sci p 249

TECHNICAL REPORTSCI PUBLICATION 249

Design Capacity ofKidney ShapedSlotted Connections

D G BROWN BEng CEng MICE

Dr W TIZANI BSc MSc PhD

Published by:

The Steel Construction InstituteSilwood ParkAscotBerkshire SL5 7QN

Tel: 01344 623345Fax: 01344 622944

P249: Design of kidney shaped slotted connections

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http://sefie.steelbiz.org/DiscussSteelbizContent.aspx?ResourceID=24901

iiP:\CMP\Cmp657\pubs\P249\P249-Final.wpd 25 April 2002

© 1998 The Steel Construction Institute

Apart from any fair dealing for the purposes of research or private study or criticism or review, aspermitted under the Copyright Designs and Patents Act, 1988, this publication may not bereproduced, stored or transmitted, in any form or by any means, without the prior permission inwriting of the publishers, or in the case of reprographic reproduction only in accordance with theterms of the licences issued by the UK Copyright Licensing Agency, or in accordance with the termsof licences issued by the appropriate Reproduction Rights Organisation outside the UK.

Enquiries concerning reproduction outside the terms stated here should be sent to The SteelConstruction Institute, at the address given on the title page.

Although care has been taken to ensure, to the best of our knowledge, that all data and informationcontained herein are accurate to the extent that they relate to either matters of fact or acceptedpractice or matters of opinion at the time of publication, The Steel Construction Institute, the authorsand the reviewers assume no responsibility for any errors in or misinterpretations of such data and/orinformation or any loss or damage arising from or related to their use.

Publications supplied to the Members of the Institute at a discount are not for resale by them.

Publication Number: P249

ISBN 1 85942 076 1

British Library Cataloguing-in-Publication Data.A catalogue record for this book is available from the British Library.

SCI Technical Reports

Technical Reports are intended for the rapid dissemination of research results as andwhen they become available and/or as ‘specialist documents’ for further discussion.They provide an opportunity for interested members to comment and offer constructivecriticisms.

Please forward your comments to Mr D G Brown, The Steel Construction Institute,Silwood Park, Ascot, Berkshire, SL5 7QN.


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FOREWORD

This Technical Report has been prepared as one of the SCI deliverables for the CIMsteelEureka 130 project. An earlier phase of the project identified that connections incorporatinga single kidney shaped slot were commonly used for relatively lightly loaded bracingmembers, although no design rules for the capacity of such details existed.

This report describes work undertaken in carrying out tests and in developing design rulesfor such connections. The determination of appropriate design rules for bracing connectionsincorporating a single kidney shaped slot was considered to be a valuable contribution tothe work within the CIMsteel project on economic connection design and detailing.

This document was prepared by Mr D G Brown of the SCI.


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This publication forms one of the deliverables of the CIMsteel project.

ACKNOWLEDGEMENTS

The Steel Construction Institute gratefully acknowledges the contribution of the CIMsteelcollaborators involved in this aspect of the project:

Dr O S Salawa Acecad Software LtdMr A F Hughes Arup AssociatesMr A J Rathbone CSC (UK) LtdMr P Quantrill Philip Quantrill (Structural Engineers) LtdMr W Park QSE LtdMr R Swift Severfield-Reeve Structures LtdMr P Purvey Taywood Engineering LtdDr G H Couchman The Steel Construction Institute Professor D A Nethercot University of NottinghamDr W Tizani University of Nottingham

Particular thanks are due to the University of Nottingham, who carried out the testing, datagathering and preliminary analysis, and to Mr C King of the SCI for his advice during theinterpretation of the results. Glosford Metal Constructions Ltd and Severfield-ReeveStructures Ltd provided the test pieces, and their generosity is much appreciated.

The SCI also acknowledges the valuable input from the members of the SCI/BCSAConnections Group and is grateful for their advice, comment and review during the progressof the project.

Additional financial support for this project was provided by the following organisations.The SCI appreciates their contribution to this research.

