Composite construction

Composite Construction
Edited by David A. Nethercot

©2004 Taylor & Francis


First published 2003
by Spon Press
11 New Fetter Lane, London EC4P 4EE
Simultaneously published in the USA and Canada
by Spon Press
29 West 35th Street, New York, NY 10001
Spon Press is an imprint of the Taylor & Francis Group
This edition published in the Taylor & Francis e-Library, 2004.

© 2003 Spon Press
All rights reserved. No part of this book may be reprinted or
reproduced or utilised in any form or by any electronic,
mechanical, or other means, now known or hereafter
invented, including photocopying and recording, or in any
information storage or retrieval system, without permission in
writing from the publishers.
British Library Cataloguing in Publication Data
A catalogue record for this book is available
from the British Library
Library of Congress Cataloging in Publication Data
Nethercot, D.A.
Composite construction / David A. Nethercot.
p. cm.
Includes bibliographical references and index.
ISBN 0-415-24662-8 (alk. paper)

1. Composite construction. 2. Composite materials. I. Title.
TA664 .N48 2003
620.1′18—dc21
2002042805
ISBN 0-203-45166-X Master e-book ISBN

ISBN 0-203-45733-1 (Adobe eReader Format)
ISBN 0-415-24662-8 (Print Edition)

©2004 Taylor & Francis


Contents

Contributors
Foreword
Acknowledgments
1

Fundamentals
D A V I D A . N E T H E R CO T

1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8

1.9
1.10
1.11
2

Introduction
History
Basic concepts
Material properties
Shear connectors
Design for ULS
Design for SLS
Composite systems
Current usage
Concluding remarks
References

Composite Beams
HOWARD D. WRIGHT

2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
2.10

Introduction
Types of beam
Basic behaviour
Ultimate strength design
Calculating the deflection
Shear connector behaviour
Continuous beams
Beams with composite slabs
Current design and future development
References

©2004 Taylor & Francis


3

Composite Columns
YONG C. WANG

3.1
3.2
3.3

Introduction
Composite columns under axial load in cold condition
Composite column under combined axial load and bending
moments at ambient temperature
3.4 Effect of shear
3.5 Load introduction

3.6 Composite columns in fire conditions
3.7 Summary
3.8 Acknowledgement
3.9 References
3.10 Notations
4

Instability and Ductility
ALAN R. KEMP

4.1
4.2
4.3
4.4
4.5
5

Introduction and elastic buckling theory
Ultimate resistance of composite columns
Continuous composite beams
Ductility considerations for compact beams
References

Composite Floors
J . B UI C K D A V I S O N

5.1
5.2
5.3
5.4

5.5
5.6
5.7
5.8
5.9
5.10
6

Introduction
Current practice
Behaviour as formwork
Composite behaviour
Dynamic behaviour
Concentrated loads and slab openings
Fire resistance
Diaphragm action
Slim floor decking
References

Composite Connections
DAVID B. MOORE

6.1
6.2
6.3

Introduction
Types of composite connections
Design principles

©2004 Taylor & Francis


6.4
6.5
6.6
6.7
6.8
6.9

Classification of composite connections
Capacity of composite connections
Ductility of composite connections
Stiffness of composite connections
Summary
References

7 Composite Frames
GRAHAM H. COUCHMAN

7.1
7.2
7.3
7.4
7.5
7.6

Introduction
Principles of frame behaviour
Frame analysis and design

Design using software
Conclusions
References

©2004 Taylor & Francis


Contributors

Graham H. Couchman
Steel Construction Institute
Silwood Park
Buckhurst road
Ascot
Berks
SL5 7QN
J. Buick Davison
The University of Sheffield
Department of Civil & Structural
Engineering
Sir Frederick Mappin
Building
Mappin Street
Sheffield
S1 3JD
Alan R. Kemp
Faculty of Engineering
University of Witwatersrand
1 Jan Smuts Avenue
Johannesburg

