Dynamic Response And Damage Mechanism Of Reinforced Concrete Structures Under Blast Loading

Recently, terrorism happens much more frequently than ever all over the world. At the same time, accidental explosions might also occur because of the aging of the gas container or the electric machine and improper storage of tinder and explosive in chemical plants. These explosions not only cause the casualties and property loss directly nearby the explosive but also damage the structural components and structure some distance away from the detonation or even lead to progressive collapse of these structures due to the blast wave. In order to study the damage mechanism and progressive collapse of building structures in blast distaster, in this dissertation, three main problems in research of dynamic response and damage mechanism of building structures under blast loading are studied. They are (1) simulation of interaction between blast wave and structural column and derivation of the formulae to estimate blast loads acting on the columns; (2) dynamic response and damage of structural columns under blast loading and the corresponding damage evaluation method; (3) new method for progressive collapse analysis of reinforece concrete frames under blast loading. The primary work and achievements are as follows:(1) Simulation of blast wave propogation and its interaction with structural columns are studied. And the method for predictimng blast loads on standalone columns is proposed. The current empirical formulae to predict blast loads are all based on the assumption that the reflection surface is big enough so that no wave diffraction occurs. They may not be suitable for predicting blast loads on a narrow wall or a standalone structural column when there are no infilled walls or there is only glass proscenia between them because of the wave diffraction. Therefore, a method to simulate the interaction between blast wave and a standalone structural column is established using the Hydrocode AUTODYN. A numerical modification method is also proposed to correct the influence of the mesh size effect on the numerical results. Parametric studies are carried out to study the influence of column parameters, such as column stiffness, ratio of the supported mass to the column mass, on the blast wave–column interaction and the blast loads acting on the column. It is found that the ratio of the supported mass to the column mass has no significant effect on the blast wave–column interaction, as well as the column stiffness. However, both the column dimension and geometry have a significant effect on the blast wave–column interaction and the blast loads acting on the column. Finally, based on the numerical results of the parametric studies, some formulae are proposed to estimate the blast overpressure, impulse, and the reflected pressure time history on both the front and rear surfaces of any standalone column.(2) Dynamic response and damage modes of reinforced concrete columns under blast loading are studied. Based on the nonlinear dynamic analysis software LS-DYNA, a method for simulating the dynamic response and damage of RC columns under blast loading is established, in which the bond slip between longitudinal steel bar and concrete is also considered. Parametric studies are adopted to investigate the effects of column parameters on dynamic response and damage of RC columns and it is found that the blast resistant capacity of the RC column can be increased through increasing the moment of section inertia, concrete strength and transvers reinforcement ratio of the column. Numerical simulations are also carried out to study the possible damage modes of RC columns under different blast loads. The results show that when the column is subjected to impulsive blast load, the column is inclined to be damaged by shear; in the quasi-static region, however, the column is likely damaged by flexural mode; and in the region of dynamic loading, the failure of the column might be a combination of shear and flexural damage.(3) Methods for evaluate the damage degree of RC columns under blast loading are studied. There is no applicable method for evaluating damage degree of structural components under blast loading available in the literature. Therefore, in this dissertation, a new damage criterion for RC column is defined based on the residual axial load-carrying capacity of the RC column. Based on the new criterion, a simplified numerical method to generate pressure–impulse diagram for RC column is proposed. Some analytical formulae to predict the pressure–impulse diagram for RC column are also derived and its applicability and accuracy are verified through comparing the pressure–impulse diagram of a RC column derived from the proposed analytical formulae with that obtained from the SDOF approach. It is shown that the proposed method gives better prediction of pressure–impulse diagram than the SDOF approach.(4) New methods for progressive collapse analysis of building structures under blast loading are studied. The current available methods in analyzing structural progressive collapse under blast loading are not practical for common engineering application because the direct simulation method is too complicated and the alternative load path method is not accurate. Therefore, a new method for progressive collapse analysis of RC frames under blast loading is proposed. The new method solves the following three problems, including: 1) determination of the critical blast scenarios for a RC frame for progressive collapse analysis; 2) deriving non-zero initial conditions and initial damage of structural members caused by blast loads; and 3) numerical simulation of structural progressive collapse with non-zero initial conditions and damaged structural members. In order to valid the proposed method, comparisons between the results of progressive collapse analysis of a typical RC frame through the proposed method, the direct simulation method and the alternative load path method are carried out. It is found that the proposed method is efficient and reliable in simulating the progressive collapse process of RC frames as compared to the direct simulation method, and gives more accurate predictions of structural progressive collapse process as compared to the commonly used alternative load path method.