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ESDEP WG 10

COMPOSITE CONSTRUCTION

Lecture 10.6.1: Shear Connection 1

OBJECTIVE/SCOPE

To describe the types and behaviour of shear connectors and to explain the load transfer between the concrete

slab and steel section.

PREREQUISITES

Lecture 10.2: The Behaviour of Beams

Lecture 10.3: Single Span Beams

RELATED LECTURES

Lecture 10.4.1: Continuous Beams I

Lectures 10.5: Design for Serviceability

Lectures 10.6: Shear Connection II and III

SUMMARY

The forces in a typical composite beam are reviewed and their action on the connectors is described. Several

basic forms of connectors are introduced and thereafter the lecture concentrates mainly on the most commonlyused of these - the welded shear stud. Design values for ultimate strength and their experimental evaluation will

be covered together with the weld qualification tests used to ensure quality control. Full connection is explained

together with the use of reductions of connector strength by recourse to moment interaction diagrams. The use of

uniformly spaced connectors and the use of through-deck welding is discussed.

1. INTRODUCTION

This lecture deals with the way that forces are transferred between the concrete slab and steel section in

composite beams. The forces are discussed together with the various types of connectors commonly used. The

most common form of connector - the welded shear stud - is described in detail and methods for predicting stud

resistance and stiffness are compared. The resistance, stiffness and spacing of the connectors affects the

behaviour of the beam and these aspects are also discussed. The use of through-deck welded connectors in

composite deck floors is also covered together with alternatives such as shot fired connectors or preloaded high

strength bolts.

1.1 The Forces Applied to Connectors

In the preceding lectures it has been assumed that the concrete and steel were fully connected together (full

connection). If there is no connection then the concrete slab and steel section slide relative to one another and thebending stresses in the section are as shown in Figure 1a.

Clearly, if longitudinal shear resistance is provided by some form of connection, so that the stresses at the

interface of the two materials are coincident, then the beam acts as a fully composite section. If it is assumed that

the fully connected composite beam acts in an elastic way then the shear flow (shear force per unit length)

between the concrete slab and the steel section may be calculated from:

T = (1)

where

V is the applied vertical shear force at the point considered.

I is the second moment of area of the section.

S is the first moment of area of either the concrete slab or the steel section about the elastic neutral axis.

Figure 1 also shows the elastic shear stress developed in the section for the conditions of both full and zero

connection.

It can be seen, from the above equation, that the longitudinal shear forces that must be carried by the connection

will vary depending upon the vertical shear present. Figure 2a shows the distribution of longitudinal shear, along

the interface between the steel section and slab, for a beam that has a rigid full connection. It must beremembered, however, that this applies only when the beam is assumed to be behaving in an elastic manner. As

the ultimate moment of resistance is reached, the steel section or concrete slab will yield or crush and a plastic

hinge will form at the critical section. The bending stresses in the beam are as shown in the dashed lines of Figure1; the distribution of longitudinal shear in the beam also changes and the connectors close to the hinge are subject

to higher loads. The dashed line, in Figure 2a, shows the plastic distribution of shear force along a uniformly

loaded beam.

In practice, connectors are never fully rigid, and there is always some slip between the slab and the steel section.

The flexibility of the connectors allows more ductility and a variation in the distribution of longitudinal shearbetween slab and steel section. The longitudinal shear force present in a composite beam with flexible connection

is shown in Figure 2b.

At ultimate load, when the plastic hinge has formed, it is likely that the end connectors will have deformed to a

considerable extent and yet still be expected to carry a high longitudinal shear load. Hence the requirement that

shear connectors must have substantial ductility to perform adequately.

In determining the resistance of the beam, it is assumed that all the connectors, even when deformed, will be

capable of resisting a longitudinal shear force. It is this shear resistance of the connectors that determines the

resistance of the beam. If sufficient connectors are provided to withstand the longitudinal shear force generated

when the full plastic resistance of the beam is developed, the beam is said to be "fully connected". It is alsopossible, as described in Lecture 10.2, to reduce the amount of connection so that the moment resistance of the

beam is limited accordingly; this is a resistance criterion and the beam is termed "partially connected".

The slip that occurs as the connectors deform has a profound effect upon the stiffness of the beam. Very flexiblebut strong connectors may allow high bending resistance but because of the substantial slip there will be a loss of

stiffness. The stiffness of the connection, in relation to the stiffness of the steel section and concrete slab, is often

termed the interaction. Consequently, a beam where the connectors are infinitely stiff is said to have "full

interaction" and one where the connection is relatively flexible is said to have "partial interaction".

It may be deduced that the strength and stiffnesses of both connector and concrete will affect the connection.

