Influences Of Slabs And Infill Walls On Seismic Performance Of Reinforced Concrete Frame Strctures

From the damage survey of Wenchuan earthquake, it is found that many RC frame structures have not obeyed the prospective yielding mechanism "strong column weak beam", so the in-depth study on seismic performance of RC frame structure is seemed to be particulary important. In seismic design of RC frame structures, moment the design values of column at column ends are usually adjusted to be bigger than that of the beam ends by amplification coefficient. So beam ends will yield before column ends under earthquake action. The amplication coefficient considerates the enhance effect of slab on negative moment bearing capacity of beam end. However, the consideration is not quite accurate, that is the one of main reasons for "strong beam weak column" yielding mechanism. Besides, that infill walls and frames as a whole to bear horizontal loads has been widely accepted, however, in seismic design of RC frame structure the interaction between infill walls and frames is not considered such that the safty of the structure is not guaranteed. In view of the above-mentioned problems, the influences of slab on the negative moment bearing capacity of beam end and the seismic performance of structures are studied based on mechanism research and numerical analysis. In addtion, in order to consider the effects of infill walls on RC frame structures, the analysis of structure dynamic properties, seismic performances under small and severe earthquakes are accomplished. The main contents of this paper are as follows:(1) Firstly, from the perspective of beam-column joints, the mechanism that slab improves negative moment bearing capacity of beam end is analyzed. Then from the perspective of structure, the influences of slab on frame yield mechanism is analyzed, and this analysis also indicates that negative moment bearing capacity of beam end can be improved by slab. The joint under low cyclic loading is simulated using ABAQUS software, and the effective flange width values in tension zone are analyzed from joint perspective.3D models with and without slab are established, and the non-linear dynamic behaviors are analyzed under severe earthquake, and effective flange width values in tension zone of slab are discussed from structure perspective.(2) Through the comparison of measured and calculated value of the basic natural vibration period of RC frame structures with or without infill walls, using experience formulas of different codes, it is found that the period should considered the influences of infill walls layout. In order to deeply investigate the influences, proper finite element model of RC frame structures with infill walls (shell element and hinged joints in comer) for dynamic analysis is discussed. And the model is verified through the comparison of measured and the finite element dynamic analysis value of natural vibration period. On this basis, the natural vibration periods of RC frame structures with different layout of infill walls are calculated. It is found that experience formulas should chose function of structure total height, and consider infill walls layout in the first floor. Simplified Rayleigh formulas method in China code is reasonable, but the reduction coefficient needs further discussion.(3) According to the interaction of infill walls and peripheral framework in horizontal loads, it is found that equivalent strut model is used to simulate the infill wall under small earthquake, and the rationality is verfied. The elastic dynamic time history analysis of the frame structures with different layouts of infill walls (pure frame without infill walls, frame with infill walls, structure, frame with non-uniform layout of infill walls in vertical and horizontal directions) are analyzed under small earthquake using equivalent strut model. The parameters of horizontal earthquake forces, top displacements, columns axial pressures, corner column strains are discussed in order to probe the rationality of pure frame model. According to the analysis results, suggestions on seismic design of RC frame structure are put forward.(4) In order to study the influences of slab and infill wall on the RC frame structure yield mechanism, the dynamic elastic-plastic time history analysis of two typical RC frame structures (with light and serious earthquake damages respectively) under Wenchuan earthquake have been performed. Three kinds of models that are model without slab, with slab, with slab and infill walls are established for each structure. Under severe earthquake, equivalent strut model for infill wall is not appropriate any more. Shell element and hinged joints in corner are used for modelling. The model of infill wall model is proved to be reasonable through the comparison of numerical simulation results and earthquake damage. For the influences of slab on the negative moment bearing capacity of beam end, RC frame structures have not obeyed the prospective yielding mechanism "strong column weak beam". Besides, pure frame model can’t reflect the weak storey, decorated by infill walls in vertical uneven, and infill walls have certain influences on the structure yield mechanism.