Accounting for biaxial bending in yield-line analysis of reinforced concrete slabs

Abstract

Yield-line analysis is conventionally undertaken using collapse mechanisms that comprise rigid regions that intersect at yield lines, which behave in uniaxial bending only. It is well established that the flexural capacity of reinforced concrete (RC) slabs subjected to biaxial bending can be less than that when they are subjected to uniaxial bending. Concerns must therefore result regarding whether the inability of conventional yield-line mechanisms to model regions of biaxial bending could lead to significantly unsafe (upper bound) results. A simple method is presented to enable zones of biaxial bending to be taken into account in yield-line analysis. In this method ‘wide’ yield-line mechanisms are used which comprise rigid regions that are connected by ‘yield lines’ of finite and constant width and with uniform finite curvature over their area. Two examples of the method are presented. From the results, it appears unlikely that the reduced flexural capacity of RC slabs in biaxial bending will have a highly significant effect on their overall capacity in many cases. Perhaps of greater significance is the variation in ductility observed for different bending cases.