| At present, the concentrically braced steel frame (CBF) is a commonly used structure system in multi-story and high-rise steel structures, and a lot of research about its aseismic performance has been conducted in China and abroad. However, there are still some key problems worth further studying due to its complex seismic response under severe earthquakes. In this paper, the low-cycle fatigue behavior of the welded I-section steel bracing members, the seismic design method and aseismic performance of the CBF are analyzed, and some useful conclusions are also suggested.Based on the low-cycle fatigue behavior tests of the welded I-section ST12 steel bracing members with pinned connections under constant amplitude reversed cyclic axial loading, the low-cycle fatigue behavior of the welded I-section Q235 steel bracing members with pinned connections was examined. The test results about the maximum lateral deformation and the maximum compressive strength of the forty-seven bracing members made of Q235 and ST12 were studied, and the reliability of the formula obtained in this paper and some codes for calculating these deformation and strength is verified. According to the assumption that the geometry properties, loading amplitude and steel yield strength are independent for estimating the low-cycle fatigue life, some empirical formulas were presented. It is found that the low-cycle fatigue life increases with the decrease in the width-thickness ratio of the flange, the increase in the brace slenderness ratio, and the decrease in the loading amplitude and yield strength of the bracing members. Because the displacement histories of the bracing members can not be predicted before a real earthquake, in order to avoid early fractures of the bracing members, besides limiting the width-thickness ratio of the flange, low yield strength steel should be adopted and the brace slenderness ratio limitation in some current codes should be enlarged. Finally, based on the energy index ratios for different damage stages, the suggestion that the low-cycle fatigue life of the bracing members can be safely predicted by the empirical formulas for stage I has been put forward.Based on the results mentioned above, several problems about the seismic design of the concentrically braces in China and abroad were analyzed and discussed, including the post-buckling resistance, limit values of slenderness and width-thickness ratios for I-section bracing members, and therefore some concrete suggestions were also put forward.A non-linear numerical analysis was carried out by the ABAQUS program to simulate the hysteretic behavior of the nineteen welded I-section Q235 steel bracing members, and in general the analysis results are in good agreement with the test ones. Numerical simulation of the low-cycle fatigue damage and crack initiation lives for the bracing members was also conducted by the FE-SAFE program based on the critical plane damage models, including the principal strain, maximum shear strain and Brown-Miller combined strain damage models. The results show that, the residual stress has no effect on the low-cycle fatigue lives of the bracing members, and the low-cycle fatigue lives predicted by the three damage models safely fall within a scatter band of four.Regarded a critical one-bay braced frame of the coal storage room selected from the main plant structure of a large thermal power plant as prototype structure, which plans to be built in the zone with the seismic design intensity of 8 degree, a 1:12 scale CBF model with pinned beam-to-column connections was made and tested on the shaking table under the action of twenty-four different earthquake ground motions. And the numerical analysis of the test model was also performed by the ABAQUS program. Dynamic properties, as well as responses of acceleration and displacement under different earthquakes, for the test and numerical models were studied. The results show that, before and after intensity 8 frequent earthquakes, the dynamic properties of the test model didn't change; after intensity 8 rare earthquakes, only a few braces and gusset plates appeared small out-of-plane deformations. Based on the test results and analysis results of the prototype structure, the computative maximum story drifts of the prototype structure under intensity 8 frequent and rare earthquakes are all less than their limit values pursuant to the GB50011 code. On the whole, the structural system of CBF with pinned beam-to-column connections has a good aseismic performance in the severe seismic region.Seismic response analysis of the four- and ten-story CBFs, located in the zone with the seismic design intensity of 8 degree, with pinned beam-to-column connections was carried out by the ABAQUS program, and the effect of the seismic design methods for the members (brace, braced beam and braced column) and the design seismic force on the aseismic performance of the structures was investigated. The results reveal that, based on some supplemental designs about the braced beams and braced columns, the dynamic responses of the structures designed by the CECS160:2004 code are similar to those designed by the GB50011 code, but the amount of steel used for the former is smaller than that for the latter, indicating that the comprehensive aseismic performance of the structures designed by the CECS160:2004 code is better and the GB50011 code overestimates the seismic force of the CBF with pinned beam-to-column connections. Finally, the suggestion for the further research to enlarge the application range of the CBF with pinned beam-to-column connections has been put forward.Seismic response analysis of the CBFs, located in the zone with the seismic design intensity of 8 degree, with rigid beam-to-column connections was also carried out by the ABAQUS program, and the effect of the seismic strengthened design methods for the moment frame on the aseismic performance of the dual system was mainly investigated. The results show that, if no brace occurs low-cycle fatigue fracture during the rare earthquakes, the aseismic performance of the dual system in which any story of the moment frame can independently resist 25 percent of the base or story shear force isn't better than that of the single system. However, based on the assumption that the brace fractures early during an artificial earthquake, the aseismic performance of the dual system in which any story of the moment frame can independently resist 25 percent of the base shear force is evidently better than that of the others. Therefore, it is suggested that the seismic design for the dual system should be performed to ensure that any story of the moment frame can independently resist 25 percent of the base shear force.The results of the shaking table tests and the seismic response analysis for CBFs also indicate that, because the aseismic characteristics for different structural system and materials can not be considered by the GB50011 code, the seismic force of the CBF with a seismic category C is overestimated by the GB50011 code.The author wishes to acknowledge the financial support of this work from National Science & Technology Pillar Program with contract number 2006BAJ01B02-01-04 and Cooperation Project of School and Enterprise-"Research and Application on the seismic design method of main plant structure of a large thermal power plant".
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