Collapse Analysis On A Typical Soft First Story Structure Subjected To Earthquake

It’s a hundred years’ puzzle why buildings collapse in earthquakes.Although there are various hypotheses or theories,all of them can hardly stand the test of earthquake damage practice.In fact,when trying to solve this problem,people have ignored another one,that is why the adjacent buildings do not fall.In this paper,a representative soft first story masonry structure is selected from the Wenchuan earthquake damage site to discuss the seismic damage mechanism.Through the comparison of actual seismic damage,model experiments and theoretical analysis,some new understandings are obtained.The main results of the paper are as follows.1.A five-story masonry structure with first soft floor（hereinafter referred to as the target building）in the extreme earthquake area was investigated in detail.The structure was mapped,the seismic damage was described,the force transmission paths of gravity and seismic force were analyzed,and the similarities and differences of seismic damage between the building and its surrounding buildings were summarized.2.Based on previous research results,the response process of the target building under earthquake action is preliminarily judged.It is pointed out that both the lateral deformation and the torsion of the structure are constrained by the lateral full masonry infill walls.Holes are opened on three longitudinal axes,and the floor slab only moves horizontally along the longitudinal direction;The seismic shear force condenses on the components with large stiffness and makes them reach the "deformation saturation" and lose the horizontal and vertical bearing capacity.As a result,the transverse walls lose the support of "sustaining wall",so that they can’t stand the test of longitudinal earthquakes and finally fall like dominoes.The overall structure presents that the ground floor collapses along the longitudinal direction and parallel to the street.3.In order to verify the prediction of the seismic response process of the target building,relevant test models are designed to simulate the longitudinal axis without wall（non-wall frame model）and with infill walls（infilled wall with openings frame model）of the ground floor.As predicted,the frame with infilled wall has large stiffness and poor ductility,which is "hard and brittle",while the simple frame has small stiffness and good ductility,which is "weak and ductile".However,when the two are mixed and matched on the same floor,they are constrained by the floor slab and have the same longitudinal lateral displacement.When the displacement is small,the "hard and brittle" component will fail.At this time,the "weak and ductile" components have not yet exerted their ductility and are crushed by the falling upper layers.4.Based on seismic damage analysis and experimental verification,some suggestions for structural seismic design are put forward: by reducing the difference in the stiffness of the components,the phenomenon of "internal force condensation" can be significantly reduced,which is reflected in the actual cases analyzed;By improving the ductility of "hard and brittle" components can realize the structure cracking without collapse under the action of super strong earthquakes,and there are corresponding cases in actual earthquake damage.