Atlas Ward Structures LtdBison Structures LtdBourne Steel LtdCaunton Engineering LtdFisher Engineering LtdSeverfield-Reeve Structures LtdWescol Structures Ltd

Together with the above organisations, the following contributed to the project with advice,comment and review:

A C Bacon Engineering LtdButler Building Systems LtdFairport Steelwork LtdGlentworth Fabrications LtdGlosford Metal Constructions LtdJames Bros (Hamworthy) Ltd

John Reid & Sons (Strucsteel) LtdNusteel Structures LtdRowecord Engineering LtdSouth Durham Structures LtdWig Engineering Ltd


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SUMMARY

This report presents details of the testing of various two bolt connections, some with plainholes and some incorporating a single kidney shaped slot. Based on the results of the tests,it is recommended that the connection capacity be taken as the capacity of a single bolt ina plain hole multiplied by a factor of 1.6. It was found that the displacement at workingload is not significantly greater than connections with two plain holes, and should notpreclude the use of such details in orthodox structures.

The recommendations apply to bracing connections in orthodox buildings with two bolts,and where the end and edge distances of the connection detail comply with certainlimitations.

For strut design, connections incorporating a kidney shaped slot should not be assumed toprovide directional restraint.


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viiP:\CMP\Cmp657\pubs\P249\P249-Final.wpd 25 April 2002

CONTENTSPage No.

FOREWORD iii

ACKNOWLEDGEMENTS iv

SUMMARY v

1 INTRODUCTION 11.1 Project background 11.2 Kidney shaped slots 11.3 Design rules 2

2 TESTING - PHASE 1 42.1 Choice of test pieces 42.2 Test arrangement - Phase 1 52.3 Results - Phase 1 72.4 Conclusions from Phase 1 tests 13

3 TESTING - PHASE 2 143.1 Objectives 143.2 Test arrangement - Phase 2 143.3 Results - Phase 2 16

4 CONNECTION CAPACITY 194.1 Capacity of connections with kidney shaped slots 194.2 Design rules for connections with kidney shaped slots 204.3 Bracing member effective length 214.4 Provision of washers 21

REFERENCES 22


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1 INTRODUCTION

1.1 Project backgroundIn an earlier phase of the CIMsteel project, a broad study of ‘novel’ connectiontypes was undertaken. The objective was to assess various connections,considering:

C Practicality.

C Cost.

C Structural efficiency.

C Safety.

Any connection types showing potential to benefit the structural steelwork industrywere to be investigated in more detail.

The initial study was completed in 1996, and an internal report presented to theCIMsteel collaborators and Project Managers(1). The report concluded that the useof kidney shaped slots in bracing connections should be researched in more detailbecause:

C Kidney shaped slots in bracing connections are a practical solution to acommon connection problem.

C Kidney shaped slots are already used by some steelwork contractors.

C No agreed rules exist for calculating the strength capacity of such details.

C The assumed capacity varies between structural designers and is, on someoccasions, the source of some disagreement.

C A relatively modest testing programme could provide sufficient data toproduce appropriate recommendations.

In parallel, all steelwork fabrication companies who were members of the SCIwere contacted with proposals for a detailed study of connections with kidneyshaped slots. Broad support for the proposals was received, and additionalfinancial support was provided by a number of companies (seeAcknowledgements).

1.2 Kidney shaped slotsKidney shaped slots are often used in bracing connections as shown in Figure 1.1.The kidney shaped slot is generally formed in the gusset plate rather than at theend of the bracing member.


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Ø22Hole

R = 70R=40

40

30°

Figure 1.1 Gusset plate incorporating kidney shaped slots

The advantages of a connection incorporating a kidney shaped slot are that:

C A standard end connection can be used for bracing members.

C Standard gusset plates which accommodate a range of bracing angles may beused.

C Slight misalignment can be accommodated.

C A two bolt connection allows one bolt to be inserted in the connection whilstlocating and maintaining alignment with a podger spanner through the otherhole.

The advantages of standardised connection details are numerous and welldocumented(2)(3)(4). Standard details encourage a batch production approach to thefabrication of the connection components, and save time in design, detailing,checking and fabrication.

1.3 Design rulesBS 5950: Part 1(5) provides certain checks for components of a connection, suchas plates, welds and bolts. These checks are generally associated with strength atthe ultimate limit state (ULS), although certain checks are in fact based onserviceability criteria (see Section 1.3.2). Rules are provided for ‘ordinarybolting’, including shear capacity, tension capacity, bearing capacity of the boltand bearing capacity of the plate. Minimum edge and end distances are alsogiven.

