###### Abstract  
The fragility of concrete-filled steel tubular (CFST) frame structures with buckling-restrained braces (BRBs) subjected to multiple earthquakes is studied in this paper. First, a fiber beam element model with rate-dependent concrete and steel material properties is developed for CFST members and, then, the effect of the strain rate on the seismic response of the CFST frame structure is investigated numerically. The influence of BRBs on the seismic response of the CFST frame structure is then comparatively analyzed. The seismic responses of the CFST frame structure with BRBs under single mainshocks and earthquake sequences are investigated, and the fragility curves are generated using probabilistic seismic demand analysis. The obtained roof displacement and inter-story drift ratio (ISDR) of the structure decreased by 10.2% and 6.9%, respectively, when obtained while considering the strain rate effect, compared with those obtained without consideration of the strain rate effect. BRBs can effectively improve the seismic performance of the CFST frame structure in that the maximum roof displacement and ISDR can be reduced by 45.1% and 43.9%, respectively. Compared with those under single mainshocks, the maximum roof displacement and ISDR of the structure with BRBs under earthquake sequences significantly increase. The fragility of the BRB structure under earthquake sequences is more severe than that under single mainshocks. Therefore, the influences of the strain rate effect and earthquake sequence should be considered to realistically evaluate the seismic fragility of CFST structures. CFST frame structure; BRBs; seismic fragility; strain rate effect; earthquake sequences 2020_0  
Many studies have shown that both steel and concrete materials are rate dependent. As the loading rate increases, the strength and deformation performance of steel and concrete differ to a certain extent from those under static loading, and the mechanical properties of reinforced concrete (RC) members and structures differ under different loading rates [7; 8; 9; 10; 11]. [PERSON] et al. [12], [PERSON] and [PERSON] [13], and [PERSON] and [PERSON] [14] reviewed and summarized the research findings relevant to the concrete strain rate and consistently concluded that the tensile and compressive strengths and elastic modulus of concrete increase with a growing strain rate. [PERSON] and [PERSON] [15] along with [PERSON] and [PERSON] [16] experimentally studied the strain rate effect of reinforcing steel under cyclic loading and found that the yield strength of the reinforcing steel increases with an increasing loading rate. [PERSON] and [PERSON] [17], [PERSON] et al. [18], and [PERSON] and [PERSON] [19] carried out experimental tests to investigate the dynamic mechanical properties of RC beams, columns, and shear walls. The results showed that the loading rate affects the energy dissipation, stiffness, and strength of RC members to certain extents. Most existing studies have focused on the influence of the loading rate on the mechanical properties of RC materials and members, yet there are relatively few studies on the effect of strain rate on structure systems under seismic excitations. [PERSON] et al. [20] studied the impact of the strain rate on the seismic response of RC structures using pushover analysis and incremental dynamic analysis (IDA). It is noted that the calculation results based on static constitutive models are more conservative and it is more reasonable to consider the material strain rate effect. Using the dynamic constitutive model of RC materials, [PERSON] et al. [21] calculated the nonlinear seismic response of an eccentric RC frame-shear wall structure, which emphasizes the necessity of considering the strain rate effect in structural seismic response analysis. A vast amount of earthquake damage data has indicated that strong earthquakes are accompanied by multiple foreshocks and aftershocks that occur in the form of earthquake sequences.