Analysis Of Seismic Performance Of Bottom Frame Masonry Structure And Lateral Stiffness Ratio Research

Bottom frame masonry structure refers to hybrid structure system housing whose layers at bottom are the frame structure or frame-shear wall structure and the upper layers are the masonry structure. The bottom of such a house is flexible room layout which can create a large space and mainly used for shops, business offices, restaurants, garages, etc; because of more cross walls and smaller housing space, the upper masonry structure is usually used for office, residential, small hostel, etc.Due to good use function, low cost and convenient construction, bottom frame masonry structure is widely used in urban villages in major cities, small towns’street construction. Because this structure is adapted to China’s economic development level, in the future for quite a long time, it will also serve as the main structure form. However, because of many cross walls, the upper masonry structure’s lateral stiffness is big, and the bottom is open, which finally form the typical chicken leg building.This soft first layer structure has poor performance in earthquakes, whose bottom layer and transition layer appeared large earthquake damage or collapse, and caused serious casualties and property losses.Therefore, it is necessary and urgent to furtherly study on the seismic response of the bottom frame masonry structure in the large earthquake, analyse the seismic performance, reveal the weak layer transfer mechanism and recommend appropriate lateral stiffness ratio limit value. Meanwhile corresponding research can provide technical references for the construction and retrofit of such buildings and evaluation of seismic performance.Supported by "Scientific and Technological Leading Talent Project"(project number:10LJRC186), this thesis mainly has done the following works:(1)By combing the bottom frame masonry buildings’damage characteristics in Wenchuan and Yushu earthquake, housing seismic weaknesses and damage law are obtained. Based on analysis of the earthquake damage law, this thesis has summed up that the lateral stiffness ratio of the transition layer and the underlying is not reasonable is the main cause of damage, and seismic countermeasures have been proposed.(2)According to basic dimensions of the intance building, establish three-dimensional finite element model, and through modal analysis by using Block Lanczos method, get the structural dynamic parameters and the mode characteristics.Analysises show that along the major axis, the longitudinal lateral stiffness is weaker than the transverse; along the floors’direction, the bottom floor vibrates strongly, which shows "large rigidity of the upper and weak of the lower part".(3)In order to evaluate the seismic performance of such a house, along both transverse and longitudinal directions, undertake a structural elastic-plastic time history analysis under a great earthquake and extract the structural displacement response and development trend of cracks. The results show that the underlying appears serious elastic-plastic deformation which is the weak layer of the structure; the wall between windows, the wall under windows and the lower part of the cross wall in the transition layer occur more cracks, which are the weak links of the upper masonry.(4)In order to explore the reasons of transition layer destruction and seek weak layer transfer mechanism, four kinds of finite element models with different lateral stiffness ratios are established by adding different size shear walls in the bottom. Through modal anlysis and structural elastic-plastic time history analysis under the great earthquake, the impact on dynamic parameters and seismic response has been obtained after adding shear wall.Analysis results show that the transition layer destruction increases after adding the shear wall, and there is need to limit the lateral stiffness ratio to prevent weak layer from transferring to transition layer of which has low material strength and deformation capacity.(5)Through elastic-plastic time history analysis of buildings with different lateral stiffness ratios under the great earthquake, based on the relative ratio between the bottom layer and the transition layer displacement angle, suggestions of lower limit value of lateral stiffness ratio have been put forward.The limit value which is within the limits of our seismic code has ideal seismic safety performance and actual application performance. (6)Based on the seismic response law of the models, this thesis summarized that the lateral stiffness degradation of concere shear wall and brick masonry in nonlinear stage is the main reason of the weak layer transfer. At the same time, according to this main factors and the practical application of infill walls in bottom suggestion of effective lateral stiffness ratio formula at elastic-plastic stage has been put forward which can provide a reference for further study on the lateral stiffness ratio limit.