2001 South Africa

©2004 Taylor & Francis

David B. Moore
BRE
PO Box 202
Watford
Herts
WD2 7QG
David A. Nethercot
Imperial College
Department of Civil & Environmental
Engineering
London SW7 2AZ
Yong C. Wang
School of Civil Engineering
University of Manchester
Oxford Road
Manchester
M13 9PL
Howard D. Wright
University of Strathclyde
James Weir Building
75 Montrose Building
Glasgow
G1 1XJ


Foreword

Composite Construction has developed significantly since its origins approximately
100 years ago when the idea that the concrete fire protection around columns might
be able to serve some structural purpose or that the concrete bridge deck might,
with advantage, be made to act in conjunction with the supporting steel beams was
first proposed. Take-up in practice and began in earnest shortly after the end of the
Second World War and progress has been particularly rapid during the past 20
years. Indeed, it is now common to ask, “Why is this not acting compositely?”
when looking to improve the efficiency of a structural steelwork design. In those
countries where steelwork enjoys a particularly high market share e.g. for high-rise
buildings in the UK and Sweden, the extensive use of composite construction is a
major factor.
Early approaches to the design of composite structures generally amounted to
little more than the application of basic mechanics to this new system. However, it
was soon realised that this particular medium possessed features and subtleties of its
own and that effective usage required that these be properly understood and allowed
for. Composite construction is now generally regarded as a structural type in its own
right, with the attendant set of design codes and guidance documents. The most
comprehensive and up to date of these is the set of Eurocodes—specifically EC4
that deals exclusively with composite construction. It is not the purpose of this textbook to serve as a commentary on the Eurocodes. Rather, it is an explanatory and
educational document, presenting the technical basis for many of the newer concepts, design procedures and applications of composite construction in buildings.
Inevitably, it makes some reference to the Eurocodes but only in the sense that their
procedures often represent formal statements of the most appropriate simplified
implementation of our current understanding. For convenience and consistency it
adopts their notation.
The authors—each an acknowledged expert in the topic on which they have
written—have selected their own way of presenting the subject matter. In all cases
the intent has been to share the technical basis and background to design so that
extrapolation and intelligent use beyond the obvious is possible. The book is not
claimed to be comprehensive or to represent a full state of the art. It should be

regarded as helpful background reading for all those wishing to acquire a better
appreciation and understanding of the major developments in the use of composite
construction for building structures.
The first Chapter of this book traces the key historical steps in the development
and understanding of Composite Construction and introduces the main fundamental
features. The next two deal with basic elements—horizontal beams and vertical
columns—showing how the combined action of the concrete and the steel member
may be synthesised to give a more efficient load resisting arrangement. A relatively
new development is the deliberate use of composite action in beam to column

©2004 Taylor & Francis


connections, thereby requiring them to be treated as partial strength and semi-rigid
for design purposes as explained in Chapter 4. Because buckling is a key item when
dealing with the response of steel members, its importance for composite
elements—especially beams—is then considered in some detail. Building floor systems now often comprise arrangements with two-way spanning composite action
and several such arrangements are discussed in Chapter 6. The final Chapter deals
with the interaction of beams, columns and joints in presenting a complete treatment
for the design of non-sway composite frames that recognises the actual behaviour
more closely than does conventional treatments based on consideration of individual
components.
This book is collaborative effort, with all the Chapter authors having made an
equal contribution. Its preparation has inevitably involved delivery against deadlines
and the required instructions. My thanks to Howard, Yong, David, Buick, Alan and
Graham for their patience and cooperation. Production has benefited from the firm
but sympathetic guidance of the publishers—particularly Alice Hudson. The coordination and final preparation of the manuscript was just one of the tasks handled so
efficiently by my PA Alice Kwesu.
David A. Nethercot

