Seismic Response Modification Factor Of Y-eccentric Braced Steel Frame Structures

The Y-eccentric braced steel frame (YECBF) sturcture with higherlateral stiffness, good ductility and energy dissipation capacity are gainingacceptance and suitable for multi-story or high-rise building in highlyseismic regions. The seismic response modification factor or structuralinfluencing coefficient is used to reduce the elastic response spectra ofSingle-Degree-Of-Freedom (SDOF) system under the fortificationearthquake to obtain the structural design earthquake action in theperformance-based seismic design of buildings, and its reasonable value isdirectly relative to the safety and economy of designed structures. Theobjective of this paper is to quantify the seismic response modificationfactor under the earthquake with seismic fortification intensity (namely themoderate earthquake) of Y-eccentric braced steel frame sturcture; the maincontents of this dissertation are as the followings:(1) To investigate the elastic-plastic performance of eccentricallybraced steel frame with Y-links, appropriate SAP2000plasticity hingemodel was set up for this structure. The reasonability of this model wasproved by means of comparison between the skeleton curve and Pushovercurve of finite element analysis of three model samples under cyclic load.To study the R values of Y-eccentric braced steel frame structure, twelveYECBF examples complying with Chinese seismic code with differentstories and different ratios of storey height to spans were established toevaluate the effects of different design parameters on the seismic responsemodification factor.(2)An improved pushover analysis named steeply-modified inertiaforce method was proposed in the quantification of R for YECBF. The improved Pushover analysis is an approximate adaptive force distributionand attempts to follow more closely the time-variant distributions of inertiaforces, in which the contribution of high modes to response and theredistribution of inertia forces because of structure yielding and theassociated changes in the vibration properties of the structure wereconsidered approximately. Based on the well known ductility factor theoryproposed by Newmark N.M. and Hall W.J., the seismic responsemodification factor, Overstrength factor and deflection amplification factorof each building were evaluated by an advanced capacity spectrum methodcombined with the global capacity envelops obtained from the proposedpushover analysis. Moreover, the effects of storey numbers and spans offrames on these factors were analyzed.(3)Based on incremental dynamic analysis (IDA), a new method forquantifying seismic response modification factors under the moderateearthquake was proposed and the detail process was explicated elaborately.The method was performed to get the seismic response modificationfactors, overstrength factors and deflection amplification factors of allbuildings. At the same time, the effects of storey numbers and spans offrames on these factors were also analyzed. Eventually, the factors of allmodels were calculated by making the roof displacements obtained by thenonlinear time history analysis under ten motions on the moderateearthquake level as the demand displacements under seismic fortificationintensity, and the values of all factors were compared with the resultsobtained by Pushover analysis and IDA analysis.(4) Based on the analysis of the results from IDA analysis andPushover analysis, the values of seismic response modification factor,overstrength factors Ω and deflection amplification factor C_d of Y-eccentricbraced steel frame structure were suggested.(5)Four Y-eccentric braced steel frames complying with the suggestedseismic response modification factors with different stories were set up.Based on the code for seismic design of buildings GB50011, the collapse-resistant capacities and safety margins against collapse of allbuildings were quantitatively evaluated with collapse fragility analysiswhich is based on incremental dynamic analysis (IDA). At the same time,the adjusted collapse margin ration (ACMR) of Y-eccentric braced steelframes were obtained and compared with the acceptable ACMR by theperformance evaluating method recommended by ATC-63. According tothe results, the acceptability of suggested seismic response modificationfactors was assessed.