Study On Earthquake Intensity Measures And Collapse Safety Margin For Super High-Rise Building Structures

Super high-rise building,characterized with its big sizes and complex functions,plays an important role in urban area,resulting in the significance of precise and effective analysis of the seismic performance of a super high-rise building.Developing reasonable ground motion intensity measures(IMs)to measure ground motion uncertainty and then analyzing structural collapse safety margin capacity have a significant positive impact on the seismic performance analysis and design of super high-rise buildings.On the one hand,the common method used in performance-based earthquake engineering is to establish a probability-based assessment method or framework,in which the first and important step is to determine a ground motion IM to characterize ground motion uncertainty.On the other hand,both the determination of a reasonable collapse safety margin baseline and the assessment of the collapse safety margin capacity considering multiple sources of uncertainties have important impetuses on the development of the quantitative assessment of the collapse capacity and performance-based earthquake engineering.In such a background,this paper focuses on two subjects,i.e.,ground motion IM and collapse margin ratio(CMR).A spectral-acceleration-based linear combination-type IM considering higher modes and a spectral-velocity-based combination-type IM considering higher modes are proposed for super high-rise buildings.The ground motion IMs suitable for estimating the earthquake demand of acceleration-sensitive elements in super high-rise buildings are selected.The calculation expressions of the acceptable values of the CMR with different confidence levels are derived.Finally,the seismic collapse risk assessment considering the modeling uncertainty is carried out for a super high-rise building.The main research contents and conclusions are as follows:(1)An attempt is made to develop a spectral-acceleration-based linear combination-type ground motion IM considering higher modes for estimating the structural damage.Based on the modal decomposition method,the proposed IM superposes the corresponding Sa(Ti)of some optimal modes,where the modal mass participation factors are used here as the weighting factors to adjust the contribution of different modes involved in the combination.A parametric analysis on the correlation between IM and the maximum inter-story drift that utilizes the generalized bend-shear coupled model yields a bilinear relation between the optimal number of modes combined and structural fundamental period.Based on two sets,i.e.,far-field and near-field pulse-type ground motions with thirty records for each,four building structures are used as analytical models for verification.The results show that this IM has relative higher efficiency,sufficiency and scaling robustness.(2)A spectral-velocity-based combination-type ground motion IM considering higher modes is proposed for estimating the structural damage in super high-rise buildings.Based on two example-frame core-tube super high-rise buildings,the correlation between spectral quantities and the maximum interstory drift ratio and maximum floor acceleration is analyzed,and then it is found that the sensitivity of spectral velocity to structural response at the long period range is superior to other two spectral quantities.On such basis,the spectral-velocity-based combination-type ground motion IM is proposed.The optimal number of lower vibration modes and corresponding combination factors of the ground motion IM are given by parameter analysis.Based on far-field and near-field pulse-type ground motions,the applicability of the proposed ground motion IM for super high-rise buildings is investigated from the perspectives of efficiency,sufficiency.relative sufficiency measure and scaling robustness.(3)The efficiency and sufficiency of scalar ground motion IMs for estimating the acceleration-sensitive nonstructural demand measures,i.e.peak floor acceleration(PFA)and floor acceleration response spectrum(FARS),are evaluated.With far-field and near-field pulse-like ground motion records acting on two super high-rise buildings.the applicability of IMs in estimating the demand measures of nonstructural components is analyzed.The appropriate IMs for FARS are notably different from those in the case of PFA,from the perspective of efficiency,as well as sufficiency.None of the collected IMs can maintain its sufficiency with respect to the magnitude and source-to-site distance in any case for FARS.The selected far-field and pulse-type near-field ground motions have a large impact on the selection of the appropriate IMs with respect to PFA and FARS.In addition,the location of a floor under consideration is also influential.(4)Based on two different approaches that incorporate aleatory randomness and epistemic uncertainty,namely the mean estimates approach and confidence interval method,the expressions for collapse-fragility-based and risk-based acce.ptable CMRs are derived,respectively.And then the acceptable values of CMR applicable to Chinese specifications are given.Compared with the mean estimates approach,the confidence interval method could be used to provide CMR acceptable values with different confidence levels to meet specific seismic targets.When the structural parameters are set to the median values,the acceptable values calculated by the mean estimates approach have only a 50%confidence level.Under the same conditions(i.e.,same uncertainties and confidence levels),the acceptable collapse fragility-based CMRs are less than the risk-based CMRs,(5)Taking the super high-rise building structure as the basic research object,the seismic collapse risk assessment considering the modeling uncertainty is carried out based on the multiple stripes analysis and the first order second moment method.The optimal ground motion levels in the multiple stripes analysis method are determined.The sensitivity of the collapse capacity of the super high-rise building to eight main parameters is analyzed,and then the modeling uncertainty coefficient is determined.Both effects of the characterization method of bi-directional ground motion intensities and the selection of ground motion IM on the aleatory randomness are investigated.The results show that the influence of the modeling uncertainty on the collapse capacity in the super high-rise building is negligible;the aleatory randomness caused by record-to-record variability is significant;an appropriate ground motion IM can significantly reduce the aleatory randomness.