1.3.1 StrengthBS 5950: Part 1 does not cover the calculation of the capacity of detailsincorporating kidney shaped slots as illustrated in Figure 1.1. Table 35 ofBS 5950: Part 1 describes dimensional limits for ‘short’ and ‘long’ slotted holes,although these are specifically for friction grip fasteners.

For ordinary bolts in ‘short’ slots (d + 6 mm for M20, d + 8 mm for M24) thefull bearing capacity may be assumed(2). No advice is available when ordinarybolts are used in slots longer than ‘short’ slots.


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1.3.2 ServiceabilityAlthough the bearing capacity of the plate and bolt is calculated at the ULS, thebearing strength of the plates given in BS 5950: Part 1 is based on limiting thedeformation at working load to an acceptable maximum. Acceptable deformationunder working load is of the order of 1.5 mm(6).

Where the end distance (measured from the centre of the hole to the adjacent edgein the direction of bearing) is less than twice the bolt diameter, the bearingcapacity of the plate is reduced.


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28

15° 15°

R = 70

R=28

R=28

15°15°

8 mm S275 8 mm S275 8 mm S275

10 mm S275 10 mm S275 10 mm S275

R = 70

R=33

15° 15°

R=70

R=70

R=33

15°15°

46

Ø22

70 70

28

70

Ø22 28

Ø22

70 70

43

22

28

46

43

Ø22

70 70

2822

28

350

46

350 Ø26

33

3326

40

Ø26

3326

3341

41

70 70 70

42

42

Ø26

Ø26 33

70

33

70 70 70

Hole

Hole

Hole Hole

Hole

Hole

Hole Hole

2 TESTING - PHASE 1

2.1 Choice of test piecesA number of objectives were set for the first phase of testing:

C To compare the performance of connections incorporating a kidney shapedslot with that of the plain hole equivalent.

C To compare the behaviour of the connections when the bolt in the kidneyshaped slot is:- in the centre of the slot- at the end of the slot

C To compare details for M20 and M24 bolts, these being the usual bolt sizesused in such connections.

Proposals for test pieces were circulated to the steelwork contractors supportingthe project. Following comment and comparison with standard ‘off the shelf’components, the test piece configurations were finalised as shown in Figure 2.1.

Figure 2.1 Phase 1 test pieces

The asymmetric test pieces were developed, as shown in Figure 2.2, to representreal situations where the bolt bears at the end of the slot. All test pieces of thesame thickness were fabricated from the same flat bar, to allow a comparison ofperformance without modification due to different material strengths.


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70

70

Figure 2.2 Development of asymmetric test piece

It should be noted that the end distances for the test pieces were less than theminimum distance given in Table 31 of BS 5950: Part 1. The minimum enddistance permitted by the Standard for the M20 samples is 31 mm (1.4D = 1.4× 22 = 31 mm). Similarly, for the M24 samples, the minimum end distancepermitted by the standard is 37 mm (1.4 × 26 mm). In both cases, the enddistance is critical in calculating the capacity of the plate in bearing, since:

Pbs = dt pbs # ½ e t pbs (Clause 6.3.3.3)

where:

Pbs is the bearing capacityd is the nominal diameter of the boltt is the thickness of the plye is the end distancepbs is the bearing strength of the connected parts

The plate thicknesses (8 mm for the M20 samples, 10 mm for the M24 samples)were also less than those used in common practice, as the bearing capacity of theplate (S275 material) is less than the bolt capacity in shear.

Both the ‘short’ end distances and the ‘thin’ plates were chosen in order toproduce a bearing failure, rather than a shear failure of the bolts. Connectionswith ‘strong’ plates, configured to produce a failure in bolt shear, were of littleinterest, as the concerns over the use of kidney shaped slots in connections relateto bearing capacity and joint displacement, not bolt shear capacity.

The plates were connected with 8.8 bolts, in single shear. The bolts were notfully threaded.

