Comparison of Seismic Behavior of Eccentrically Braced Frames with Vertical and Knee Links in Retrofitting the Reinforced-Concrete Buildings with Intermediate Moment-Resisting Frame Systems

Abstract

Given the fact that some reinforced-concrete (RC) buildings have not met resistance against seismic loads, it is mandatory to adopt a suitable and economically cost-efficient method to retrofit them. One appropriate method is application of steel Eccentrically Braced Frames (EBFs). This article is aimed at evaluation of seismic performance of retrofitted reinforce-concrete buildings by EBFs with single and knee links. For evaluation of modeled buildings, different editions of the Iranian Seismic Code (Standard No. 2800) were reviewed. To this end, three Moment-Resisting Frames (MRF) of four-, eight-, and twelve-story buildings with medium ductility were modeled where they were designed for seismic loads based on Standard 2800, 2nd edition. In order to evaluate buildings under modified seismic loads, models were seismically reloaded based on the Standard 2800, 3rd edition. Reanalysis showed that the stress ratios exceeded 1 in most columns. Therefore, buildings were retrofitted using EBFs with knee and single vertical links and their seismic performance were evaluated using nonlinear static analysis. Results indicate that knee bracing systems are more efficient than bracing systems with vertical link in increasing rigidity and controlling displacements, while they significantly reduce ductility. For example, application of knee bracing system in the twelve-story building can cause an 11% reduction in displacement in comparison to bracing systems with vertical links.