Seismic Performance Evaluation Of Steel Concentrically Braced Frames Based On Reserve Capacity

Steel concentrically braced frames(CBFs)are used extensively in buildings as a lateral resistance system around the world due to their high stiffness and strength.The poor ductility and energy dissipation capacity of CBFs have been clearly demonstrated in past researches due to brace buckling and failure under seismic loads.The frame of structure as the reserve system provide reserve stiffness and reserve strength after brace fracture.The reserve system has a great influence of seismic behavior of CBFs.CBFs in moderate seismic regions of the United States are typically designed using a response modification coefficient,R,equal to 3.which allows for seismic detailing to be ignored by ASCE 7-10.Therefore,it is also called low-ductility CBF such as its brace has large width-to-thickness ratio and slenderness ratio.Low-ductility CBFs are widely used in MSRs of the United States due to the designing convenient and economical.CBFs in China are not classified by ductility.The CBFs are designed more conservative which lead to high cost of steel structures and restricted the development of steel structures in China.It is critical to provide a kind of economical CBFs with enough seismic performance in our country.To solve this problem,this paper focused on the seismic performance of low-ductility CBFs,and a series of theoretical research and experiment study are performed.The detail research constant are shown as following:(1)First,the seismic performance of Single degree-of-freedom(SDOF)system CBF was studied.The parameters of reserve system were introduced and the ductility demand spectra was presented.The influence of parameters for ductility demand was evaluated.Afterwards,the R-?-T model of SDOF low-ductility CBF system was constructed.The veracity of the presented model was verified by comparing the model with the mean ductility demand spectra established by nonlinear time history analysis.(2)The collapse prevention performance of SDOF low-ductility CBF was investigated based on incremental dynamic analysis(IDA).Reserve strength and reserve stiffness are considered in the CBF model during IDA.The dynamic instability occurrence was defined as the collapse evaluation criterion,where the ductility demand is defined as the collapse ductility.The collapse ductility spectrum was constructed to reflect the ductility when structural dynamic instability occurred with different reserve parameters.The design threshold periods were identified based on the ductility demand spectra and the collapse ductility spectra.The reasonability of the calculated threshold periods of CBFs were verified by the vulnerability analysis results according to the recommended method from FEMA P695 standards.(3)The braces failure location and weak story based on pushover analysis of multi-story CBFs.The equivalent SDOF system parameters were identified by the pushover curve.The development and assessment of a simplified procedure for estimating the collapse performance and displacement response of multi-degree of freedom CBFs considering reserve capacity by using Pushover analysis and R-?-T model was presented.The reserve capacity parameters representing strength and stiffness ratios between reserve system and primary system are established.This simplified evaluation method is also called 'reserve capacity method'.A reasonable accuracy is demonstrated by comparing the results of ductility demands and roof displacements of proposed method with the results obtained by non-linear dynamic analyses of two 5-story test buildings.The proposed method offers a simple,yet efficient,tool that is capable of predicting seismic performance of multi-degree of freedom CBFs.(4)A 3-story 2-bay 1:6.5 scaling low-ductility CBF shake table test was performed subjected to a stiff soil earthquake and a soft soil earthquake with increasing intensity,repectivelly.The seismic performance,damage revaluation,and reserve capacity was evaluated.In addition,the collapse prevention performance was evaluated by the reserve capacity method,which practicability was verified.(5)The design process between China and USA was compared.Building design examples of 3-story CBF structure were given,which contains a low-ductility CBF example and an OCBF example.A 3-story low-ductility CBF according to Chinese standards was designed,and the scaled model was tested on the shake table.Damage and residual drifts of reserve system were not obvious found after tests.The results gave many efficiency suggestions of design philosophy of CBFs in China.