©2004 Taylor & Francis


Acknowledgments

Considerable effort has been made to trace and contact copyright holders and
secure replies prior to publication. The authors apologise for any errors or
omissions.
Extracts from Eurocode 4, Eurocode 3 and BS 5950 Part 3: 1990 are reproduced with the permission of BSI under licence number 2002SK/0204. Eurocodes
and British Standards can be obtained from BSI Customer Services, 389 Chiswick
High Road, London W4 4AL. (Tel + 44 (0) 20 8996 9001).
Figures from Steel Construction Institute publications are reproduced with
kind permission from the Steel Construction Institute.
Acknowledgments are also required for the following:
Chapter One—Fundamentals
David A. Nethercot
Figure 1.1 reproduced with kind permission from the ASCE from: Moore, W.P.,
Keynote Address: An Overview of Composite Construction in the United States,
Composite Construction in Steel & Concrete, ed. C.D. Buckner & I.M. Viest,
Engineering Foundation, 1988, pp. 1–17.
Figures 1.2 and 1.3 reproduced from: David A. Nethercot, Limit States Design
of Structural Steelwork, Spon Press.
Figures 1.4, 1.5, 1.6 and 1.8 reproduced from: Johnson, R.P., Composite
Structures of Steel & Concrete Volume 1 Beams, Slabs, Column & Frames for
Buildings, 2nd edition, Blackwell Scientific Publications.
Figure 1.16 reproduced from: Lam, D., Elliott, K.S & Nethercot, D.A., Structures and Buildings, ICE Proceedings
Chapter Two—Composite Beams
Howard D. Wright
Figure 2.4 reproduced from: Mullett, D.L., Composite Floor Systems, Blackwell
Science Ltd.

Chapter Three—Composite Columns
Yong C. Wang
Tables 3.4, 3.5 and 3.6 are reprinted from Journal of Constructional Steel Research, 51,
Kodur, V.K.R., Performance-based fire resistance design of concrete-filled columns,
pp. 21–36, 1999, with permission from Elsevier Science.

©2004 Taylor & Francis


Chapter Five—Composite Floors
J. Buick Davison
Figure 5.20 reproduced from: Composite Slab Behaviour and Strength Analysis.
Part 1 calculation Procedure, Daniels, Byron J., Crisinel, Michael, Journal of Structural
Engineering, Vol. 119, 1993—ASCE.
Figure 5.26 reproduced courtesy of Corus plc.
Chapter Six—Composite Connections
David B. Moore
Figures 6.1, 6.13, 6.16, 6.17 and 6.18 reproduced with kind permission of Building
Research Establishment Ltd.

©2004 Taylor & Francis


CHAPTER ONE

Fundamentals
David A. Nethercot

1.1 INTRODUCTION
The term “composite construction” is normally understood within the context of

buildings and other civil engineering structures to imply the use of steel and
concrete formed together into a component in such a way that the resulting
arrangement functions as a single item. The aim is to achieve a higher level of
performance than would have been the case had the two materials functioned
separately. Thus the design must recognise inherent differences in properties
and ensure that the structural system properly accommodates these. Some form
of interconnection is clearly necessary.
Since its introduction, the utilisation of composite action has been recognised as an effective way of enhancing structural performance. In several parts
of the world a high proportion of steel structures are therefore designed compositely. Design codes, textbooks, specialist design guides, descriptions of projects
and research papers directed to the topic exist in abundance; many of these are
referred to in the present text.
This opening chapter covers the general background to the subject necessary
for a proper appreciation of the following six chapters, each of which concentrates on a specific topic. Its coverage is thus much broader and its treatment of
particular aspects of composite construction rather more elementary and less detailed
than will be found elsewhere in the book. Readers already possessing some
knowledge of the subject may well prefer to go directly to the chapter(s) of interest.
Because the book focuses on the use of composite construction in building
structures it does not attempt to cover those structural phenomena that are
unimportant in this context. Thus items such as fatigue, temperature effects,
corrosion, impact response etc., which need to be properly addressed when
designing bridges, offshore structures, tunnels, military installations etc., are not
included.
1.2 HISTORY
In both the opening Address (1) to the first in the series of Engineering Foundation
Conferences on Composite Construction (2–5) and a Keynote paper to a US–
Japan Symposium on the subject (6), authors from the same US consulting firm
have traced the very early use of composite construction in North America. The
year 1894 is stated as the period in which concrete encased beams were first
used in a bridge in Iowa and a building in Pittsburgh. The earliest laboratory
tests on encased columns took place at Columbia University in 1908, whilst

