Progressive Collapse Assessment

Abstract

Progressive collapse is a collapse where local failure leads to a disproportionate collapse. Due to a focus on ease of erection in the construction process and more and more optimisation of design through advanced analysis techniques buildings are believed to have become more vulnerable to loads outside the design envelope over the past decades and are thus more vulnerable to progressive collapse. When a building is subjected to local failure the load resisting behaviour is quite different than the behaviour considered in conventional linear elastic design. To design a building resistant to progressive collapse in a cost efficient and aesthetically attractive way consideration of these non-linear effects is required. The purpose of this research is to investigate structural non-linear behaviour of building structures and develop design rules or strategies to economically design building structures resistant to progressive collapse. Focus is on RC structures in static loading conditions. In order to do this first the progressive collapse phenomenon itself was considered. Three design approaches were distinguished: the event control approach aimed at improving the level of protection of a building, the specific local resistance approach aimed at increasing the hardness of a building and the alternate load path approach which aims at improving the robustness of building. The latter approach has been elaborated. Alternate load paths can be developed roughly in four manners. By arch action, suspension action, Vierendeel action and catenary action. Ductility of the structure and its connections is important to enable these alternate load paths. For arch and catenary action special detailing of structural ties is needed, especially catenary action depends highly on the elongation capacity of these ties. When Vierendeel action occurs, redistribution of internal moments is an important effect. This effect relies highly on the ductility of the RC structure. To assess the effect of non-linear effects numerical determination of limit loads was performed for a case study. Three non-linear effects creating an overcapacity compared to linear assessment were distinguished in the case study: strain hardening of the reinforcement steel, moment redistribution and occurrence of torsional moments in hollow core slabs.