2.2 Test arrangement - Phase 1The arrangement used for the first phase of the tests is shown in Figure 2.3.

The testing was carried out using a 2000 kN INSTRON universal testing machine(UTM) and a SOLATRON data logger. Backing plates were added to the testplates to ensure that the test plates were installed in the testing machine parallelto the applied force. The connections were assembled and free slack removedbefore tightening the bolts. The bolts were made finger tight, followed by 1/20turn with a spanner. This tightening was adopted in order to minimise any


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frictional resistance between plates. For the M20 assemblies, the load was appliedat the rate of 15 kN per minute up to the design shear capacity of the bolts(184 kN). For the M24 assemblies, the load was applied at the rate of 20 kN perminute up to the design shear capacity of the bolts (264 kN). In each test, theapplied axial load, the total displacement (displacement between the grips of theUTM), and the displacement of the ‘free’ end of the slotted plate (see Figure 2.3)were recorded by the data logger at 20 second intervals.

Figure 2.3 Phase 1 test arrangement

Table 2.1 gives the programme for the first phase of tests.

Table 2.1 Phase 1 test programme

Bolt Hole/Slot Location ofbolt in slot

Numberof tests

Results(Figure No.)

M20M20M20

PlainSlotSlot

-Central

End

333

2.52.62.7

M24M24M24

PlainSlotSlot

-Central

End

333

2.82.92.10


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2.3 Results - Phase 12.3.1 Test resultsFigure 2.4 shows a sample of each plate type after testing.

Figure 2.4 Sample test pieces after testing

The plots of load versus displacement for each test are shown in Figures 2.5 to2.10, corresponding to the tests described in Table 2.1.

0 1 2 3 4

0

50

100

150

200

250

300

Displacement (mm)

Load

(kN

) Test 1

Test 2

Test 3

Figure 2.5 Load vs. displacement for M20 bolts, plain holes


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0 2 4 6 8

0

50

100

150

200

250

300

Displacement (mm)

Load

(kN

) Test 1

Test 2

Test 3

Figure 2.6 Load vs. displacement for M20 bolts, bolt central in slot

0 2 4 6 8

0

50

100

150

200

250

300

Displacement (mm)

Load

(kN

) Test 1

Test 2

Test 3

Figure 2.7 Load vs. displacement for M20 bolts, bolt at end of slot

0 2 4 6

0

50

100

150

200

250

300

Displacement (mm)

Load

(kN

) Test 1

Test 2

Test 3



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0 2 4 6 8

0

50

100

150

200

250

300

Displacement (mm)

Load

(kN

) Test 1

Test 2

Test 3


0 2 4 6 8

0

50

100

150

200

250

300

Displacement (mm)

Load

(kN

) Test 1

Test 2

Test 3

Figure 2.10 Load vs. displacement for M24 bolts, bolt at end of slot

2.3.2 Design capacitiesThe ultimate limit state (ULS) design capacities according to BS 5950: Part 1 forthe connections with plain holes are calculated below, based on 8.8 bolts and S275steel. To determine an equivalent maximum working load at which to comparedisplacements, the ULS capacities are divided by a factor of 1.4, reflecting theusual situation where kidney shaped slots are used in bracing systems resistingwind loads.

M20 bolts with 8 mm plate

Shear capacity = 2 × 92 kN = 184 kN

Bearing in plate = (20 × 8 × 460 + ½ × 28 × 8 × 460) × 10-3

= 125 kN

ULS design capacity = 125 kN


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Working load = = 89 kN1251 4.

M24 bolts with 10 mm plate

Shear capacity = 2 × 132 = 264 kN

Bearing in plate = (24 × 10 × 460 + ½ × 33 × 10 × 460) × 10-3

= 186 kN

ULS design capacity = 186 kN


2.3.3 Capacities based on measured material propertiesMeasured material properties are shown in Table 2.2.

Table 2.2 Material properties

Plate Property Sample 1

Sample 2

Sample 3

Average

8 mm plate Modulus of elasticity(N/mm2)

204 210 190 201

0.2% proof stress(N/mm2)

272 276 280 276

Ultimate tensilestrength (N/mm2)

587 594 590 590

10 mm plate Modulus of elasticity(N/mm2)

219 215 216 217

0.2% proof stress(N/mm2)

314 316 312 314

Ultimate tensilestrength (N/mm2)

552 548 547 549

Based on the average measured material properties, the bearing strength pbs, maybe calculated from the formula given in Table 33 of BS 5950: Part 1.

