Catenary Action in Steel Framed Structures with Autoclaved Aerated Concrete Infill & Buckling Restrained Braces

Abstract

The progressive collapse of structures is considered a major concern for designers since the Ronan Point incident. Preventing and mitigating progressive collapse has become a serious issue for all types of buildings. Progressive collapse is disproportionate failure of the structure due to the failure of a relatively small part of it. This may result in either damage to a large portion or collapse of the whole structure. Collapse of portions of a structure generates lateral force demand in the lateral frames. Catenary action is a load mechanism that is developed to resist the additional loads as a result of sudden column loss and prevent disproportionate collapse. The beams above the removed column will resist the loads by flexural action until plastic hinge formation. Subsequently, the beam resists the loads by catenary action. The developed catenary (axial) forces in steel beams play a big role in the resistance of progressive collapse along with the adjacent lateral load resisting system. OpenSees (Open System for Earthquake Engineering Simulation), a software developed by the Pacific Earthquake Engineering Research (PEER) Center of the University of California at Berkeley, was used to analyze the models. Autoclaved aerated concrete (AAC) infilled walls have shown superior resistance to axial loads compared to other infill materials. Previous research has focused on the impact of AAC on the seismic behavior of steel frames when subjected to lateral loads. Infilled steel frames have shown remarkable performance in resisting earthquake loads. The impact of AAC infill on the performance of steel frames subjected to lateral seismic forces suggests that AAC infill might affect steel frames that are subjected to progressive collapse loads as well. Detailed study on the impact of AAC infill on the catenary action demands in steel framed structures was conducted. Results showed that AAC infilled frames had a higher load carrying capacity compared to bare steel frames. Buckling restrained braces (BRBs) have been widely adopted as a dependable lateral load mechanism since the early 1990s. BRBs are particularly popular in highly seismic regions due to their superior performance in resisting earthquake loads compared to regular braced frames. The impact of BRBs on the performance of steel frames subjected to lateral seismic forces suggests that BRBs might also be beneficial to steel frames that are subjected to progressive collapse loads. A study on the impact of BRBs on the catenary action demands in steel framed structures was carried out. Results showed that buckling restrained braced frames can increase the resulting axial forces in steel beams. The contribution of the concrete slab to catenary action development was also investigated. The results indicate that accounting for the concrete slab significantly increases the load-carrying capacity of the steel frame at earlier stages of deflection. A change in the concrete slab thickness affects the contribution of the slab in the axial force development in the beams. The results of this dissertation will help designers and code developers around the world understand the impact of AAC infill, BRBs and concrete slabs on the catenary action demands in steel structures.