Robust design of steel and concrete composite structures

Accidental events, such as impact loading, are rare events with a very low probability of occurrence but their effects often leads to very high human losses and economical consequences. An adequate design should not only reduce the risk for the life of the occupancy, but should also minimize the disastrous results and enable a quick rebuilding and reuse. A robust design prevents the complete collapse of the structure when only a limited part is damaged or destroyed. Design against disproportionate collapse is usually based on the residual strength or the alternate load path methods. Identification of an alternate path may lead to an effective and cost efficient design for progressive collapse mitigation by redistributing the loads within the structure. The continuity of the frame and of the floor represent essential factors contributing to a robust structural response. They in fact enable development of 3D membrane action. A European project focusing on robustness of steel and steel and concrete composite structures subjected to accidental loads is still ongoing. In the framework of the project the authors concentrated their studies on the redundancy of the structure through slab-beam floor systems as well as through ductile joint design. At this aim, two 3D full scale substructures were extracted from a reference building and experimentally investigated with the purpose to get an insight into the mechanisms allowing the activation of the alternate load paths resources, when a column collapse. The paper illustrates the main features of both the specimens tested and the experimental campaign. The preliminary results of the tests are presented and discussed