The major force acting on the connector is one of direct shear. The shear force is generally assumed to be

greatest at the level of the weld between the steel section and the connector itself. The area and shear strength ofthe connector and weld must, therefore, be adequate to carry the forces generated. It is unlikely that any

substantial deformation will take place due to this shear.

However, relative movement between the slab and steel section does occur. The mechanism for this movementcan be seen in Figure 3. The concrete may crush at the connector base allowing some deformation of the

connector itself. However, at the head of the connector the confining concrete is not so highly stressed and this

part of the connector remains in its original position. The result is bending deformation in the connector, whichcan be seen clearly in Figure 3. Long connectors are more likely to deform into this characteristic "S" shaped

pattern and therefore tend to be ductile. Short stocky connectors tend to be brittle and are therefore undesirable.

Most codes of practice require stud connectors to be at least three or preferably four times longer than their

diameter.

The major force resisted by the concrete is one of bearing against the leading edge of the connector. It has

already been mentioned that the concrete in this region is likely to crush allowing bending deformation to occur in

the connector. The bearing resistance of the concrete in this region is dependent upon its volume as well asstrength and stiffness. In fact, where there is sufficient concrete around the connector, the bearing stress may

reach several times the unconfined crushing strength of the concrete.

There is also likely to be direct tension in the connector. The different bending stiffnesses of the slab and the steelsection, coupled with the deformed shape of the connectors, gives rise to the tendency for the slab to separate

from the steel section. It is, therefore, usual for connectors to be designed to resist this tensile force.

In most composite beams the connectors are spaced along the steel section and, therefore, provide a resistance

to longitudinal shear only locally to the top flange. The longitudinal shear force must, therefore, be transferred

from the narrow steel section into the much wider slab. This transfer is normally achieved using bar reinforcement

that runs transverse to the beam line. These bars are normally placed below the head of the stud and extend intothe slab, as shown in Figure 4.

To summarise, the connection must be capable of transferring direct shear at its base, resisting bending forces

and creating a tensile link into the concrete. The concrete must have sufficient volume around the connector and

be of sufficient strength to allow a high bearing stress to be resisted; bar reinforcement is often provided to

ensure adequate lateral distribution of longitudinal shear.

1.2 Basic Forms of Connection

Early forms of shear connector were shop welded, using conventional arc welding. Various forms of connector

welded in this way are shown in Figure 4. The most common types are the hoop connector and T connector

which serve to show the complexity of the forming and welding operation necessary. The popularity of

composite beam construction has led manufacturers to develop very simple forms of shear connector such as thePerfobond strip [1].

Despite the plethora of connection types available, the shear stud connector has now become the primary

method of connection for composite beams. The stud can be forge welded to the steel section in one operation,

using micro-chip controlled welding equipment. These machines, operating at current settings of up to 2000

amps allow operators to weld approximately 1000 studs per day. The most advanced machines allow studs to

be welded through galvanised steel sheeting. This ability has enabled the economic advantages of composite floordecks to be fully exploited. Figure 5 shows a typical shear stud before and after welding and the sequence of

weld current required.

Such complex welding technology does have disadvantages when used on construction sites. The weld relies on

the two surfaces being clean, free of mill-scale and, above all, dry. These conditions are often difficult to achieve

especially when the studs are welded through a galvanised steel sheet; in this case, the weld current is maintained

for a sufficient period to burn away the zinc galvanising, which would otherwise cause imperfect welds. Welding

22mm, rather than the more common 19mm studs, through deck also demands care. An alternative to through

deck welding is to punch holes in the steel deck and then weld the studs directly to the steel section. A more

reliable weld is obtained in this way but the construction process is made more complex.

2. DESIGN VALUES FOR SHEAR CONNECTORS

The load-carrying mechanism of stud shear connectors is complex and analytical methods for predicting the

shear resistance are not applicable. Instead the resistance of the connectors may be determined using empiricalformulae or from tests.

The empirical formulae relate to stud and concrete resistance; the design shear resistance is therefore given by

the smaller of:

stud shear resistance:

PRd = 0,8 fu (pd2/4) / gv (2)

where the shear resistance of the connector is related to the tensile strength of the steel fu, using a

factor of 0,8;

concrete resistance:

PRd = 0,29 a d2 Ö(fckEcm)/gv (3)

where:

d is the diameter of the shank of the stud (not greater than 22mm).

fu is the specified ultimate tensile strength of the stud material (not greater than 500N/mm2).

fck is the characteristic cylinder strength of concrete at the age considered.

Ecm is the mean value of the secant modulus of concrete.

and a is given by:

for 3 £ h/d £ 4; a = 0.2 [(h/d) + 1]

where h is the overall height of the stub,

for h/d > 4; d = 1.

The partial safety factor gv is normally taken as 1,25.

