Simulating Method For Failure Process Of RC Structures Based On RBSM

Reinforced concrete(RC)structures has been widely used in various fields of civil engineering,atomic engergy engineering,ocean engineering and mechanical manufacturing because of its high mechanical properties and superior economic indexes.According to relevant documents,the amount of concrete and steel bars used in China are respectively about 900 million cubic meters and 20 million tons per year,which is in the forefront of countries in the world.It is foreseeable that reinforced concrete will be one of the most important engineering materials for a long peirod of time in China.During the service of reinforced concrete structures,there will always be a variety of adverse factors,such as material aging,overloading,foundation settlement and so on,which will damage some parts of the structure.The continuous accumulation of minor damage is likely to lead to the final collapse of the structure which is the main source of heavy casualties and major economic losses.The key point to expand the living space of personnel and reduce the loss of property is to learn the damage and collapse process of RC structures,which makes it possible to ensure the structure in safe condition or collapsing in a reasonal manner.Numerical simulation is an effective tool for studying the failure process and failure mechanism of reinforced concrete structures.The failure process of reinforced concrete structure is a process of transforming a continuous medium into a discontinuous medium,and involves the problems of material nonlinearity,geometric nonlinearity,contact collision,discontinuous displacement field description and synergistic action of reinforced concrete.The numerical simulation of these questions is one of the most challenging topics in the field of numerical simulation at present.The research in this field is not only of great scientific significance,but also of great practical value.It can provide an effective research method for the anti collapse design of buildings as well as the evaluation and maintenance of the bridges in the service.Considering continuum assumption of displacements between adjacent elements,the transformation of continuous medium to discontinuous medium for continuous numerical methods,such as Finite Element Method(FEM)and Meshfree Method,must be achieved by various crack models,which makes it a complicated task to simulate the failure process of a RC component.Discontinuous numerical methods,such as Discrete Element Method(DEM),Particle Flow Code(PFC)and Discontinuous Deformational Analysis(DDA),don't rely on the continuum assumption of displacements and allow the adjacent blocks separating at the interface,which makes them efficient and convenient numerical tools to simulate the cracking processes of components.However,the stiffness of the interface springs in these methods are usually set according to experience,and the contact detection between blocks is a complicated and time-consuming task,so these methods are poor in accuracy and low in efficiency.Rigid Body Spring Method(RBSM)has both advantages of continuous and discontinuous numerical methods,and can show realistic behavior of components from cracking to failure.Recently,the theoretical research and project applications of RBSM mainly focus on rack mass and slope engineering,and very little,if any,published literatures describe the use of RBSM to analyze the failure process of RC components.In this dissertation,a modified RBSM capable of simulating the failure process of concrete and reinforced concrete components is presented,and the main research contents of the dissertation are following:1)Based on the basic principle of RBSM,a rigid spring model suitable for concrete material is presented.The spring stiffnesses of interface element of concrete components is derived by accumulating element deformation to the interface of adjacent blocks.The integral equilibrium equation of concrete components is constructed by applying the principle of minimum potential energy.The stiffness matrix of interface element and load matrix of blocks are obtained by applying simplex integral method.The implementation method of boundary conditions in RBSM is studied,and the corresponding programs are complied based on FORTRAN Language.The effectiveness of the proposed method is verified by the analyses of an elastic cantilever beam,and the results show that the results of elastic cantilever beam from RBSM is very close to the analytical solutions.Mesh shape and size have little influence on the numerical results in elastic analyses.2)The Mohr-Coulomb strength criterion has been improved,and the shear surface for concrete materials has been determined.Direct shear test of concrete material is designed,and the shear strength indexes of common used concrete are studied.The failure surface of concrete materials is given by combining the uniaixal strength of concrete with Mohr-Coulomb strength criterion.In order to avoid the conflict of failure surface,the shear failure surface under tensile stress is modified.Compared with the existing test methods to determine the shear strength index of concrete,the proposed method is simple and has low requirements for test equipments.The obtained data are reliable.The shear failure surface of concrete determined by the proposed method is more reasonable,which can provide reference for the study of shear failure behavior of concrete components.3)A rigid body model for analyzing the failure process of concrete materials is presented.Random mesh grids based on Voronoi diagram is adopted to generated the discrete model of concrete,which can effectively ease the effect of mesh shape on crack propagation.The nonlinear constitutive model of concrete which is applicable for RBSM is defined based on the uniaxial stress-strain curves of concrete,and the aggregate interlock effect of concrete is considered.The comparison between experimental and numerical results shows that the combination of Voronoi mesh and nonlinear constitutive model of concrete can greatly improve the ability of RBSM to analyze the failure behavior of concrete components.The nonlinear constitutive model slows the cracking process of concrete down,which makes that the crack has enough time to choose the most possible propagation path and can avoid the extensive problem caused by sudden stress releasing.Moreover,the randomness of Voronoi mesh can well reflect the influence of concrete aggregate on crack propagation path,which makes the failure process of concrete components closer to the fact.4)Two kinds of the reinforcement models considering the bond-slip action between concrete and reinforcement are proposed for the rigid body spring method to take into the defference between the mian reinforcement and constructional reinforcement.The two nuermical models for steel bars,are namely bar element model and smeared element model according to the distribution characteristics of steel bars in RC components.In bar element model,steel bars are modeled as a series of bar element which can be freely positioned in the domain.Each node of bar element has two Degree of Freedoms(DOFs)which are attached to the corresponding concrete block through a linkage element.Bar element model has clear mechanical concept,and can accurately locate the reinforcement.Moreover,bar element model is able to consider the bond-slip effect between concrete and reinforcement through the linkage element.In smeared element model,reinforcement is evenly dispersed into rigid blocks as a homogeneous axial material,and then it is concentrated on interfaces of adjacent blocks and presented as a uniform distribution of springs.Information input of reinforcement in smeared element model can be conducted by specifying the reinforcement ratio and direction,which greatly simplified the preprocessing of steel bars and is applicable to simulating the reinforcement with large and uniform distribution in RC components.5)A 3-D RBSM for RC structures is proposed in this dissertation.In the study,the two-dimensional concrete model and reinforcement model are extended,and the local coordinate systems for concrete interface elements and steel bar elements are particularly explained.Considering the difficulties in data organization and numerical integration for 3-D RBSM,an efficient vertex sorting method and a corresponding integration method based on simplex integration method are proposed.In order to verify the efficiency of the proposed method,numerical analyses of a T-shape cantilever beam and a RC simple beam are conducted.Results show 3-D RBSM proposed in this dissertation makes up the shortcomings of 2-D RBSM in simulating complicate spatial RC components,and the ultimate strength,crack distribution and load-deflection curve predicted by 3-D RBSM are close to the experimental results.Moreover,the proposed vertex sorting method and integration method are efficient,which effectively solve the problems of 3-D RBSM in data organization and numerical integration.