Beam and slab floor systems— composite design by yield line theory

Abstract

This paper describes how beam and slab floor systems may be designed using yield line theory. For given geometry and loading, the slab design is carried out so as first to prevent a “slab alone” failure, beams are then selected so that the desired load factor is provided against a combined beam and slab failure mechanism. This is accomplished by considering three basic modes of failure, A, B and C or D in such a way that collapse would occur by any of these basic mechanisms at the same applied load, thus producing an economical design. In designing the floor system any one of five different degrees of composite action may be considered between the beams and slab. These different degrees of composite action involve such parameters as the inclusion or not of composite action in hogging and sagging bending, and the effect of slab reinforcement on composite fully plastic moments. Experimental results are presented to show that the design method satisfactorily predicts the mode of failure and the load at which it occurs, provided that the correct degree of composite action is taken. A number of design examples were considered and the effects of degree of composite action, concrete strength, slab thickness, sides ratio etc. on structure weight are determined, thus enabling general recommendations to be made on efficient design of floor systems.