composite beams were first tested at the Dominion Bridge Works in Canada in

©2004 Taylor & Francis


1922. By 1930 the New York City building code recognised some benefit of
concrete encasement to steelwork by permitting higher extreme fibre stresses in
the steel parts of the encased members. Welded shear studs were first tested at
the University of Illinois in 1954, leading to publication of a design formula in
1956 and first use for both bridge and building projects that same year. In 1951
a partial interaction theory was proposed, also by the team from Illinois. Metal
decks first appeared in the 1950s, with the first recorded use of through deck
stud welding on the Federal Court House in Brooklyn in 1960. It was not until
1978, however, that this arrangement was recognised in the AISC specification.
Early usage in Japan has been recorded by Wakabayashi (7), who refers to
the use of concrete encasement to improve both fire and earthquake resistance,
dating from about 1910. Termed “steel reinforced concrete” or SRC, this form
of construction quickly became popular for buildings of more than 6 storeys. Its
integrity was demonstrated by the good performance of structures of this type in
the great Kanto earthquake of 1923. Research on the topic in Japan did not start
until the 1930s; codes came much later, with the first SRC code produced by
the Architectural Institute of Japan (AIJ) appearing in 1958. Work on a design
guide for composite bridges started in 1952 but the document was not published
until 1959. Beginning with the earliest beam tests in 1929 and column tests in
the same year, research studies flourished in the 1950s and 1960s, leading to a
heavy concentration since then on understanding the behaviour of composite
construction when subjected to seismic action.
One of the earliest substantial documents devoted to composite construction was the book by Viest, Fountain & Singleton (8). Published in 1958,
it referred to early usage of the technique in the United States by 1935 and a
patent by Kahn dated 1926 (Figure 1.1). The book was written to complement the

American Association of State Highway Officials (AASHO) 1957 Specification

Figure 1.1 Kahn Patent of 1926

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that covered the use of composite beams for bridges. In addition to referring to
several North American examples of composite bridges constructed in the
1940s and 1950s, it includes an Appendix outlining the use of composite beams
in building construction. A full set of Rules covering the design of composite
beams was provided in the 1961 American Institute of Steel Construction
(AISC) Buildings Specification.
Parallel developments had been taking place in Europe—especially as part
of the post-war reconstruction in Germany. Reporting on this in 1957, Godfrey
(9) refers to “research in Germany, Switzerland and elsewhere” providing the
basis for their “Provisional Regulations for the Design of Girders in Composite
Construction” published in July 1950. Four years later the topic was addressed
more formally in DIN 1078. In a follow-up paper Sattler (10) reported on numerous examples of the use of composite construction for both bridges and buildings
in Germany. This extended to the use of concrete filled tubes as columns but for
beams relied on far more complex forms of shear connection than the welded
studs which were at that time already being introduced into the United States by
the Nelson company. In the discussion to these papers, British researchers
Chapman & Johnson referred to both research in progress and buildings under
construction that had been designed compositely at Imperial College and
Cambridge University. Reports on this work appeared a few years later (11–15),
papers on cased stanchions (16), early UK composite bridge applications (17)
and background studies for buildings (18) having appeared in the late 1950s.
Thus by the mid 1960s the structural engineering community in the UK
was appreciative of the merits of composite construction. It had for example

