretes that the values of the ratios between the compressive strength a t an age of j days and the compressive strength a t an age of 28 days are in accordance with the following table, which is valid for normal temperatures (15-20°C).

Age of concrete (days) 3 7 28 90 360

Normal 0.40 0.65 1.00 1.20 1.35

Rapid- hardening 0.55 0.75 1.00 1.15 1.20

Portland cement

I 176

RECOMMENDATIONS

Section 18. Workmanship and construction

1 8.1 RESPONSIBILITY Where several parties are involved in the design and construction of a composite

structure the responsibilities of the individuals or organisations appointed to under- take the design, co-ordination, satisfactory completion and safe execution of the works should be clearly defined at the start of the project.

18.2 SEQUENCE OF CONSTRUCTION I

~

The sequence of construction should be considered as an integral part of the design process, for example, when calculating the stresses or deflections in a composite section, and should be clearly indicated and described on the final design plans and instructions to site.

I Consideration should be given to the speed and sequence of concreting to I

prevent damage to partly matured concrete as a result of limited composite action occurring from deformation of the steel beams under subsequent concreting operations.

day cylinder strength, Pc, the basic strengths of shear connectors and elastic properties and limiting compressive stresses in the concrete should be based upon the cylinder strength at the time considered, f,, except that no reduction in stiffness of the concrete need be made if:

0.75 f', < f, < f',. Where the cylinder strength of the concrete at the time considered is not less

than 15 N/mm2, the basic strengths of shear connectors may be determined from Section 6.

In prestressed composite beams, it is recommended that partial prestressing as well as full prestressing of the concrete slab should not take place until the concrete has reached the required compressive strength. To ensure this precondition tests should be carried out, otherwise the procedure should be chosen in accordance with CEB/FIP Recommendations.

must be designed for this condition as well as for the final condition.

stages of erection.

Where the composite section carries load before the concrete has attained its 28-

Where a partially cast slab is assumed to act compositely the shear connection

The time-dependent effects of creep and shrinkage should be considered for all

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RECOMMENDATIONS

18 3 STABILITY OF STEELWORK Usually stiffeners or cross-frames are required in order to ensure stability of the

steelwork, particularly before the section acts compositely. The stiffness of shuttering or other similar formwork material is not sufficient to provide the necessary lateral support.

buckling if jacking down of supports is used as the method of prestressing either the steelwork or the composite structure. In this context, it may be advisable under certain circumstances to measure the induced reactions and to compare them with the calculated values, furthermore special consideration should be given to the stability of lifting frames, jacks, packing materials etc. Horizontal forces caused by bearing friction or resistance to longitudinal support movements should also be considered.

The same attention to detail should be given to the calculations for safety against

1 8.4 SUPPORT CONDITIONS DURING CONSTRUCTION All support movements which occur during the various stages of construction

should be calculated in advance and carefully controlled on the site. Upon completion of the structure it is essential that a detailed check be made of all support conditions, with particular emphasis given to any deviations from the planned erection procedure which may have taken place. Careful consideration should be given to the effects of future creep and shrinkage when predicting final support conditions.

18.5 TEMPERATURE EFFECTS DURING CONSTRUCTION Due to heat of hydration produced during the concrete setting process, loading

are induced into the structure, the magnitude and nature of which are not normally covered by Code recommendations. If temperature load cases of this nature are not adequately investigated in design calculations,the concreting sequence as well as the measures taken to protect the fresh concrete from the weather should be so chosen that the structure does not suffer damage as a result of overstressing.

18.6 ANCHORAGE OF PRESTRESSING CABLES

Special care should be exercised in the choice of both tensioning and anchorage locations when concentrated prestressing forces are to be increased or decreased. If the tensioning locations are actually in the concrete flange, undesirably large prestressing recesses in the form of notches or holes become necessary in order to facilitate positioning of stressing jacks and to provide sufficient leeway for cable extensions. This type of tensioning arrangement should only be chosen when the resultant disruptive influence on the stress distribution within the flange can be distributed over a relatively wide area. In other cases the cables should be drawn out of the flange into built-up sections or ribs on the underside of the flange.

