Hybrid braced frame with buckling-restrained and strong braces to mitigate soft story

The steel concentric braced frames (CBFs) are the common structural system for buildings. Unlike high strength and stiffness, brace buckling in CBFs results in high nonlinear behavior, which causes damage concentration in one or a few stories during a severe earthquake. This paper proposes a hybrid CBF bracing system to reduce soft-story mechanisms and improve the structural behavior of the CBF system. The proposed system is composed of a buckling restrained brace (BRB) or an energy dissipation device anda strong brace (SB) in one span. The strong braces are designed to have an elastic performance during an earthquake. The direction of BRB and SB are changed, inversely, story by story in a zigzag form. This configuration leads to paradoxical results: stiffness and ductility. This improves both the soft story mechanisms and the buckling capacity of CBFs. In this study, a set of 6-story structural models with different bracing patterns including inverted-V, multi-X, zipper, hexa (hex-agonal bracing configuration), and strongback braced frames were evaluated through nonlinear static (push -over) and dynamic analyses (IDA). The collapse performance of structural models was assessed by comparing the seismic behavior of conventional CBFs with similar bracing configuration based on FEMA P695 criteria. From the evaluation of collapse fragility curves, the hybrid bracing system had more safety margin against col-lapse during severe earthquakes than conventional CBFs. (c) 2021 Elsevier Ltd. All rights reserved.

Automated summary

This paper introduces a hybrid CBF bracing system that can reduce soft-story mechanisms and improve the structural behavior of the CBF system. Evaluation of a series of structural models shows that the hybrid bracing system has higher collapse safety margin than conventional CBFs during severe earthquakes.