A Procedure for Assessing Low-Cycle Fatigue Life of Buckling-Restrained Braces

Abstract

Buckling-restrained brace frames (BRBFs) have been a popular seismic force-resisting system in the US since it was first introduced to the AISC Seismic Provisions and ASCE 7 in 2005. The steel core of a buckling-restrained brace (BRB) yields in tension and buckles plastically in a high-mode buckling pattern in compression under cyclic loading. Prequalification testing of BRBs in conformance with the AISC 341 requirements relies on a highly idealized loading protocol that is not representative of the expected response during an earthquake. Although testing has demonstrated that BRBs have excellent energy dissipation capacity, a low-cycle fatigue model will permit designers to evaluate the remaining life of BRBs after a seismic event. To develop and validate such model, 18 full-scale BRBs in groups of three different yielding strengths (1,110; 2,220; and 3,330 kN) were cyclically tested to fracture with the following loading conditions: (1) symmetric cycles with constant strain amplitudes (±0.25% to ±3%), (2) asymmetric cycles with constant strain amplitudes and non-zero-mean strains, (3) symmetric cycles with variable strain amplitudes, and (4) simulated earthquake responses. Two low-cycle fatigue models for BRBs (a standard model and an alternative model for ease of practical use) were developed based on 11 symmetric, constant-amplitude tests. An assessment procedure combining the proposed fatigue models, rainflow cycle counting method, and Miner’s damage index was proposed to predict the fatigue life of BRBs subjected to random cyclic loading histories, the accuracy of which was verified by the variable-amplitude test results.