18.7 CONSTRUCTION ACCURACY AND QUALITY CONTROL OF MATERIALS

The CEB/FIP Recommendations for structural concrete and the ECCS Recom- mendations for Steel Construction apply to composite construction.

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18.8

a) : b) i

Figure C18.1 a) Tension test; b) Bending test, c) Reversal bending test, d) Hammer blow bending test.

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RECOMMENDATIONS

If deflections of the steel beam by pouring of the concrete are significant their values shall be given in the design calculation and on the drawings. Generally this may not be necessary for beams in buildings of conventional structures but will usually be necessary for bridges or similar structures. Sometimes it may be of a certain importance to take into account the influence of an eventual slip in joints, possible effects of residual mill stresses or welding stresses or other causes of increased or reduced deflections.

The measured actual deflections shall be compared with the theoretical values. Deviations from the desired form may be compensated by thicker dimensions of concrete or wearing coat only if the additional loading is considered in the statical analysis.

18.8 SHEAR CONNECTORS The welding of block-type connectors, anchors and hoops should be in accor-

dance with the relevant Clauses of the ECCS Recommendations for Steel Construc-

With regard to friction grip bolting the necessary measures to be taken during

The following recommendations are made for the welding of the stud connectors.

tion. I

the construction have already been described in Section 6.

The proper values of the current strength and welding period should be determined on the basis of trial weldings, supplemented by one or more of the following tests:

tension test, bending test, reversal bending test, hammer blow bending test. Only after these tests is it recommended to start welding on the steel structure.

The quality of the stud-welding there should be checked by visual inspection. Any defective studs shall be replaced. In addition up to about 5% of the studs should be bent by hammer blow over 15". Neither the noise made by the blow, nor visual inspection shall indicate any crack in the welding. The studs may be left in the bent position.

pay attention to the proper working of the welding equipment. At the beginning of each new shift and after long interruptions it is advisable to

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i

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RECOMMENDATIONS

18.9 PRECAST CONCRETE SLABS FORMING THE FLANGES OF COMPOSITE BEAMS

The necessary measures to be taken during the construction have already been described in Section 14.

18.10 COMPOSITE FLOORS WITH PROFILED STEEL SHEETS During construction steel sheets should be fixed in order to ensure connection

between the slab and beam, to keep them in position after laying and to transmit horizontal forces. The sheets should be connected together with seam fasteners at a maximum of 500 mm centres. Sheet edges should also preferably be fixed at the same centres.

beams and 50 mm over intermediate beams.

beams the following conditions shall apply:

The minimum bearing of the profiled steel sheet shall be 40 mm along the edge

Where shear stud connectors are to be welded through the sheet to the supporting

1. The thickness of paint on the joist shall not exceed 50 microns. 2. When the sheet is ungalvanised the gross thickness shall not exceed 1.5 mm

and any corrosion present shall be minimal. 3. The overall thickness of a galvanised sheet shall not exceed 1.25 mm and

galvanising shall not exceed 30 microns on each face of the sheet.

4. The paint or the plastic coating underneath the sheet shall be removed 5 . Wet conditions shall be avoided. To avoid the corrosion of the steel decking all surface damage shall be made good.

Corrosion protection shall be generally increased when steel decking is to be used in a highly humid atmosphere.

18.1 1 CONSTRUCTION OF COLUMNS The concrete must be carefully chosen and its quality controlled. Material

property tests should be carried out under representative conditions at all stages of preparation and casting. Special attention needs to be paid to the achievement of satisfactory compaction particularly in the case of concrete-filled hollow steel tubes.

Vent holes should be provided at the base of concrete filled composite columns as a precaution against fire. The use of steel fibre-reinforced concrete as a filling for such columns is acceptable as a means of providing adequate fire protection provided that sufficient evidence exists to substantiate its fire resistance. Where a substantial amount of reinforcing bars are used in either type of column, consideration should be given to the use of small size aggregate.

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composite structures

Documents

general section

design general

shear connection general

composite construction

deflections section

creep section

limit state requirements

mechanical shear connection