The low-cycle fatigue tests of welded I-section bracing members

The low-cycle fatigue behavior and energy dissipation capacity of the welded I-section bracing members with different geometry properties are investigated by 21 pin-ended bracing tests under axial harmonic cyclic loading with constant amplitudes of 6 times the tension yield deformation of specimens. It is found that the fatigue damage propagating to fracture in the flanges of the bracing members can be divided into 3 stages, the macroscopic surface crack initiation, the penetrated crack formation and the penetrated crack propagation. Some empirical formulas to estimate the fatigue life and cyclic energy dissipation capacity of the bracing members are also presented based on the experimental data. The statistical analysis indicates that the fatigue life to surface crack generation significantly depends on the inelastic local buckling and will increase with decreasing width-thickness ratios of the flanges and increasing slenderness ratios of the bracing members. The test results show that the bracing members with better low-cycle fatigue resistance also have better energy dissipation capacities. In order to avoid early fractures of the bracing members in concentrically braced frames during a severe earthquake, besides limiting the width-thickness ratios of the plates, the slenderness ratios of the bracing members limited by the current codes should be relaxed in the practical design.