| Chinese seismic design codes adopt fixed force reduction factor R with more andmore relaxed seismic fortification measures for regions from intensity9to intensity7,which does not follow the R-μ principle. At present, there are only few studies onevaluating the rationality of seismic measures provided by the current seismic code forframe structures in Intensity Region7(0.10g), the proportion of which is relatively largenationwide. Therefore to evaluate the effectiveness of seismic measures for structures inIntensity Region7(0.10g) is of prime necessity. Different structural analysis programsutilize various basic assumptions and methodologies of algorithm and based on differentselecting and scaling methods the input ground motion records would not be identical,both of them would lead to variation of the analyzing results. Considering the problemsmentioned above, this paper utilized the PERFORM3D program to conduct thenonlinear dynamic time-history analysis of frame structure located in Intensity Region7(0.10g) and re-evaluate the effectiveness of seismic measures for structures in thisregion in current Chinese design code based on the predicted response results.As an important input parameter in nonlinear dynamic analyses of structures,damping affects the structural response dramatically, therefore it is better to beprudential when choosing rational damp value and model. In addition, the input ofground motion records is of great impact on the response of structures. Selectingreasonable ground motion records is primary for predicting structural response duringearthquake event by time-history analysis.Based on the ideas mentioned above, the research work accomplished by his paperis as follows:①The damping mechanism of structures is briefly introduced. The measuringresults of damping both at home and abroad and the damping ratio used by differentcountries when designing and evaluating structures are summarized. Referring to recentacademic reports, suggestions about reasonable damping ratio utilized in the design andevaluating structures are provided. On the basis of research findings both at home andabroad, different expressions of Relay and Modal damping model and their impacts onanalysis results are summarized.②Through the nonlinear dynamic analysis of two structural models with the inputof ground motion records utilizing PERFORM3D, the impact of different damping ratios and models on analysis results are introduced.③Referring to common methods of selecting ground motion records used in Chinaand United States, the probability of applying Uniform Hazard Spectrum andConditional Mean Spectrum in China is analyzed. Considering the context of this paper,the Uniform Hazard Spectrums of4sites located in Intensity Region7(0.10g) areprovided through probabilistic seismic hazard analysis and their comparison with thespectrum in Chinese design code is provided, then the target spectrum for selectingground motion records is determined.④A typical reinforced concrete frame in Intensity Region7(0.10g) has beendesigned strictly conforming to the current Chinese codes, and nonlinear dynamicanalyses under7groups of rare earthquake excitations using the software ofPERFORM-3D have been carried out. Based on the structural seismic behavior, therationality of seismic measures provided by the current seismic code has beenevaluated.Based on the research, main conclusions are as follows:①It is advised to still adopt5%of critical damping when designing reinforcedconcrete structures. Also, the appropriate damping values are proposed in nonlinearresponse analysis of typical buildings: below30stories, the maximum critical dampingratios range up to2%(for steel) and4%(for reinforced concrete), whereas above30stories, the critical damping ratios range from0.5%to2%(for steel) and1%to3%(forreinforced concrete). Once the structures step into inelastic work, Rayleigh dampingcomputed by initial stiffness will over estimate damp effect. A combination of Rayleighdamping and modal damping is commended to be used if the analysis software employsthis type of Rayleigh damping. In terms of the damping expressions in PERFORM-3D,a reasonable damping model that combines modal damping with a small amount ofRayleigh damping is proposed.②Damping ratio is of great influence on structures. The impact of damping modelon tall buildings cannot be neglected and its impact on structures with few stories islittle.③The mean spectrum of4Uniform Hazard Spectrums has little difference withthe Chinese code spectrum in the period range T1~1.5T1(T1is the fundamentalstructural period), therefore the code spectrum can be used as target spectrum whenscaling ground motion records. The method for selecting ground motion records arebased on target spectrum. When selecting, the spectrum of ground motion records are controlled to match the target spectrum, especially in the period range T1~1.5T1.④The nonlinear response performance of the structure under rare earthquakes wasverified preliminarily. The maximum story drift hasn’t reached the limitation providedby Chinese codes and is far from the value proposed by PEER-Guidelines. Beam-hingemechanism hasn’t formed and the number of column hinges is higher than that of beamhinges. Compared with the experiment results, the column damage is not serious. Thebeam damage is so slight that it will only weaken the lateral load resistance capacity andlateral stiffness to a certain extent. The elements with plastic deformation have notcaused serious damage and will not bring direct hazard to the structure. Therefore, theoverall structure has good seismic behavior. In conclusion， seismic measures for thezone of Intensity Region7(0.10g) provided by the current seismic code is reasonable.To sum up, the performance of this model under rare earthquake is satisfied then theseismic measures for reinforced concrete frame structures in Intensity Region7(0.10g)are effective. While plastic hinges occurs in almost all the base columns with the inputof7groups of ground motion records, the moment resisting ability of these location aresuggested to be enhanced. |
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