Seismic Performance Of Isolated Step-terrace Frame Structure Under Long Period Ground Motions

The step-terrace structure has become a typical architectural form which is widely used in mountainous areas.Compared with ordinary frames on plain field,the step-terrace structure has quite different dynamic response characteristics due to the non-uniform vertical stiffness.The step-terrace structure has weak upper embedding stories and obvious torsional effects,which is found in the Wenchuan earthquake and Sikkim earthquake.Focusing on these poor seismic performance,the isolation technology can be used effectively.However,the fault zones and the large sedimentary basins usually near the mountainous areas,which provides conditions for the breeding of near-fault pulse long period ground motions（NF type）and far-field long period ground motions（FF type）.Considering the significant influence of the long period ground motion is often ignored in practical design,and the seismic performance research of isolation step-terrace structure under long period ground motion is quite lack,so this thesis aims to study the dynamic response characteristic of the isolation step-terrace structure under two types of long period ground motions,and clarify the damage mechanism.The main contents and conclusion of this thesis are summarized as follows:（1）Characteristics research and extraction of long period ground motion:Firstly,the near-fault and far-field ground motions are compared with the ordinary motions through time history,frequency spectrum and energy.It is found that the long period components of the exist in the velocity pulse and the long period coda wave respectively.At the same time,the PGA of the far-field motions is significantly lower,which is easy to be ignored,while the energy accumulation of the near-fault ground motions is quite faster.Secondly,based on the HHT method,the velocity pulse and the coda wave are extracted from the original waves,and the T_pof pulse and the characteristic period T_fof coda are obtained.（2）Dynamic response of isolation step-terrace structure under long period motions:Based on the seismic design of a plane step-terrace frame structure,three isolation forms are established:Lead-Rubber Bearing（LRB）,Natural Rubber Bearing（LNR）,and the hybrid isolation.Comparing the seismic force results between the time history analysis method and the code response spectrum method,it’s found that the result of code response spectrum is dangerous while accounting the long period ground motions;and the results shows that the seismic force of upper floor of isolation structures is quite smaller than the anti-seismic structure,especially at the upper embedding columns,but the force of the downstairs part has a slight amplification;And the base isolation can reduce the maximum acceleration response and the interlayer displacement angle of the upper structure,but the same response of the embedding parts is relatively increased,especially under the two types of long period motions.What’s more,the results show that the maximum displacement of the isolation bearings under long period earthquake is much larger than the ordinary,which can easily exceed the limit displacement,and the LRB isolation structure can effectively control the displacement of bears.（3）Analysis of the mechanism of isolation step-terrace structure under long period motions:By contrasting the displacement response spectrum of the processed waves and the original waves,which throughs light on the control effect of the long-period components on structural response.And the nonlinear analysis results of LRB model under processed waves and original waves are compared.The result shows that:when the velocity pulse is extracted of the NF motions,the maximum displacement and energy hysteresis of the bearings decreases obviously.In particular,when the pulse period T_pis close to the isolation period T₁,the decrease can be the greatest;when the coda is extracted from the FF motions,the displacement of the bearings decrease a little,while the energy hysteresis decrease significant.Especially,the peak displacement of some FF motions can be lager,which can be due to the resonance-like phenomenon of the structure.Generally speaking,the NF motions mainly cause the bearings reach the first exceed damage because of the large displacement,and the energy hysteresis of bearings under FF motions should be paid attention to.（4）Seismic fragility analysis of the isolation step-terrace structure under long period motions:Firstly,based on the IDA method,choosing Sa（T₁）as IM index,maximum displacement of isolation bearings and the interlayer displacement angle of superstructure as EDP index,the fragility curves of isolation bearing and superstructure are obtained respectively.The results show that:when the structure reaches the ultimate collapse state（CP state）under NF motions,the corresponding Sa（T₁）is the minimum,while the ordinary motions is the maximum,it means that the NF motions can seriously threat the isolation structure;in addition,the CP state of the isolation bearing is always priorly reached to that of the superstructure,so the bearing is the key part of the isolation structure under the action of long period motion.Finally,in order to further evaluate the fragility of the structure,the vector IM which can represent different long period ground motions are proposed,and the correlation and discreteness are used to evaluate each IM index,then the fragility surface under different indexes is constructed.The results show that:[Sa（T₁）,PGA/PGV]is the relatively optimal vector IM,and Sa/PGA,I_D,t₈₀/t_eare also acceptable,and the fragility surface can reveal more ground motion information,which don’t need to classify different kinds of ground motions in the fragility analysis.