Chapter 10 - Composite Beams
© 2006, 2007, 2008 T. Bartlett Quimby
Introduction To Composite Beams
Last Revised: 11/04/2014
In building and bridge structures, steel beams are frequently used to support concrete slabs. If there is an adequate connection to transfer longitudinal shear between the slab and the beam, then the two elements act together as a composite beam. The composite beam has increased section properties, making the composite beam stiffer than the steel beam by itself.
The elements of a basic composite beam are shown in Figures 10.1.1 and 10.1.2. The system consists of a steel beam, a concrete slab of some width and some kind of connector that transfers longitudinal shear between the beam and the slab.
The combination of a steel beam and a connected concrete deck creates a member that is much stronger and than just the steel beam by itself. This is particularly practical to do since it is common to use steel beams to support concrete decks. Simply adding a connection between the two allows for much smaller steel beams in many cases.
The key to this added strength and stiffness is the increase moment of inertia of the composite cross section.
Note, however, this advantage is only true when the concrete is subject to compressive forces. Concrete is a brittle material whose tensile strength is quite variable and fractures (or cracks) with little or no warning. As a result, concrete is assumed to have no tensile strength and is ignored when it is in tension. On the other hand, concrete is very good in compression. This means that composite action is only of benefit in POSITIVE moment regions where concrete is on the compression side of the beam. In negative moment regions, where the concrete is on the tension side of beam, the concrete adds nothing to the strength or stiffness of the steel beam.
Since composite action is of most benefit to simply supported, single span beams where all the moment on the section is positive. For continuous beams, the maximum moments tend to be negative and located over supports. Beams sized for these negative moments will tend to more than adequate to handle the positive moments so composite action is not required.
Making use of composite behavior can significantly reduce the required size of a steel member, thus saving the extra cost associated with connecting the slab to the beam.
This chapter looks at the behavior of composite beams and the SCM Chapter I3 limit states of shear, flexure, and deflection as they apply to composite beams.