Evaluation and Enhancement of Robustness for Reinforced Concrete Buildings

Abstract

Since the events of September 11, 2001, the engineering community has devoted increased attention to evaluating the vulnerability of multistory buildings to disproportionate collapse, including substantial research and standards development. This paper reviews recent research conducted by the NIST on the robustness of RC buildings, including experimental validation of both high-fidelity and reduced-order modeling approaches using measurements from subsystems and an entire building tested under column removal scenarios. A robustness index was developed using pushdown analysis of a structure with initial damage to allow an evaluation and comparison of the disproportionate collapse potential of various structural systems using the validated modeling approaches. It is shown that the robustness index is sensitive to differences in the strength and detailing of the structural system, providing higher values for buildings with more stringent design requirements. Further, the paper presents a novel approach for enhancing the robustness of RC beams through local debonding of tensile reinforcing bars at the joints, where significant cracking occurs after a column loss. Experimental and computational studies were conducted to demonstrate the efficacy of this approach. It is shown that debonding of the bottom bars for a length of two beam depths on each side of a central column results in an improvement of more than 30% in the peak vertical load-carrying capacity under a column removal scenario.