Failure Mechanism Under Strong Earthquake And Fragility Analysis Of RC Frame Stuctures On The Slope

As China’s central and western areas are dominated by mountainous terrain, RC step-terrace frame structure has been widely used as a typical kind of mountain building. The structure of the step-terrace frame is serious irregular in vertical because of its special unequal-height grounding method. In Wenchuan earthquake investigation, a large number of step-terrace buildings are collapse, which reminds us to rethink the key issues that may be ignored or unrecognized; therefore, it is necessary to study the seismic performance of step-terrace frame structure.In this paper, the seismic performance of structures under rare seismic is studied. And the research subjects are RC step-terrace frame structures with different grounding methods which are uncontrolled, distributed, reinforced, base isolation and story isolation. In view of the influence of uncertainty and nonlinear coupling on the structural seismic response, the method, incremental dynamic analysis(IDA), is chosen and the different IM, DM parameters are analyzed based on probability statistics. Meanwhile, based on the IDA analysis to study the fragility of step-terrace structure, a certain failure probability of structure is gained under different earthquake intensity, and CMR coefficient is used to evaluate the anti-collapse ability.The main conclusions of this article are as follows:In the uncontrolled structure, there is a serious lateral stiffness mutation between the off-layer part and the grounding-layer. The earthquake response of the former is small, while the later and the above structure is big, which results in the upper grounding column damaged easily. The distribution structure, the reinforcement structure, the base isolation structure and the story isolation structure can improve the stiff irregular in the uncontrolled structure; the story isolation structure and the distribution structure have a better result, though the story isolation structure can enlarge the earthquake response of step-terrace part.In the uncontrolled structure, the largest internal force resulting from seismic action occurs in the upper grounding pillars, and for the story isolation structure it occurs in bottom pillars; the internal force is even distributed relatively in the remaining structures; under the rare seismic action in seismic fortification intensity VII area, all five kind of structures are not damaged seriously and appear the beam hinge mechanism, and there is no column-hinge though the lower part of the upper grounding pillars enter the plastic status; when the inputted ground motion intensity reaches the value corresponding to the ultimate state of the structure, all five kind of structures are controlled by column-hinge damage, while beam-hinge damage only plays a role on local failure.In the IDA curve of the uncontrolled structure exists a turning point around which the slope of the IDA curve declines sharply, that indicates some suddenly collapses happen within a relative small deformation; the failure mechanism of the story isolation structure is the damage of the lower part of the isolation layer owing to the weak layer transferring to step-terrace part, which leads to the bearings damaged prematurely without making full use of them; for the remaining structures, their seismic performance is more better with a relatively flat slope of the IDA curves; through the comparative analysis of IM parameters, Sa and PGA, it is found the Sa parameter can reduce the discreteness of statistical results effectively; it is reasonable to adopt the PGA parameter when it comes to long period structures because the application of Sa parameter need a further study and as the Sa parameter is closely related to the first period of structure, it does not recommend to use Sa parameter in the comparison of seismic performance of structures with different periods.By the study of the variations of the slope of the 50% IDA curve using the maximum story drift angle as DM parameter in all five kind of structures, the values of four kind of performance level points, IO, DC, LS and CP, are gained; through fragility analysis, the discrepancies of seismic performance of different structures are reflected in the level of LS and CP. And the exceeding probability of the structures are decreasing in the following order, no controlled structure, story isolation structure, reinforcement structure, distribution structure and base isolation structure, which is still the increasing order of the security; the sequence of the resisting collapse capacity is as follows: base isolated structure, distribution structure, story isolation structure, reinforcement structure and no control structure.