Study On Seismic Collapse Capacity Of Moment Frames Supported By Foundations With Multiple Elevations Based On IDA Method

Assessing the seismic collapse capacity of moment frames supported by foundations with multiple elevations(MFFME)in mountainous areas is the premise of seismic collapse capacity improvement and seismic design optimization of MFFME.It is also essential for strengthening the seismic disaster prevention ability in mountainous region.Previous research referred to MFFME has well discussed its seismic performance,seismic design and detail measures.Nonetheless,very few studies quantitatively assess the seismic collapse capacity of MFFME.Compared with moment frames on plain field,the upper embedding stories of MFFME are the soft and/or weak stories due to the multiple elevations foundation,which also cause the eccentricity for the inconsistency between the center of mass and stiffness,resulting in obvious torsional effects.Therefore,it is a necessity to conduct quantitative seismic collapse analysis of MFFME,and adopt 3-D model to assess its seismic collapse capacity and collapse margin ratio,which can provide reference for seismic collapse safety performance and seismic design of structures in mountainous areas.Based on the current Chinese codes,this thesis designed 10 study cases of 2-D and 3-D plain field frames and MFFME in seismic fortification intensity 7 degree region.Seismic collapse analysis of MFFME cases has been carried out by adopting incremental dynamic analysis(IDA).Single IDA curve,IDA fractile curve and seismic fragility curve of 2-D and 3-D models have been studied under two different collapse limit states.Seismic collapse capacity and collapse margin ratio(CMR)have also been calculated.Based on the IDA results,this study also provided the peak interstory drift,statistics of collapse modes and corresponding probability for each models under two collapse limit states.The main innovations in this thesis include: 1)conducted a seismic collapse analysis of 3-D models of MFFME based on IDA method,and a comparison study of 3-D and 2-D model analysis results;2)adopted two different collapse limit states(i.e.upper embedding side collapse and lower embedding side collapse limit state)to define the collapse of MFFME,and compared their corresponding seismic collapse analysis results;3)carried out a comparison study of the seismic collapse capacity and CMR of MFFME under ground motions in different directions.Major findings can be concluded as follows:(1)The MFFME reached the slignt damage limit state earlier than frames on plain field due to its upper embedding weak stories.However,compared with plain filed frames,the seismic collapse capacity and CMR of MFFME show no significant reduction.(2)The results of MFFME 2-D and 3-D models assessing the seismic collapse safety performance were consistent mostly.Nevertheless,when using frames on plain field at collapse limit state 1(i.e.upper embedding side collapse limit state)as reference,the CMR value of 2-D MFFME were larger than that of 2-D plain field frame,while the 3-D results of MFFME were smaller than 3-D plain field frame.Therefore,it is likely that 2-D model analysis might overestimate the CMR of MFFME.It is suggested to adopt 3-D models to calculate the CMR of MFFME.(3)The results of seismic collapse safety performance assessment differed significantly under two collapse limit states defined in this thesis.When using collapse limit state 2(i.e.lower embedding side collapse limit state)as the real structure collapse,the calculated CMR of MFFME largely increased compared with frames on plain field.Meanwhile,with the number of stories under upper embedding end increased,the CMR of MFFME enlarged.These results indicated that adopting collapse limit state 2 may cause a delay of the real collapse searching,resulting in overestimation of seismic collapse safety;thus,collapse limit state 1 might be a more rational choice for seismic collapse analysis of MFFME designed in this study.(4)The results of seismic collapse capacity and CMR varied when 3-D model input ground motion directions differed.It is likely that overestimation of CMR occurs when only one direction ground motion records are considered.Therefore,bi-direction ground motion might be a better choice for seismic collapse analysis of MFFME.(5)When using collapse limit state 1 to define collapse,the main collapse mode of MFFME lay in the first upper embedding story,and the different direction input ground motions caused no significant distinction on the collapse mode results.However,when adopting collapse limit state 2,the collapse modes and corresponding probabilities of 2-D and 3-D models differed,and new collapse modes arised when 3-D MFFME were subjected to bi-direction ground motions.