Study On Effective Numerical Model For Damage And Progressive Collapse Analysis Of RC Structures Under Blast Loading

In recent years,terrorist attacks and accidental explosions occur frequently.These sudden explosions will not only cause casualties directly,but also lead to damage or even progressive collapse of building structures nearby the source of explosion,and further intensify the disaster degree.Due to the rapid development of urban modernization in China,the urban construction and population are becoming more and more concentrated.Once the building structures collapse under blast loading,it will cause heavy casualties and severe social influence.Meanwhile,the effectiveness of blast resistant design,progressive collapse resistant design and post-blast emergency assessment of building structures depends on accurate and efficient models for damage and progressive collapse analysis.In order to promote the development of bast and progressive collapse resistant design theory of building structures,thus improving collapse resistant safety for structures,a systematic research is carried out from the material constitutive model,the component element model and the structural collapse model based on characteristics of collapse for reinforced concrete structures,and the damage simulation of materials,components and structures is achieved.And eventually,an effective numerical model for progressive collapse analysis of reinforced concrete structures under blast loading is established.The primary work and achievements include the following several aspects:(1)In the framework of classical continuum mechanics,the introduction of strain softening behavior of concrete will lead to the pathologically theoretical solution,the mesh-dependent numerical solution and zero energy dissipation of the physical failure process.In order to overcome the mesh-dependent problem in the numerical simulations,the nonlocal integral formulation is introduced into the Mazars local damage model to establish the Mazars nonlocal damage model,and the numerical integration scheme of the model is deduced,and then implemented in the general finite element code LS-DYNA.The static tension process and wave propagation of a onedimensional rod are simulated and analyzed,and the results show that the model can simulate the damage evolution process of the rod well,moreover,the simulated results are independent of the mesh size,and the validity of the model is verified.Meanwhile,the softening behavior of a reinforced concrete column under static load and the dynamic response of an impacted reinforced concrete beam are simulated and analyzed.The results agree well with the experimental data,and the mesh-dependent problem in descending branch of the force-displacement curve is eliminated successfully,which indicate that the model can objectively simulate the damage process of reinforced concrete members,and lay the foundation for simulation of damage or even progressive collapse of reinforced concrete structures.(2)The mesh-dependent problem of the post-blast residual capacity of reinforced concrete columns is studied.The whole process of a reinforced concrete column subjected to initial axial load,blast load and axial displacement loading is simulated under four different mesh sizes.The results show that the mid-span displacement response of the column can converge,while the residual capacity of the column increases with the decrease of mesh size.And the research findings indicate that the main reason for this problem is the lack of a characteristic scale in the concrete material model,which leads to mesh dependence of local damage.The strain rate effect is introduced into the Mazars nonlocal damage model and adopted in the calculation and analysis of the reinforced concrete column.The results show that the Mazars nonlocal damage model considering strain rate effect can eliminate the mesh-dependent problem of post-blast residual capacity of reinforced concrete columns,and provide a reliable guarantee for the damage and collapse risk assessment of the reinforced concrete structures after explosions.(3)An effective element model for analysis of dynamic response and damage of reinforced concrete beam/column components under blast loading is established based on the traditional fiber beam element,the modified compression field theory(MCFT),the Mazars nonlocal damage model considering strain rate effect and the three-axis isotropic elastoplastic model.A reinoforced concrete column under blast loading is simulated and analyzed,and the results show that the model can simulate the shear failure of the reinforced concrete column well,and it is in good agreement with the numerical results calculated by high-fidelity physics-based(HFPB)model and experimental results.A series of simulation analyses of a reinforced concrete beam subjected different load cases are carried out,and the results show that the simulated results under both the impulse type load and the general dynamic load agree well with the HFPB model,and the damage process of the beam can be well simulated.In addition,the computational time is compared between the effective model and the HFPB model,and the results show that the effective model not only has the computational accuracy the same as HFPB model,but also has the computational efficiency improved greatly,which provides the possiblility of damage or even progressive collapse analysis of reinforced concrete structures under blast loading.(4)The macro-based joint model is established by using the fiber beam element,the discrete beam element and the rigid link element in the finite element code LSDYNA,which is based on the observed phenomenon of longitudinal reinforcement slip and shear failure of the joint plate in collapse test of the reinforced concrete beamcolumn sub-assembladge.And the constitutive relation for the longitudinal reinforcement considering bond slip is deduced.The beam-column sub-assemblages are simulated by the proposed model,and the results show that the model can simulate the resistance of the compression arching mechanism and the catenary mechanism in the process of structural collapse,and can also reasonably reflect the rupture of the longitudinal reinforcement.Condiering the important role of reinforced concrete slabs on progressive collapse resistance of structures,a floor system model suitable for structural collapse analysis is established based on the fiber beam element,the rigid link element and the layered shell element.The collapse test of a beam-slab substructure is simulated by the proposed floor system model,and the results show that the model can simulate the whole process from the initial compression arching mechanism to the later catenary mechanism and even the rupture of steel bars.(5)An effective numerical model for progressive collapse analysis of reinforced concrete structures under blast loading is established based on extending the twodimensional macro-based joint model to the three-dimensional space and the previous research work on material and component levels.A 6-story reinforced concrete frame structure is taking as an example,a series of simulation analyses of the dynamic response and progressive collapse process are conducted under different column removal scenarios and blast loading conditions.The results show that the simulated results are in good agreement with the results calculated by the detailed model,and even the collapse process is identical within the two models,thus the validity of the proposed model is verified.A series of parametric analyses are carried out under the structural collapse case,and the results indicate that the steel layer at top of the floor can reduce damage of the slabs during the large deformation process,thus improving the collapse resistance of the reinforced concrete structure.In addition,the calculation time of the proposed model and the detailed model are compared,and the results indicate that the proposed model can improve the computational efficiency of about 50 times.A fast evaluation method for the progressive collapse resistant performance of reinforced concrete structures is proposed based on the proposed model,and the unltimate collapse resistance of the structure can be determined by simply conducting a static pushdown analysis.In this dissertation,the material constitutive model,the component element model and the structural collapse model are all integrated into the general finite element code LS-DYNA through developing subroutines.Excellent pre-and post-processing speed,numerical stability and efficiency can be obtained when simulating the damage and progressive collapse of reinforced concrete structures under blast loading,which provides a powerful tool for research of progressive collapse mechanism and design theory of reinforced concrete structures.