been employed for a number of Government designed buildings (19), essentially
in the form of composite beams but with the novel feature that these utilised
precast lightweight aggregate concrete panels and planks (20). Early ideas for
composite columns that saw the 1948 edition of BS 449 simply permit the
radius of gyration to be taken as something greater than that for the bare steel
member had, as a result of the 1956 tests (21), been extended to using the full
concrete area. The later BRS column tests (22), including considerations of
beam-column behaviour, saw the development of soundly based interaction formulae approaches to design. Ref. 22 includes an interesting reference to several
very early sets of tests on concrete encased columns conducted during the period
1912–1936. Much of this British work was then brought together into the first
comprehensive composite code, CP 117, published in 3 parts (23), covering:
simply supported beams in buildings, beams for bridges and composite columns.
For building structures this was later replaced by the Part 3 of the limit states
based code BS 5950, although only the Part 3.1 dealing with simply supported
beams and Part 4 covering composite slabs were ever completed (24).
Fascinating accounts of research into various aspects of composite
construction conducted during different parts of the period 1940–1990 in the
USA, UK, Japan, Australia and Germany are available in the third Engineering
Foundation Volume (4).
1.3 BASIC CONCEPTS
The essence of composite construction is most readily appreciated by considering
its most commonly used application, the composite beam. To begin with a very
simple illustration, consider the beam consisting of two identical parts shown in

©2004 Taylor & Francis


Figure 1.2 Basic mechanics of composite action

Figure 1.2. In the case of Figure 1.2a both parts behave separately and move

freely relative to each other at the interface, whilst in the case of Figure 1.2b
both parts are constrained to act together. For case a longitudinal slip occurs as
indicated by the movement at the ends, whereas in case b plane sections remain
plane. It is readily demonstrated using elastic bending theory that case b is
twice as strong and four times as stiff as case a. Now consider the steel/concrete
arrangement of Figure 1.3a. The two parts are now of different sizes and possess

(a) cross-section

(b) Neutral axis in slab

(c ) Neutral axis in steel section
Figure 1.3 Stress block representations

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different stress–strain characteristics. Assuming for the purpose of the illustration
that the neutral axis of the composite section is located at the concrete/steel
interface and that full interaction is ensured so that no slip occurs, the distributions
of strain and a corresponding stress block representation of stresses at the
assumed ultimate condition will be as shown in Figures 1.3b and 1.3c respectively.
Using the latter, considerations of cross-sectional equilibrium permit the
moment of resistance to be readily calculated. Although the member’s neutral
axis will clearly not always fall at the interface, good design will attempt to
locate it close to this position as representing the most efficient use of the
strengths of the two different materials (concrete acting in compression and
steel acting in tension).
For such cases the resulting equilibrium calculations to determine the
moment of resistance are only slightly modified.

The use of plastic methods to determine strength, as employed in the above
illustration, are now commonplace when dealing with composite construction.
Although extensive elastic treatments exist, it has been found that, providing
certain rules are observed e.g. relating to potential instability in parts of the
steel section, the ability of the shear connection to resist the interface slip etc.,
then a relatively simple plastic approach is both easier to use and leads to
higher resistances.

1.4 MATERIAL PROPERTIES
When designing composite elements it is usual to adopt the same properties for
steel and for concrete as would be the case when designing structural steelwork
or reinforced concrete. Thus codes of practice covering composite construction,
such as EC4, normally simply summarise the relevant sections from the steelwork and concrete documents—EC3 and EC2 in the case of EC4.
Within the limit states framework of design, it is customary to work with
characteristic values of material strengths. Whilst these are normally defined
with reference to a suitable fractile in the assumed statistical distribution of the
material strengths, for structural steel nominal values are usually taken as characteristic values due to the quality control processes used in its manufacture and
thus the basis on which it is supplied. The design values to be used in structural
calculations are derived directly by dividing the characteristic by the appropriate partial factor. In the case of shear connectors it is customary to work with
component strengths—defined on the basis of testing—rather than material
strengths.
As an illustration, the relevant sections of EC4 covering: concrete, steel
reinforcement, structural steel, metal decking and shear connections, together
with their key recommendations, are summarised in Table 1.1.
Concrete is specified in terms of its compressive strength, as measured in
a cylinder test, fck. Grades between 20/25 and 50/60 are permitted. Characteristic tensile strengths are also provided; for lightweight concretes tensile values
should be modified by the correction factor:
η = 0.30 + 0.70 (ρ/2000)
in which ρ is the oven-dry unit mass in kg/m3.

