Strut-Relieved Single Steel Haunch Bracing System for Mitigating Seismic Damage of Gravity Load Designed Structures

Abstract

A large number of existing reinforced concrete structures, constructed before the 197s, are poorly designed and inadequately detailed. The design and detailing of these structures are based on the prevailing guidelines and practices to cater for the gravity load demand. Such structures are extremely vulnerable during seismic events. An efficient, easy to implement, and cost-effective intervention to those structures is the immediate requirement to mitigate the damage due to possible seismic events. The present study deals with the development of an innovative single metallic bracing system fixing to the bottom of the beam-column subassemblage. This is made to protect the week joint region of gravity load designed (GLD) structures by allowing the force path to bypass from beam to column. A steel haunch system was developed and implemented in beam-column subassemblages representing the GLD scenario. Analytical formulations were developed for evaluating the force flow mechanisms in rigid and flexible systems. Experimental studies on upgraded beam-column subassemblages were carried out to check their performance under reverse cyclic loading. The key behavioral parameters such as hysteresis behavior, energy dissipation, and stiffness degradation were evaluated. It was found that the single steel haunch system was very effective in improving the performance of the subassemblages. The steel haunch system was further improved by introducing a strut relieving mechanism, which showed encouraging performance. At drift corresponding to the 2% reduction in the strength, it was found that the single haunch and strut relieving bracing systems were capable of dissipating almost 46% and 6%, respectively, more energy than that of the GLD specimen. The study points out the feasibility of seismic damage mitigation of poorly designed building stock using the innovative steel haunch system.