Study On The Failure Mechanism And Torsion Effect Of Bottom Frame Structure In High Seismic Intensity Zone

Masonry building with frame and seismic-wall in the lower stories is a special structure form accords with China's economic development at the present stage.Bottom frame structure is widely used in street building of small and medium-sized town of our country construction due to its low engineering cost,short construction period,and the flexibility of spatial arrangement in bottom layer.However,the bottom frame structure is a complex structural system which is composed of the frame-shear wall structure in underlayer and the masonry structure in upper layer.The stiffness of the structure changes greatly in vertical direction,the stress of transition layer is complex,and the seismic performance is not good enough.In Wenchuan earthquake,a lot of bottom frame structures which were located in high seismic intensity zone caused serious damage or even collapsed.Among them which also appeared serious damage phenomena due to torsion under strong earthquake action,which was quite rare in previous earthquake damage survey.Therefore,analyzing the seismic damage mechanism of the bottom frame structure and studying the control factors of the torsion effect has important significance to improve the seismic capacity of the bottom frame structure in high intensity region.This paper took a severely damaged bottom frame residential building in Wenchuan earthquake as example.By using ABAQUS software to establish finite element model,and elastic and elastic-plastic time history analysis was carried on.The failure mechanism of the bottom frame structure and reason of torsional effect was studied.On this basis,a series of comparison models were designed for nonlinear earthquake response calculation to study the various influence factors on seismic performance and torsion effect,which could provide references for the revision and further improvement of the seismic design code.This paper mainly completed the following work.Firstly,characteristic of earthquake disaster of bottom frame structure in Wenchuan earthquake was summarized,and the causes of damage were analyzed.The similarities and differences of seismic damage between the bottom frame structure and masonry structure were introduced.The design development and trend of the provisions of the bottom frame structure was summarized at the Chinese seismic code including "78 code","89 code","01 code" and "10 code",the reasons for the change and its influence on the actual engineering structures were expounded.Secondly,the finite element modeling method of wall-filled frame component was researched.According to the seismic investigations in field,a new improved equivalent diagonal bracing model of a frame-filled wall was proposed based on the traditional equivalent diagonal strut model.The numerical simulation results and the experimental results were compared,indicating that the simplified modeling method which was presented in this paper was more accurate and more conforming with the actual earthquake damage.On the basis of mechanical model,calculation formulas of the infilled shear bearing capacity and the filling wall stiffness were given through theoretical derivation.Thirdly,Tianhe Xinyuan No.4 Building in city of Mianzhu in Wenchuan earthquake was taken as an example to study the failure mechanism of the bottom frame structure in high intensity region.Though the earthquake damage surveys,the field modal testing,the weak floor distinguishing and lateral stiffness ratio checking,the ABAQUS finite element model was established to simulate the nonlinear seismic response.The analysis of the calculation results described the reason of serious damage due to torsional effect.Design suggestions were given to improve the seismic performance and restrict torsion effect of bottom frame structure under horizontal seismic force.Fourthly,on the base of existing seismic damage example,variable conditions of design finite element models were established for comparative time history analysis of nonlinear seismic response.On three aspects including vertical ground motion,in-plane stiffness uneven distribution and vertical stiffness uneven distribution,the influence of characteristic of ground motion and structure on the seismic performance of bottom frame structure was investigated.The control parameter was proposed to limite the torsion effect of structure.On the design point of view,the measures and suggestions to reinforce seismic capacity of bottom frame structure were given.