©2004 Taylor & Francis

(1.1)


Table 1.1 Material properties specified in EC4

fck, fctm, fctk0.05, fctk0.95, total long-term free shrinkage
strain, Ecm modular ratio
fsk, ft, minimum ratio of ft, /fsk, εu as given in EN 10 080
Reinforcing steel
fy, fu or refer to EN 10 080
Structural steel
Profiled steel sheeting fyb
Shear connectors
Refers to Chapter 10 of EC4 to obtain PRd, δ v & PRk
fu/fy < 1.2, εu on a gauge length of 5.65 A o < 12%
Concrete

Nominal values for the total long-term free shrinkage strain due to setting
are provided for typical environments. Creep may normally be covered by using
an effective modulus equal to Ecm/3 when dealing with deflections due to long-term
loads, with Ecm for short-term loads being the tabulated secant modulus for the
particular grade. Some cautions on the extent to which shrinkage and creep
effects are adequately treated by present simplified approaches have been
provided by Leon (25).
Characteristic yield strengths of plain and ribbed bars fsk are provided in
EN 10 080. For structural steel, as previously mentioned, nominal values of
yield strength fy and ultimate strength fu are given, with grades Fe 360, 430 and
510 being covered. These should be used as characteristic values. Nominal

yield strengths for profiled steel sheeting for use as characteristic values when
dealing with composite slabs are also provided.
In the case of shear connectors the characteristic resistance PRk is to be
based on a 5% fractile of test results using a homogenous population of specimens. The design resistance PRd is then equal to PRk/δ v . Further discussion of
the behaviour of shear connectors, including details of the test methods used to
obtain PRk, is provided in Section 1.5 of this chapter.
1.5 SHEAR CONNECTORS
Several early forms of shear connector, used principally for bridges—especially
in the United States and Germany—are illustrated in Figure 1.4. By comparison
with today’s near universal use of welded, headed shear studs of the type shown
in Figure 1.5, they are cumbersome and expensive but provide significantly
higher strengths. Studs range typically between 13 & 25 mm in diameter, although
since the welding process becomes significantly more difficult and therefore
expensive for diameters exceeding about 20 mm, 19 mm studs are by far the most
commonly used. Since the resistance developed by a stud depends (among other
things) on the thickness t of the flange to which it is welded, a limit of d/t of 2.5
is specified in EC4. The steel used to manufacture studs typically has an
ultimate tensile strength of at least 450 N/mm2 and an elongation of at least 15%.
Stud resistances, depending on size and other factors, of up to about 150 kN are
achievable using simple welding procedures. Studs have equal strengths in all
directions and provide little interference to the positioning of reinforcement.
Stud strengths are normally obtained from “push-off” tests, in which
a load–slip curve is determined using a standard test arrangement. Several variants
of these have been recommended, designed to both replicate more closely the

©2004 Taylor & Francis


Figure 1.4 Early forms of shear connector

Figure 1.5 Headed shear stud

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Figure 1.6 Push-off test arrangement

conditions experienced by studs in the compression region of a composite beam
and to ensure greater consistency of results from notionally identical arrangements and procedures. Figure 1.6 illustrates the standard EC4 test arrangement.
Johnson (26) lists those factors which influence the load–slip relationship
obtained from a push-off test as:
1. number of connectors in the test specimen,
2. mean longitudinal stress in the concrete slab surrounding the connectors,
3. size, arrangement, and strength of slab reinforcement in the vicinity of
the connectors,
4. thickness of concrete surrounding the connectors,
5. freedom of the base of each slab to move laterally, and so to impose
uplift forces on the connectors,
6. bond at the steel-concrete interface,
7. strength of the concrete slab, and
8. degree of compaction of the concrete surrounding the base of each
connector.
A typical load–slip relationship is given as Figure 1.7. This exhibits significant
ductility or deformation capacity, a property that is necessary for the redistribution of forces between shear connectors that is an essential requirement of the