pbs = 0.65 (Us + Ys)

where:Us is the ultimate tensile strengthYs is the yield strength (equivalent to 0.2% proof stress)

Thus for the 8 mm plate pbs = 0.65 (590 + 276) = 563 N/mm2

and for the 10 mm plate pbs = 0.65 (549 + 314) = 561 N/mm2


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Bearing capacities, based on measured properties, are therefore:

M20 bolts in 8 mm plate

Bearing in plate = (20 × 8 × 563 + ½ × 28 × 8 × 563) × 10-3

= 153 kN

Working load = = 109 kN1531.4

M24 bolts in 10 mm plate

Bearing in plate = (24 × 10 × 561 + ½ × 33 × 10 × 561) × 10-3

= 227 kN

Working load = = 162 kN2271.4

2.3.4 Ultimate capacitiesTesting was halted before gross deformations occurred, and the plateau of the loadvs. displacement curve (and hence the ultimate capacity) was therefore not found.This is particularly apparent for the M24 samples (Figures 2.8, 2.9 and 2.10).

However, it can be seen that for all tests the maximum applied load wassubstantially greater than both the plain hole design capacity and the plain holecapacity based on measured material properties.

Testing of the M20 samples was generally stopped at 186 kN (being more than thedesign bolt shear capacity of 184 kN), although one sample was loaded to 210 kN.All M20 samples therefore demonstrated an ultimate capacity of at least 1.5 ×(design capacity with plain holes).

The M24 samples were tested up to an applied load of 264 kN (being the designbolt shear capacity). All M24 samples therefore demonstrated an ultimate capacityof at least 1.4 × (design capacity with plain holes).

Based on the measured material properties, the M20 samples demonstrated anultimate capacity of at least 1.2 × (capacity with plain holes) and the M24samples an ultimate capacity of at least 1.2 × (capacity with plain holes).

2.3.5 Serviceability performanceTable 2.3 indicates the displacement for each of the M20 samples, recorded at thedesign working load of 89 kN.

Table 2.4 indicates the displacement for each of the M24 samples, recorded at thedesign working load of 133 kN.

From these Tables, it will be seen that the displacement at working load for thespecimens with slots (with the bolt central to the slot) are 30% and 45% more thanthe plain hole equivalents, for M20 and M24 bolts respectively. The additionaldisplacement is, however, small in absolute terms (0.45 mm for M20 and0.83 mm for M24). It is considered that the additional deformation would not be


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detrimental to the performance of the type of structural systems likely to use suchconnections, noting that ordinary bolts in clearance holes have the potential for4 mm movement across the connection before any plate deformation takes place.

Table 2.3 Displacement at design working load, M20 samples

Specimen Test Displacement(mm)

Averagedisplacement (mm)

Plain holes 123

1.401.761.27

1.48

Slot, bolt central in slot 123

1.701.592.51

1.93

Slot, bolt at end of slot 123

1.702.301.81

1.94

Table 2.4 Displacement at design working load, M24 samples


Averagedisplacement (mm)

Plain holes 123

1.611.772.17

1.85

Slot, bolt central in slot 123

2.252.912.89

2.68

Slot, bolt at end of slot 123

2.502.462.63

2.53

It is assumed that with the thicker plate provided in normal practice (10 mm withM20 bolts and 12 or 15 mm with M24 bolts), the additional deformation wouldbe less. It is assumed that further reduction in deformation would occur if the enddistances were increased from the minimum allowed.

In the calculation of average displacement in Tables 2.3 and 2.4 above, no attempthas been made to reduce the effect of the initial slip seen in, for example, Test 2,Figure 2.5 and Test 3, Figure 2.6.

The average displacements at working loads based on measured material propertiesare shown in Table 2.5.

Removing the initial slip from Test 2 of Figure 2.5 reduces the averagedisplacement for the plain hole M20 samples from 1.73 mm to 1.6 mm. Thiscorrelates well with the BS 5950: Part 1 approach to bearing capacity, as discussedin Section 1.3.2. It should be noted that for all samples, the end distance was lessthan the minimum required by BS 5950: Part 1, as noted in Section 2.1, whichmay have contributed to increased deformations.


