Analyses Of Structural Vulnerability And Collapse Modes Considering The Plastic Properties Of Materials

Truss and frame structures are widely used in the real world.Under accidental loads the key components of such structures may fail with the loss of bearing capacities,resulting in the progressive collapse of the entire structures.Therefore,it is important that the structural vulnerability should be analyzed in the interest of exploring the possible collapse mode.The methods based on the importance coefficients of components have been developed for such purpose,which assumes that structural components are made of brittle materials without considering the influence of plastic properties of materials on the bearing and deformation capacities of components.Thus the final failure modes of structures could deviate from the actual modes.Due to it,this thesis proposes an improved method for analyzing the importance coefficients of components,which involves the effects of plastic properties on the stiffness and bearing capacities of components before and after damage.By this means,rational structural collapse modes can be obtained.The main contents and research fruits of this thesis are described as follows.Firstly,existing methods for calculating the importance coefficients of structural components are introduced and their drawbacks are revealed by some examples.Then an improved method presenting the plastic importance coefficients of components is proposed that can simultaneously consider the elastic and plastic properties of materials during the vulnerability analysis.For different structures under different external loads,the stiffness of components experiencing plastic failure is modified in order to obtain stiffness coefficient matrices for all components and transfer coefficient matrices for plastic deformations.The coefficients related to structural bearing capacity are subsequently updated based on which the plastic importance coefficients of components are calculated.Meanwhile,according to the proportion of failed components and their failure sequence,4 levels are defined for the entire collapse of a structure.The final collapse modes are then determined on the basis of these levels.In the aspect of method validation,simple truss structures were first analyzed to obtain the plastic importance coefficient of each component and the final collapse modes,which were compared with the results given by the existing methods.Then the proposed method was applied to a reinforced concrete frame under vertical dead and live loads.The plastic importance coefficients of all the beam and columns were calculated and the collapse mode of the frame was explored based on the failure sequence of the components.Then the analysis results were compared with those from the existing methods and also with the observations from the laboratory test.By this means,the rationality of the proposed method has been verified.Lastly,the vulnerability of the same frame structure under horizontal seismic loads was analyzed.The plastic importance coefficients of all the beam and columns were calculated in order to obtain the failure sequence of these components and the final collapse mode of the frame.By comparing to the analysis results of the frame under conventional loads,the collapse mechanism of such frame structure under accidental loads can be found.In general,the proposed method can be regarded as an extension of the existing vulnerability analysis method.It effectively considers the contributions of the plastic properties of materials to structural stiffness and bearing capacities.Therefore,rational collapse modes close to actual situations can be explored.