A Study Of Critical Element Of The Structure Duringthe Progressive Collapse And Sensitivity Analysis

The progressive collapse of the buildings is an extremely harmful disaster, which always leads to serious loss of life and property. It has been one of the hot-spots of the civil engineering study. Finding out the critical element of the structure during the progressive collapse, and determining the sensitivity of the progressive collapse to the design parameters, is meaningful to the resisting progressive collapse design. When the critical element and the sensitive parameters of the structure are ascertained, the resisting progressive collapse design can be done purposefully and targeted, so that the disaster can be controlled with the most efficiency. However, in the world wide, the explicit method to decide the critical element has not been put forward so far. Instead, several structural measures were provided to prevent the disaster. In this article, the formulas of the critical element were deduced, setting out from the working characteristic of steel beam during the stage of catenary and the energy method respectively. What’s more, with the help of finite element AB AQUS software, an exploratory study about the sensitivity of the progressive collapse to the design parameters was done to a reinforced concrete frame. The main work can be expounded in the following respects:1. Based on the analysis of working characteristic of steel beam during the stage of catenary, the formula of maximum permissible displacement of the steel beam was deduced. And then, the concept of "Characteristic Removing Factor (μ)" of the column was drawn out, which was used to calculate quantitatively the importance index of a certain member during the progressive collapse. A dynamic nonlinear analysis was done to a practical project with the finite element ABAQUS software, imitating the working conditions of removing corner column, middle column of long side, middle column of short side and inner column. The analysis shows that, the element importance index can be reflected by the method of Characteristic Removing Displacement to a certain degree and the result is consistent with the general project experience. What’s more, the method can be applied to structured programming calculation.2. The energy method is a good entry to the design to resist progressive collapse. As an evaluation method for the element importance, the element index was defined as the change of deformation energy during the process of removing the certain element, which was visual to understand. And on that basis, the Neumann Series was introduced to the calculation of element importance index for simplifying. This method avoids the step to solve the assembled stiffness matrix of the structure repeatedly after damage. The calculation efficiency was improved obviously with high precision. In addition, the concept of "Extending Importance Index" was further proposed, which could be used to do the comparative analysis of the danger of the structure when several elements were damaged simultaneously. The analysis result of a practical project shows that, the method is rational and applicability.3. With the help of finite element ABAQUS software, a group of computation models of reinforced concrete frame were built, whose design parameters (number of floor levels, seismic fortification intensity, web reinforcement ratio, etc.) were different from each other. The degree of seriousness of the progressive collapse was presented by the Alternate Path Method. In addition, a comparative analysis was further completed. The analysis result showed that the progressive collapse was more sensitive to number of floor levels and web reinforcement ratio, rather than the seismic fortification intensity.