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Table 2.5 Average displacements at working load, measured materialproperties

Sample Working load(kN)

Specimen Average displacement (mm)

M20 109 Plain holes 1.73

Slot, boltcentral in slot

2.34

Slot, bolt atend of slot

2.34

M24 162 Plain holes 2.28

Slot, boltcentral in slot

3.27

Slot, bolt atend of slot

3.01

2.4 Conclusions from Phase 1 testsWhilst the results from Phase 1 of the testing were encouraging, it was recognisedthat more information would be beneficial for the following reasons:

C Testing had ceased prior to the ultimate capacity.

C Testing had not demonstrated how the applied load was shared between bolts.

It has long been recognised that design bolt capacities are quite conservative,allowing for an uneven distribution of applied load within large bolt groups, andallowing for a broad variation of bolt strength. It was therefore considered thatif ‘overstrong’ bolts had been used in the Phase 1 tests, the single bolt in the plainhole might have been carrying the majority of the load, with the bolt in the slotrelatively ineffective. If this was the load distribution and, in practice,‘understrength’ bolts (though still within specification) were used, the connectioncould fail at a lower load than that predicted by the Phase 1 tests.

The combination of ‘understrength’ bolts and no (or little) distribution of appliedload between the two bolts was considered to be an unlikely scenario, but onewhich should be considered. It was therefore decided to carry out a second phaseof tests to investigate the distribution of applied load between the bolts, and toconsider the behaviour of connections with more orthodox details i.e. platethickness and end distances.


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3 TESTING - PHASE 2

3.1 ObjectivesThe objectives of the second phase of testing were:

C To investigate the distribution of the applied load between bolts.

C To investigate the behaviour of connections with typical plate thicknesses andend distances.

3.2 Test arrangement - Phase 2In order to determine the distribution of load between the bolts, it was concludedthat the most appropriate solution, considering cost, time constraints andpracticality, was to instrument the samples with strain gauges to determine thestrains in different parts of the specimens. From this, it was hoped to deducewhat proportion of the total load had been transferred by each bolt. For practicaland economic reasons, it was decided to instrument a single thick plate with plainholes, which could be used for each of the samples to be tested. Figure 3.1illustrates the test arrangement.

10 mm plate20 mm plate

Strain gauges

Figure 3.1 Phase 2 test arrangement

Strain gauges were placed at two cross sections in the locations shown inFigure 3.2.


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60

35

9570

70

35

Figure 3.2 Location of strain gauges

From the first phase of testing it was clear that, as expected, the slotted connectionwith the bolt in the centre of the slot produced the maximum joint displacement.It was therefore decided to test only this configuration of slotted connection in thesecond phase, comparing behaviour with the plain hole equivalent. Due toeconomic pressures, the second phase of testing was also limited to connectionswith M20 bolts, assuming that the results would be equally applicable tocommensurately sized M24 details. The test pieces are shown in Figure 3.3. Therevised plate thickness and end distances (compared to the Phase 1 samples)should be noted. The testing in Phase 2 was carried out using the same equipmentand procedures employed in Phase 1 (Section 2.2) except that the loading rate wasincreased to 20 kN per minute. Table 3.1 gives the programme for the secondphase of tests.

15°15°

R = 70

10 mm S275 10 mm S275

46

46 Ø22

70

Ø22 Ø22

R=40

40 40

350 350

40 3840

22

Hole

Hole Hole

70 70 70 70

Figure 3.3 Phase 2 test pieces

Table 3.1 Phase 2 test programme

Bolt Hole/Slot Location ofbolt in slot

Number oftests

Results(Figure No.)

M20 Plain - 3 3.4

M20 Slot Central 3 3.5


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3.3 Results - Phase 23.3.1 Test resultsThe plots of load versus displacement for each series of tests are shown inFigures 3.4 and 3.5, corresponding to the tests described in Table 3.1.

0 1 2 3 4 5 6 7 8 9 10 11 12 13 140

50

100

150

200

250

300

350

Displacement (mm)

Load

(kN

) Test 1

Test 2

Test 3


0 1 2 3 4 5 6 7 8 9 10 11 12 13 140

50

100

150

200

250

300

350

Displacement (mm)

Load

(kN

) Slot, 1

Slot, 2

Slot, 3


3.3.2 Analysis of the strain gauge dataDespite extensive data, it was not possible to propose a robust model which wouldsatisfactorily explain the strain gauge readings. Variations in strain due to localeffects and plate bending meant that any proposal regarding levels of stress indifferent parts of the plate had to accommodate an unacceptably high level ofexperimental scatter. No conclusions regarding the distribution of load betweenthe two bolts in the connections could be drawn from the strain gauge results.


