Composite gravity beams subject to slab lateral movement

https://doi.org/10.1016/j.jcsr.2020.106162Get rights and content

Highlights

  • •

    Composite beam behaviour under gravity and lateral force is investigated theoretically and numerically.

  • •

    A method is proposed to design composite beams under a combination of gravity and lateral forces.

  • •

    Lateral forces may reduce or eliminate composite action in composite beams.

  • •

    Providing a minimum 25% composite action may not be adequate to prevent shear stud failure.

  • •

    Gravity and lateral force concurrent action should be considered in composite beam design.

Abstract

The behaviour of shear studs and slabs on thirty-three composite beams with different details and differing gravity loading are investigated subject to slab lateral (in-plane) forces. The finite element analysis showed that shear studs, connecting the concrete slabs to the steel beams, yielded under a combination of gravity loading, and lateral forces. This yielding led to a decrease in the degree of composite action and an increase in beam vertical deflection. The presence of initial shear stud stress due to gravity load also caused the lateral force resistance of a shear stud group within a bay to be less than the sum of all shear stud strengths, because some shear studs fractured before others reached their capacity. A method to estimate shear stud group lateral force resistance considering this effect of gravity loading was developed. This method was found to be conservative, estimating a shear stud group lateral strength of 0.8–1.0 times that found from FEM analyses. Due to the possible reduction of composite action under lateral forces, it is recommended that the bare steel beam be designed to carry the gravity un-factored dead and live loads. Furthermore, to limit the loss of composite action, it is recommended that the shear stud group demand from lateral forces alone be less than 50% of the shear stud lateral force capacity obtained using the developed method.

Introduction

Composite beams typically consist of a concrete slab, supported by, and connected to a steel beam. Composite action developed between the slab and beam through mechanical attachments (called shear connectors) increases the beam flexural stiffness and strength. Therefore, smaller steel beam section sizes compared to the non-composite situation are possible. Composite beams are usually primarily designed to carry gravity loads. Stresses in the steel, concrete and shear studs, due to these gravity loads are different for propped and unpropped construction.

During earthquake events, in-plane diaphragm forces due to inertia, transfer, compatibility, slab bearing and interaction with other structural elements need to be transferred to the structural frames [1]. In general, force transfer directly from the diaphragm into the column should not be used as it is not a reliable load path. This is because if a gap is provided around the column during construction, the diaphragm forces need to be transferred through friction and mechanical attachments (e.g. shear studs) into the beam, and then from the beam into the structure vertical lateral force resistance (VLFR) system. Even if there is no construction gap around the column, slab horizontal inertia forces in the direction of the column cannot transfer directly into the column through slab bearing. This is because as the frame sways, due to the beam-column joint rotation, the column moves away from the slab that wants to bear on it, creating a gap [13].

As lateral forces need to be transferred from the slab to the beam through shear studs, there are concerns that: (i) the shear stud group lateral force resistance may be reduced due to the presence of gravity loads as the displacement (slip) capacity of some shear studs is exceeded, (ii) the composite action effect controlling beam gravity strength and stiffness may be reduced due to shear stud yielding due to horizontal forces, and (iii) the transfer behaviour may depend on whether the composite beam construction is propped or unpropped.

Previous rigorous studies do not seem to have been performed regarding this issue in the past, and there are contrary design recommendations. Some recommendations are that the design of the shear studs for lateral force does not depend on the gravity loading, while other recommendations indicate that both the lateral and gravity demands should be summed together when determining the number of shear studs.

For designers to have confidence in the performance of composite steel-concrete beams which carry gravity loads as well as lateral seismic forces, and at the same time design an economical structure, there is a need to understand the shear stud behaviour and composite beams.

This study addresses this need for composite beams subjected to lateral and gravity forces by answering the following questions:

  • 1)

    What parameters influence the likely lateral strength of a group of shear studs on a simply supported beam subject to gravity and lateral loading?

  • 2)

    How does shear stud yielding affect the beam composite action?

  • 3)

    For uniformly distributed shear studs, can the peak lateral strength be estimated?

  • 4)

    Are there some simple recommendations for the design of composite beams that will limit the possibility of beam failure during design level shaking, and for keeping the building serviceable after a significant event?

Section snippets

Literature

Very little has been written about the lateral force resistance of shear stud groups on a composite beam. Mullett [14] advised, without stating any research, that he considered that it is not normally necessary to provide additional shear connectors to carry diaphragm in-plane forces except close to points of local transfer of shear forces such as adjacent to VLFR systems. Burmeister and William [7] also advised that for beams designed as composite members to carry gravity loads, additional

Shear stud demands

In addition to slab-beam interface slip caused by in-plane diaphragm forces due to (i) inertia, (ii) slab bearing, (iii) transfer effects, (iv) compatibility, and (v) inclined support forces, as described by Alizadeh et al. [1], beam local deformations may cause additional effects. Local issues resulting from beam deformations are due to (i) gravity loading which causes stresses in the shear studs as shown in Fig. 1a, (ii) horizontal slab forces acting at a location higher than the neutral axis

Concept testing

The considerations above are stated as a hypothesis. This is tested below using thirty-three FE models including non-composite, partially-composite and fully-composite beams with different span lengths and section heights.

Behaviour evaluation

  • i)

    Shear stud failure

Fig. 13 shows the lateral force-slip behaviour of models with low (10%) composite action with and without gravity load. It may be seen that:

  • a)

    in the absence of gravity load, W, the total lateral force resistance of shear studs was equal to the sum of all shear stud strengths. Increasing W tended to decrease the ultimate lateral force resistance and stiffness. This is because higher W caused larger shear stud slip, Smax, as described in Section 3.3.

  • b)

    for the weaker 180UB beam

Limiting beam vertical deflections in low damage structures

When the objective of structural design is low-damage construction, structural components should be designed to avoid major damage during the expected earthquake shaking level. A simple way of achieving this may be by controlling the beam vertical deflection as a result of composite action loss. Numerical models are used to determine the lateral force resistance corresponding to an arbitrary 10% increase of vertical deflection, PΔ10%, for all models studied. The value of PΔ10% divided by the

Conclusions

The behaviour of composite beams subjected to gravity and lateral forces was considered. It was found that:

  • 1)

    Composite beam local deformations due to gravity loading, horizontal slab load being offset from the neutral axis, and beam end rotations may affect composite beam interface sliding and shear stud stresses. Only gravity loading effects are significant for typical gravity beams. The ratio of maximum total lateral force resistance of a group of shear studs spread uniformly over a beam of a

References (15)

There are more references available in the full text version of this article.

Cited by (0)

View full text
© 2020 Elsevier Ltd. All rights reserved.