Instead of using the above formulae the designer may also obtain stud resistance values from tests. Full beam

tests are expensive and a model test known as the "push-out" test is often used. This test is shown in Figure 6,together with a typical graphs of load against slip from the test. The resistance is, of course, dependent upon the

concrete cube strength and is also reduced if the concrete is made from lightweight aggregate.

Shear studs carry very high loads and are normally made from drawn steel rod. Most codes quote steel

properties for stud shear connectors; in addition to a high yield value the studs must be ductile and a minimumelongation is often specified. The necessity for ductility has already been explained.

The weld itself must be very well formed to ensure that the connector can carry the design loads. Elaborate weldqualification tests have been devised in America [2], to ensure that both stud and welding equipment are capableof providing consistent weld quality. Site methods of testing the weld quality are less complex, and involve

bending or impact loading a certain percentage of the studs. Any studs breaking off should be replaced andfurther tests carried out.

3. APPLICATION OF DESIGN VALUES

In Lectures 10.2 and 10.3, it was established that, when the ultimate moment resistance of the beam is reached

at a critical section, the connection is required to carry a force equal to that acting in the concrete slab betweenthe critical section and the support. This force is the smaller of the axial resistance of the concrete slab or thesteel beam. If each connector can carry a force of PRd at failure, then the total force that all the connectors can

resist between the critical section and the support is:

Fc = N.PRd (4)

where

N equals the number of connectors between the critical section and the support.

Fc is the force in the concrete slab.

PRd is the design shear resistance of each connector.

The ultimate resistance of stud connectors is relatively consistent and the partial safety factor gv of 1,25 is often

considered more than just a material factor. The factor has been incorporated as a result of the work ofinvestigation of the possible results of connector failure [3]. Figure 7 shows the results of this work; in thisdiagram the moment resistance, expressed as a fraction of the fully composite resistance moment (as calculated

in Lecture 10.3) is plotted for beams with varying numbers of shear connectors. The number of shear connectorsmay be expressed as a fraction of the number of shear connectors required for complete interaction (r in the

figure); the value r is known as the degree of shear connection.

For very low degrees of shear connection the concrete contributes little to the resistance of the composite beam,

and the moment resistance is that of the steel section alone (A-B). Higher numbers of connectors allow the slabto contribute more to the resistance but failure occurs as a result of connectors failing in shear (B-C). When more

than approximately 50% of the connectors required for full connection are provided (r = 0,5), the beam may failin flexure with the concrete crushing, steel yielding and the connection deforming (C-D). It should be noted that

despite a flexural failure the resistance moment of the beam is less than the fully plastic resistance moment(denoted by the lower dashed line in Figure 7). The transition between shear failure and flexural failure willdepend upon the ductility of the connectors. Beams formed with very brittle connectors will fail in shear even at

very high degrees of connection.

In order to ensure that the full plastic resistance moment is achieved it is necessary to provide more connectors

than the theoretical calculated value would suggest. It could reasonably be assumed that the fully plasticresistance moment would be achieved when the connection ratio is equal to one. However, Figure 7 shows that

the connection ratio needs to be about 1,5 before this moment is approached.

If strain hardening is taken into account, the stress levels in the steel section increase, the composite beam can

take a higher moment, and the interaction curve will rise to that shown on Figure 7 as E-F-G. To ensure that thecomposite beam failed in a flexural mode and that the fully plastic resistance moment was achieved, it has beensuggested that the connection ratio should be 1,25 [3].

The graph in Figure 7 is non-dimensional and, therefore, does not depend upon the material properties of thesteel section, concrete slab or shear connectors. It can be concluded, therefore, that the 1,25 value is not a

material safety value, but a factor to take into account the reduction in flexural resistance that occurs as a result ofconnector flexibility.

4. SPACING OF SHEAR CONNECTORS

If the beam is assumed to behave elastically and is being designed so that elastic stresses are not exceeded in thesteel section, concrete slab or shear connection, it is preferable to space the connectors along the beam so that

the higher shear loads near the supports or concentrated load positions are resisted by a closer spacing ofconnectors. This spacing ensures that each connector carries an equal share of the longitudinal shear force (seeLecture 10.2).

However, it is now more common to design the beam at the ultimate limit state, using factored loads and a plasticsection analysis. In this case, as shown in Figure 2, the shear flow along the beam is more uniform and a uniform

spacing of connectors is more appropriate.

Most beams carrying uniform loads are designed with a uniform spacing of shear connectors. For this purpose

the connectors must be ductile and allow some straining associated with the relative movement between slab andsteel section.