©2004 Taylor & Francis


Figure 1.7 Typical load–slip relationship

usual plastic method of designing the shear connection in composite beams. For
studs with h/d > 4 EC4 requires designers to use the lower of the values for stud
strength PRd of:
2

0.8 f u ( πd /4 )
P Rd = --------------------------------γv
2

(1.2)
1/2

0.29d ( f ck E cm )
P Rd = --------------------------------------------γv

(1.3)

These cover the two possible modes of failure: reaching the maximum load
when the concrete fails or shearing of the stud. Using γv = 1.25 and fu = 450 N/mm2,
the first equation will normally govern for concrete grades in excess of C30/37.
An explanation of the basis and significance of equations 1.2 and 1.3 may be
found in ref. 26.
In addition to resisting horizontal shear at the steel/concrete interface,
headed shear studs also prevent any tendency for the slab to lift off the steel
resulting from the deformations occurring in the system. EC4 covers this design
requirement generally by means of an uplift check using a nominal tensile force
of 10% of the design shear resistance of the connectors. Headed studs automatically satisfy this.
For forms of shear connector other than headed studs some design expressions are given in Section 6 of EC4. However, for proprietary components
e.g. mechanically fastened connectors, including the recently introduced
Perfobond arrangement, it is customary to refer to the design data supplied by

the manufacturers themselves. This will have been obtained from the results of

©2004 Taylor & Francis


Figure 1.8 Mechanically-fixed shear connector

push-off tests; Section 10.2 of EC4 covers the design of a suitable test
programme, the conduct of the tests and the assessment of the results so as to
derive characteristic strengths.
In situations where site welding of shear studs is unattractive, an alternative
is to use mechanically fastened shear connectors. One such product is illustrated
in Figure 1.8. The advantages associated with the use of shear connectors
attached using the shot-firing process as compared with welded studs have been
stated as (27):
1. the fixing process is not sensitive to protective coatings on either the
beams or profiled-steel sheeting, and is unaffected by moisture or
inclement weather,
2. the fastening can be performed by semi-skilled labour after a short
training period,
3. the fastening pins are easily checked visually,
4. the connectors are secured by powder-activated fastening which eliminates the need for an electricity supply,
5. and the whole system is compact.
Tests indicate that such components typically only develop around 40% of the
resistance of the standard 19 mm welded stud. They also tend to be more
expensive.
A further development is the use of a strip of connector material of the
form shown in Figure 1.9. First proposed in Germany (28), this has been given
the name “Perfobond”. Interaction is developed by concrete engaging with the
perforations, the strip being cut and attached by welding as required to the

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Figure 1.9 “Perfobond” shear connector

beam flange. Push off tests (28) have been used to devise a design equation for
the case where concrete strength (rather than yielding of the Perfobond strips)
governs:
PRd = 1.6ld2 fck/γv

(1.4)

Further studies (29, 30) have suggested that the influence of other variables
e.g. hole size and spacing, should be included in the strength formula. They
have also raised some concerns that with certain combinations of variables
the Perfobond arrangement may not deliver the level of ductility required by
EC 4 to permit shear connector design to be based on the use of a plastic
approach. A refinement of Perfobond replaces welding with the use of powder actuated fasteners (31), also modifying the profile by using an array of
slots. In a further development the same form of fastening has been used
with pieces of metal deck profile (31), a modification of the earlier idea
(32) illustrated in Figure 1.10. Further developments in the form of curved

Figure 1.10 Shear connection using sections of decking

©2004 Taylor & Francis


strips have also been proposed and a limited number of push-off tests
performed (33).