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3.3.3 Design capacitiesThe ULS design capacity for the connections with M20 8.8 bolts in plain holes(10 mm, S275 steel) is:

Shear capacity = 2 × 92 = 184 kN

Bearing in plate = 2 × 20 × 10 × 460 × 10-3=184 kN

Design capacity = 184 kN


3.3.4 Ultimate capacitiesPhase 2 tests continued until gross deformations began. Ultimate capacities weretherefore determined, and are shown in Table 3.2.

Table 3.2 Ultimate capacity of connections tested in Phase 2

Specimen Test Ultimate capacity (kN)

Plain holes 123

320299318

Slot, central bolt 123

272268274

The samples with plain holes demonstrated a minimum ultimate capacity of 1.63× design capacity. The samples with a slot demonstrated a minimum ultimatecapacity of 1.46 × (design capacity with plain holes).

3.3.5 Serviceability performanceTable 3.3 indicates the displacement for each of the tests, recorded at the workingload of 131 kN.

Table 3.3 Displacement of Phase 2 connections at working load


Average displacement(mm)

Plain holes 123

1.15 (1.85*)2.772.36

2.33

Slot, central bolt 123

2.353.603.10

3.02

* Test 1 of the series with plain holes shows a displacement discontinuity at approximately180 kN, which is not apparent in any other results (see Figure 3.4). No explanation is apparent.The displacement of 1.85 mm which is indicated in Table 3.3 was determined by extrapolatingback the curve above 180 kN, following the general form demonstrated by all other results. Thevalue of 1.85 mm was used in calculating the average displacement.


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From the above results, it can be seen that displacement at working load for theconnections with a slot is some 30% greater than for the samples with plain holes.Both values are greater than 1.5 mm previously indicated as the limiting value.For reasons already explained in Section 2.3.5, it is not considered that thesedisplacements would be detrimental to the performance of structural systemsincorporating such connections.


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4 CONNECTION CAPACITY

Based on the results obtained from the two phases of tests described in this report,it is possible to recommend design rules for two bolt connections incorporating asingle kidney shaped slot. The derivation of the design capacity for suchconnections is described in Section 4.1.

4.1 Capacity of connections with kidney shapedslots

In an orthodox two bolt connection with plain holes, it is assumed that at theultimate limit state (ULS), the applied load is shared equally between the bolts.From Figure 3.4 and Table 3.2 it can be seen that, for the test specimens, themaximum capacity at the ULS for an M20 connection with plain holes was320 kN.

Assumed load per bolt = 320/2 = 160 kN

It may be assumed that in a connection with a slot, the presence of the slot doesnot affect the capacity of the bolt in the plain hole. Therefore the total capacityis equal to the capacity of the bolt in the plain hole, plus some contribution fromthe bolt in the slot.

From Figure 3.5 and Table 3.2, the minimum capacity at the ULS for an M20connection with a slot was 268 kN.

Assuming the contribution from the bolt in the plain hole was 160 kN, then thecapacity of the connection may be expressed as:

(plain hole capacity) × 268160

or(plain hole capacity) × 1.68

It is proposed that the factor of 1.68 be reduced to a recommended factor, fordesign purposes, of 1.6, to allow for additional variation in performance, over andabove that implied by the use of maximum plain hole capacity and minimumslotted hole capacity from the test results.

The proposal for connection capacity given above is based on results forconnections where the bearing capacity of the plate is similar to the bolt capacityin shear (i.e. M20 8.8 bolts in 10 mm S275 plate). In these circumstances thetests demonstrated that a ductile failure will occur. The possibility of a non-ductile failure, due to the use of a relatively strong plate (i.e. relatively thick, orhigher grade steel) compared to the bolt strength, was discussed in Section 2.4.

Having considered this possibility, it was concluded that if ‘strong’ plates are usedin a connection, the presence of a slot has less effect on the connection capacity.As the influence of the slot reduces, it was concluded that the capacity of theconnection would approach that of a connection with plain holes, where theconnection capacity would be taken as twice that of a single bolt.