A further reason for the use of the partial factor gv, when determining the design resistance of connectors, can be

seen when the uniform spacing of connectors is examined in relation to the applied moment diagram. Figure 8

shows the applied bending moment for a uniform loading and the moment of resistance generated by the beam.The major component of bending resistance is provided by the composite action and, as the shear connectors

are uniformly spaced, the resisting moment varies linearly from a maximum at mid span to a minimum at thesupport. The shaded portions of the applied bending moment diagram are, therefore, greater than the moment

resistance of the beam. If the connector resistance is assumed to be only 0,8 of its full value (1/gv), then the

actual resistance moment generated will be increased sufficiently to ensure safety.

The uniform spacing of connectors makes detailing much easier but care needs to be taken if heavy concentrated

loads are to be applied to the beam. In these cases connectors should be spaced broadly in accordance with theshear flow along the beam. A minimum spacing of 600mm for connectors is normally recommended to ensure

that the shear flow along the beam is not too irregular.

5. SHEAR CONNECTORS IN SLABS FORMED USINGPROFILED STEEL SHEETING

The use of profiled steel sheeting in composite slabs has revolutionised office construction, mainly due to the easewith which "Through Deck" shear studs can be welded. There are three possible causes for concern when usingthis form of construction:

1. The first is the quality of the weld. Modern welding equipment has been developed that can safely weldthe stud through galvanised steel sheets of thicknesses up to 1,5mm.

2. The presence of troughs in the sheeting, when running transverse to the main span, reduces the area ofconcrete effective in resisting compression forces, as shown in Figure 9. In this case it is necessary to

discount the area of concrete in the troughs when calculating section properties.3. The reduced volume of concrete around each shear connector means that the connector is less well

confined than when in a solid concrete slab. A reduced resistance and stiffness results. The reduction in

resistance is dependent upon the size and shape of the profiled sheeting. There has been some attempt toprovide design formulae quantifying the reduction in stud resistance for varying profiled steel sheet

geometries. These formulae are generally empirical and depend on the orientation and height of the deck,

and on the height and position of the stud connector. Push-out tests which incorporate the particulargeometry of profiled steel sheeting are a preferable way of determining the connector resistance in thesesituations.

6. ALTERNATIVE FORMS OF CONNECTION

Although welded shear studs are the major type of connector used, there are situations where alternatives are

more applicable. For example, alternatives may be preferable on construction sites where only a small number ofconnectors are required and the hiring of expensive generators cannot be economically justified.

An alternative in these situations, is the shot-fired connector shown in Figure 10. This connection can be fixed

using modified cartridge guns which do not require any power supply. The connection itself fails when the pinspull out and is, therefore, weaker than the welded shear stud. When used in pairs these connectors can provide

sufficient resistance for small beams and for beams designed with partial shear connection (see Lecture 10.6.2).

Another situation where welded connectors are inappropriate is the case where precast concrete slabs are used

compositely with the support beam. In this case high strength friction grip bolts (HSFG) have been used to fix theslabs to the steel flange, see Figure 10. HSFG bolts rather than simple bolts are needed so that, at theserviceability stage, no slip between the concrete slab and steel section can take place. Consequently, for

serviceability design, the slip resistance of the HSFG bolt should be used and for the ultimate moment design, thebearing or shear resistance of the bolt is used.

7. CONCLUDING SUMMARY

The shear connectors between the slab and steel section in a composite beam are required to resist shear,bending and tensile forces. Transverse bar reinforcement is often required to ensure adequate transfer of

shear into the slab.The stiffness, resistance and amount of confining concrete around each connector affects the performance

of the connection.Welded shear studs are the most common form of connector.Connector resistances may be determined using empirical formulae or "push-out" tests.

The design resistance of a connector is normally assumed to be 0,8 times the ultimate resistance.Studs may normally be uniformly spaced along the beam unless high local shear loads exist.

Slabs formed with profiled steel sheeting may be used with composite beams as long as the concrete in thetroughs is ignored when calculating section properties and the possibility of reduced connector resistance

is taken into account.Alternative forms of connection include the shot-fired connector and HSFG bolts.

8. REFERENCES

[1] Andra, H.P., "Economical Shear Connectors with High Fatigue Strength", Proc. IABSE Symposium onMixed Structures, Brussels, 1990.

[2] ANSI/AWS, "American Welding Society Structural Welding Code Steel", 1985, (Appendix K) Doc. D1 1-

85.

[3] Yam, L.C.P. and Chapman, J.C., "The Inelastic Behaviour of Simply-supported Composite Beams of Steel

and Concrete", Proc. I.C.E., 41, 651-684, Dec 1968.

9. ADDITIONAL READING

1. Eurocode 4: "Design of Composite Steel and Concrete Structures": ENV1994-1-1: Part 1.1: General rule

and rules for buildings, CEN (in press).

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