Quantifying the seismic performance factors of half-elliptic-braced steel moment frames (HEB-MFs)

Abstract

Chevron or inverted V-braced frames (IVBFs), despite their high stiffness and strength, exhibit weak post-buckling behavior. In these systems, the buckling of the brace’s compressive member in a particular story causes an unbalanced vertical force at the mid-span of the beam, leading to the concentration of damage in that story and, ultimately, the collapse of the structure. This paper aims to propose and investigate a novel bracing system called the half-elliptic-braced steel moment frame (HEB-MF) to overcome the drawbacks of conventional IVBFs. This system is composed of two quarter-elliptic bracing members. The elliptical geometry of these bracing members not only mitigates the unbalanced force and deflection at the beam’s mid-span but also enhances energy absorption. At the outset, the hysteric performance of this bracing system is evaluated in comparison with the IVBF system. Afterward, considering eight three-dimensional archetypes with varying numbers of stories in two distinct seismic design categories (SDC Cmax and Cmin), the seismic performance factors (SPFs) of this system are determined based on the FEMA P695 procedure, and the fragility curve is plotted for each archetype. The results reveal that employing this system not only effectively reduces the unbalanced force and beam deflection but also enhances ductility and energy absorption compared to the IVBF systems. Furthermore, this system’s response modification, over-strength, and deflection amplification factors have been obtained as 7.5, 3, and 7.5, respectively.