Composite beams - Shear connection part 1

1. INTRODUCTION

This lecture describes 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 previous 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 the bending 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 be remembered, 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 Figure 1; 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 shear between 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 is 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 also possible, as described in the Behaviour of beams lecture, 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 flexible but 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 of the 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 movement can 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, which can 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 as strength 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 steel section, coupled with the deformed shape of the connectors, give 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 into the 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 connectors 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 shear stud connector has now become the primary method of connection for composite beams. The stud can be welded to the steel section in one operation using micro-chip controlled welding equipment. These machines allow operators to weld approximately 1000 studs per day. The most advanced machines allow studs to be welded through galvanised steel sheeting when applied on erection site. This ability has enabled the economic advantages of composite floor decks 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 detailing is made more complex. Depending on national regulations and recommendations, some limitations have been introduced to limit the risk of corrosion of the galvanised deck sheets after welding of the studs (e.g. for open multi-storey carparks).

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