ABSTRACT
This chapter explains the theory of how to design unpropped beams safely. Composite beams are vulnerable to lateral torsional buckling when supporting wet concrete. During ultimate limit state design, it is necessary to check the strength and stability under wet concrete, as well as when loaded by the fully factored dead and imposed loads. Beams with the profiled metal decking spanning along the direction of span are laterally unrestrained when supporting wet concrete. When a composite beam flexes, the slab acts like a compression flange. In order to determine bending strength, it is necessary to establish the width of slab that contributes to the flexural strength. Composite beams can be either propped or unpropped during casting of the slab. If unpropped, then wet concrete is supported by the steel section alone. Contractors prefer this, because propping costs money; however, unpropped construction is more complicated to design. A lack of understanding by engineers has occasionally led to beams being overstressed.