1.5.1 Influence of Slab Type
Equations 1.2 and 1.3 were devised from test data for solid slabs. Other
arrangements are, of course, possible. Of particular importance in building construction is the type of composite slab illustrated in Figure 1.11, in which the
concrete is cast directly on top of metal decking (more properly referred to as
profiled steel sheeting). This provides permanent formwork during the curing
operation and then acts as bottom reinforcement to the slab spanning transversely between the beams. Full details of its use are presented in Chapter 5
covering floor systems.
The presence of the sheeting means that the system of forces to which a
shear connector, attached by through deck welding to the beam’s top flange in
the trough region, is subjected differs from that experienced by a stud in a solid
slab. Figure 1.12 illustrates this. The most important feature is that the large
fraction of load carried by the weld collar is now very significantly reduced. As
a result stud strengths in composite slabs should not be obtained directly for
equations 1.2 and 1.3.
EC 4 addresses this through the use of a pair of reduction factors, k1 and kt
one each for slabs with ribs parallel to or perpendicular to the beam:

Figure 1.11 Use of metal decking

©2004 Taylor & Francis


Figure 1.12 Forces on shear studs

bo h
k 1 = 0.6 -----  ----- – 1 ≤ 1.0
hp  hp 
0.7
k t = ---------Nr

b
----ohp

h- 
 ---– 1 ≤ 1.0
 hp 

(1.5)

(1.6)

Most of the terms in equations 1.5 and 1.6 are defined in Figure 1.13; h should
be limited to hp + 75 mm, Nr is the number of studs in one rib where it crosses
the beam and must not exceed 2.

Figure 1.13 Definition of terms used in equations 1.5 and 1.6

©2004 Taylor & Francis


Figure 1.14 Effect of position of stud

The basis for these formulae comes largely from test programmes
conducted in North America (32). Their suitability for all design situations is of
some concern and in both the United States (33) and the U.K. (34) attempts are
being made to improve upon them. Since these involve the giving of explicit
recognition in the design expressions to a larger number of influencing factors,
the resulting equations are, inevitably, more complex. Particular concerns centre
around the practical point that in order to provide additional stiffness to the
decking when acting to support the wet concrete a longitudinal stiffener is often

formed in the trough. This prevents central positioning of the studs, thereby
creating the “strong” and “weak” orientations of Figure 1.14. Other influencing
factors include sheeting thickness, sheeting material strength and the use of
studs arranged in pairs. A point that would appear to have received little attention
is the popularity of the re-entrant deck profiles of Figure 1.15 for which very
little test data are available.
Another slab variant, based on removing the need to support the slab
whilst it hardens, is the use of precast concrete slabs (hollow core units or hcu)
with a small amount of in situ concrete placed over the beam’s flange as illustrated in Figure 1.16. For this arrangement studs may be welded off-site as part
of the steel fabrication process. Push off tests (36) specifically configured so as
to provide data on this arrangement have suggested that when PRd is governed
by stud strength (equation 1.2) no change is necessary and that when concrete
failure controls equation 1.3 should be modified to:
2

1⁄2

0.29αβεd ( ωf E ck )
P Rd = -----------------------------------------------------γv

(1.7)

In which α = 0.2 (h/d + 1), 1.0
fck = average concrete cylinder strength, taken as 0.8 × average cube strength
of the in situ and precast concrete
Eck = average value of the elastic modulus of the in situ and precast
concrete
β = 0.5 (g/70 + 1) < 1.00 is a factor to account for the gap width g and
g > 30 mm

©2004 Taylor & Francis


Figure 1.15 Re-entrant decking profiles

ε = 0.5 (φ/20 + 1) < 1.0 is a factor to account for the diameter of the transverse high tensile tie steel reinforcement and φ > 8 mm
ω = 0.5 (w/600 I) is a factor to account for transverse joints
W = width of hollow core unit.
Since equation 1.7 is based on the use of 125 mm × 19 mm TRW-Nelson
studs in a 150 mm deep slab, caution is necessary before extending its application
to other arrangements. A valid criticism of all existing methods for predicting
stud strength is their reliance on test data—whether directly from push-off tests
or, in some cases, by working back from actual beam tests. So far attempts to
devise a theory based on the application of mechanics to the topic have not
been successful, although recent application of F.E. techniques to model the

©2004 Taylor & Francis