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The converse situation of ‘strong’ bolts was also considered, since the sharing ofload between bolts is partly due to bolt deformation. A ‘strong’ bolt in the plainhole could therefore attract a larger proportion of the load than implied by the 1.6factor above. It was concluded that if a ‘strong’ bolt did attract additional loaduntil deformation of the plate commenced, this could be accommodated by theincreased shear capacity of the ‘strong’ bolt.

It was therefore concluded that the factor of 1.6 × single bolt capacity may beused in all cases.

4.2 Design rules for connections with kidneyshaped slots

For a two-bolt connection transferring axial load between two members, whereone of the holes is kidney shaped, the following rules are recommended, inaddition to those of BS 5950: Part 1,:

Fastener spacing and edge distancesThe minimum spacing according to Clause 6.2.1 should apply.

All edge and end distances, as defined in Clause 6.2.3, should be not less than 2d,where d is the nominal diameter of the bolt.

The length of the kidney shaped hole should be a maximum of 2d, and such thatthe angle 2, as shown in Figure 4.1 does not exceed 30E.

There is no restriction on the location of the bolt within the kidney shaped slot.

θ = 30° maximum

2d maximum

Figure 4.1 Maximum length of slot

Connection capacityThe capacity of a two bolt connection with a single kidney shaped slot should betaken as:

1.6 × (capacity of a single bolt in plain hole)

where the capacity of a single bolt in a plain hole is the lesser of the values givenby BS 5950: Part 1, Clauses 6.3.2 and 6.3.3.


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4.3 Bracing member effective lengthSince the kidney shaped slot allows rotation of the connection, the connectiondetail cannot be assumed to provide directional restraint equivalent to that of twobolts in plain holes.

When effective lengths for hollow section bracing are determined from Table 24of BS 5950: Part 1, a connection incorporating a kidney shaped slot should not beassumed to provide any directional restraint in the plane of the gusset.

When angles are used as bracing and the connections incorporate kidney shapedslots, the provisions for single bolt connections in Table 28 of BS 5950: Part 1should be adopted, though the 80% reduction in compression resistance stated inNote 3 to Table 28 need not be applied.

4.4 Provision of washersClause 6.1.4(ii) of the National Structural Steelwork Specification(7) (NSSS)specifies that a plate washer or heavy duty washer be used under the bolt head andnut when bolts are used to assemble components with oversize or slotted holes.It is understood that the origin of this clause concerned slots which were providedto permit movement in service. Shouldered bolts would normally be used in suchjoints.

The use of kidney shaped slots in bracing connections is a different situation to theone intended to be addressed by Clause 6.1.4(ii) of the NSSS. Ordinary washersare considered to be satisfactory for use in bracing connections with kidney shapedslots.


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1. CARLSSON, M., MALIK, A.S., BROWN, D.G.Novel ConnectionsSCI Report RT560(Internal report - unpublished)

2. THE BRITISH CONSTRUCTIONAL STEELWORK ASSOCIATION LTDand THE STEEL CONSTRUCTION INSTITUTEJoints in simple constructionVolume 2: Practical applicationsBCSA & SCI, 1992

3. THE STEEL CONSTRUCTION INSTITUTE and THE BRITISHCONSTRUCTIONAL STEELWORK ASSOCIATION LTDJoints in steel construction: Moment connectionsSCI & BCSA, 1995

4. GIRARDIER, E.V.Construction Led Design (series of four articles)Steel Construction (BCSA), Feb. 1991, Aug. 1991, Feb. 1992New Steel Construction (SCI/BCSA), Feb. 1993

5. BRITISH STANDARDS INSTITUTIONBS 5950: Structural use of steelwork in buildingPart 1. Code of practice for design in simple and continuous construction:hot rolled sectionsBSI, 1990

6. MALIK, A.S. (Editor)Introduction to steelwork design to BS 5950: Part 1SCI, 1988

7. THE BRITISH CONSTRUCTIONAL STEELWORK ASSOCIATION LTDand THE STEEL CONSTRUCTION INSTITUTENational Structural Steelwork Specification for Building Construction (3rdEdition)BCSA & SCI, 1994

REFERENCES


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sci p 249

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