Institute Of Geophysics,China Earthquake Administration Doctoral Dissertation

The collapse mechanism of structures subjected to earthquake is such a difficult issue that civil engineers around the world have conducted research on it over 100 years.In the early 1960s,they all agreeed that weak columns and strong beams may lead to the“side sway mechanism”.However,the facts showed a big difference between the reality and expectation.Therefore,it is urgent to carry out futher research on the collapse mechanism mainly based on damaging earthquake field.Many collapsed and non-collapsed buildings in the Wenchuan earthquake were compared and analyzed,and a"hypothesis"was initially proposed.Then it was verified through the shaking table tests and quasi-static tests.Finally,a new understanding of the collapse mechanism which can be summarized as"internal forces concentration"was proposed.Contents and conclusions can be summarized as follows.1.According to the construction drawings of nearly 20 typical buildings in two extreme earthquake regions(Beichuan and Yingxiu)in Wenchuan earthquake,configurations of collapsed and non-collapsed buildings were compared.The results showed that the difference of the lateral stiffness of the vertical members on each axis in the ground floor is the key to the collapse of the structure.Then the collapse mechanism based on"internal forces concentration"was preliminarily proposed.2.Two RC frame models with and without shear walls were designed.Then the accelerations,displacements,strains and the failure of them were obtained by carrying out the shaking table tests.The internal force of each column in the ground floor showed that the infilled walls changed the distribution of internal force among the columns.In particular,the columns constrained by the half-height continuous infilled walls were subjected to 6-8 times the seismic shear force of the unconstrained ones due to their large lateral stiffness,which verified the rationality of the collapse mechanism based on"internal forces concentration".3.The shear walls were built beside the corridor columns with weak lateral stiffness,and the internal force of the columns on each axis tended to be balanced.Finally,the total shear force ratio(?F_A:?F_C)was reduced from 7.4:1 to 0.7:1,avoiding the collapse mechanism based on"internal forces concentration".Obviously,this method can greatly improve the anti-collapse ability of RC frame with eccentrically placed open corridor.4.Based on the earthquake damage of multi-storey masonry structures with first storey used as shops,the quasi-static tests of three groups of combinations(frame without infilled walls,frame with infilled walls built ahead and frame with infilled walls built afterwards)were carried out.The results indicated that lateral stiffness of different types of wall-column composite members differed greatly.Therefore,the internal force distribution among them were unbalanced in earthquake.A few constructional elements would be destroyed firstly due to the large internal force,leading to the collapse of building.At the moment,the elements with high ductility had not yet worked.