1

Composite Construction

Introduction to composite construction of buildings

2

General

These two materials complete one another:

Steel and concrete

Concrete is efficient in compression and steel in tension

Concrete encasement restrain steel against buckling

Concrete provides Protection against corrosion and fireSteel bring ductility into the structure

3

Composite construction refers to any members composed of more than one material. The parts of these composite members are rigidly connected such that no relative movement can occur.

The main composite elements in buildings are1.Steel Concrete Composite Beam

2.Composite Slab

3.Composite Columnn

4

Steel Concrete Composite BeamComposite beams are normally hot rolled or fabricated steel sections that act compositely with the slab. The composite interaction is achieved by the attachment of shear connectors to the top flange of the beam. These connectors generally take the form of headed studs.

5

The composite action increases the load carrying capacity and stiffness of the beam by factors of up to 2 and 3.5 respectively.

It is normally designed to be unpropped during construction, and must be sized to support the self-weight of the slab, and other construction loads, in their non-composite state.

6

size of the steel section is governed by serviceability considerations because composite beams tend to be used for long span applications

Check that beam deflections during construction will not lead to significant additional concrete loads (due to ponding) that have not been allowed for in the design

The bending resistance of the section is normally evaluated using ‘plastic’ principles

7

The plastic moment resistance is calculated using idealized rectangular stress Blocks.

It is assumed that stresses of fyd and 0.85 fcd can be achieved in the steel and concrete respectively

8

Composite beams are generally shallower (for any given span and loading) than non-composite beams, and they are used commonly in long span applications.Consequently, deflections are often critical.

9

The Various types of composite Beams

10

Shear Connectors

These connectors are designed toTransmit longitudinal shear along the interfaceprevent separation of steel beam and concrete slab at the interface

11

most common type of shear connector used in composite beams for buildings is a 19 mm diameter by either 100 mm or 125 mm long welded stud.

12

The property of shear connector most relevant to design is the relation-ship between the shear force transmitted, P, and the slip at the interface, s This load-slip curve should ideally be found from tests on composite beams.

13

Composite slabs• consist of profiled steel decking with an in-situ

reinforced concrete topping.

• The decking(profiled steel sheeting) not only acts as permanent formwork to the concrete, but also provides sufficient shear bond with the concrete so that, when the concrete has gained strength, the two materials act together compositely

• span between 3 m and 4.5 m onto supporting beams or walls

14

If the slab is unpropped during construction, the decking alone resists the selfweight of the wet concrete and construction loads. Subsequent loads are applied to the composite section.

If the slab is propped, all of the loads have to be resisted by the composite section.

are usually designed as simply supported members in the normal condition

15

Profiled steel sheeting

• yield strengths ranging from 235 N/mm2 to at least 460 N/mm2

• depths ranging from 45 mm to over 200 mm

• .8 mm and 1.5 mm thick

The various shapes provide Interlock between steel and concrete

frictional mechanical

16

• decking may also be used to stabilise the beams against lateral torsional buckling during construction.

• stabilise the building as a whole by acting as a diaphragm to transfer wind loads to the walls and columns

• temporary construction load usually governs the choice of decking profile

17

COMPOSITE COLUMNS

18

A steel-concrete composite column is a compression member, comprising either a concrete encased hot-rolled steel section or a concrete filled tubular section of hot-rolled steel.

19

The presence of the concrete is allowed for in two ways.

• protection from fire• It is assumed to Resist a small axial load • to reduce the effective slenderness of the steel

member, which increases its resistance to axial load.

The bending stiffness of steel columns of H-or I-section is much greater in the plane of the web (‘major-axis bending’) than in a plane parallel to the flanges (‘minor-axis bending’).

20

There is no requirement to provide additional reinforcing steel for composite concrete filled tubular sections.

The ductility performance of circular type of columns is significantly better than rectangular types.

corrosion protection is provided by concrete to steel sections in encased columns

21

The plastic compression resistance of a composite cross-section represents the maximum load that can be applied to a short composite column.

While local buckling of the steel sections may be eliminated, the reduction in the compression resistance of the composite column due to overall buckling should definitely be allowed for.

22

Joints Example of vertical shear transfer

between beam and columnremoved after concreting

bracket with shear connectorscontact pieceweld seam

reinforcement

bracket for the lower flange

shot-fired nails

23

Aspects for using composite structures:

Architectural

Economical

Functionality

Service and Flexibility

Assembly

24

Aspects for using composite structures

Architectural:

Longer spans

Thinner slabs

More slender column

More generous opportunities for design

25

Aspects for using composite structuresEconomical:

Reduction of height reduces the total of the building --> saving area of cladding

Longer spans with the same height --> column free rooms

Additional storeys with the same total height of building

Quicker time of erection: Saving costs, earlier completion of the building Lower financing costs Ready for use earlier thus increasing

rental income

26

Aspects for using composite structures

Functionality: Fire protection by using principles of reinforced

concrete in which the concrete protects the steel

27

Aspects for using composite structures

Service and building flexibility: Adaptable structures

Modification during the life of the building Modify services without violating the privacy of

other occupants Accommodation of service facilities

in the ceilingwithin a false floorin a coffer box running along the walls

28

Aspects for using composite structuresAssembly:

Working platforms of steel decking Permanent shuttering Reinforcement of profiled steel sheetings Speed and simplicity of construction Quality controlled products ensure greater accuracy

29

Construction methods

Traditionally two counteracting methods of construction could be observed both connected with special advantages but also disadvantages worth mentioning. Conventional concrete

construction method Construction in steel

+ freedom of form and shapes

+ easy to handle

+ thermal resistance

- time-consuming shuttering

- sensitive on tensile forces

+ high ratio between bearing capacity and weight

+ prefabrication

+ high accuracy

- low fire resistance

- need of higher educated personal

30

Construction methods

Composite Construction

comparing these two methods a combination of both presents the most economic way

+ higher bearing capacity

+ higher stiffness

+ plastic redistribution

31

Examples

Millennium Tower (Vienna - Austria)

55 storeys Total height 202 m Total ground floor 38000 m2 Capital expenditure about 145 million Euro Time of erection: 8 months

32

Examples

Millennium Tower (Vienna - Austria)

Composite columns

Concrete core

Composite Slim floor beams

Concrete slab

42,3 m

Composite frame

42,3 m

33,0

5 m

33

Examples

Millennium Tower (Vienna - Austria)

Total time of erection: 8 monthmax. speed 2 to 2.5 storeys per week!

34

Examples

Parking deck “DEZ” (Innsbruck - Austria)

Erection of composite columns over 2 storeysAssembly of prefabricated concrete slabs

35

Examples

Parking deck “DEZ” (Innsbruck - Austria) 4 storeys Ground dimensions 60 x 30 m Max. span length 10.58 m with

26 cm slim floor slab (= l/40)