Analysis For Punching Shear Failures Character Of Thick Flat Raft Foundation

As the most important and complicated part of structural design, foundation design affects the security, cost and applicability of structure directly. At present, thick raft foundations have been applied widely for high-rise buildings, but some problems still exist as follows: the mechanics characteristic of raft foundation is very complicated;the reliability is difficult to be determined and the analysis of punching shear has not gained a satisfactory result. Taking thick raft foundation as object of study in this thesis, the selection of critical section at collapse destroy is analyzed and the effect of reinforcement ratio on ultimate stress is discussed. Trough program of ADINA for finite element analysis, raft distortion, subgrade reaction as well as reinforcement stress is analyzed when destruction of interior, side and comer slab-column connections occurs.Under comparison of critical section with different codes, nominal shear posses no physical meaning but calculational meaning instead. The final purpose is to make calculated strength closer to precision value, so the design codes of punching shear vary among different countries. The analysis results show that the calculated values with our code are far less than the experimental values, and the critical perimeter can be properly increased The critical section of nonrectangular column is supplied in this paper and the formulation of present code is applied to calculate the ultimate bearing capacity, which is matched with experimental value. Thus, this critical perimeter can be used as reference for the design codes of punching shear failures.In the process of calculating punching shear strength with technical code for tall building box foundations and raft foundations (JGJ6-99), the influence of reinforcement ratio is not taken into account. By comparing experimental and calculated values of ultimate bearing capacity, the feasibility of finite element simulation is determined, and then, the failure mechanism of slab-column connection in the situation of different reinforcement ratios is proposed. The comparison results show that the curvefitting of BS8110 is far more excellent than others. Taking BS8110 as reference, it's more reasonable for explicit formula to replace implicit formulaDuring the process of ADINA analysis on a test model, nonlinear material properties, nonlinear contacting of subgrade with foundation, connections of concrete and steel are taken into account. Bearing performance of slab-column connections is analyzed according to the values of effective stress and shear stress, and the results show that ultimate bearing capacity of interior slab-column connection is more than that of side position, and the capacity of side position is more than that of corner position. It also can be shown that the reinforcement stress follows the contrary principle as ultimate bearing capacity does. On the same level of reinforcement ratio, adding proper amount of reinforcement radio in corner and side position is effective to improve the reliability of raft foundation.