Research On Numerical Simulation Of Masonry-infilled RC Frames Using XFEM

Masonry-infilled reinforced concrete(RC)frames,as one of the most common forms of building constructions,have been built in many parts of the world.In the structural analysis and design,infill walls are generally treated as non-structural components and their effects on the seismic behavior of structures are ignored.However,they will interact with the bounding frame when the structure is subjected to a strong lateral load in the event of an earthquake.This interaction has a significant impact on the seismic performance of the structures,and it may alter the lateral load bearing capacity of buildings,and induce some failure mechanisms that are undesirable for the seismic performance of the structure.Despite significant research effort dedicated to such buildings during the last decades,there are neither clear understanding on the seismic performance of infilled frames nor a reliable numerical approach for evaluating the behavior of such structures.The main challenge is because of the complexity of interaction between the infill and the frame.Hence,it is necessary to develop a reliable analysis approach to reveal the interaction between the infill and the bounding frame and access the behavior of such structures.In this thesis,extended finite element method(XFEM)is improved to better simulate the cracking behaviors of quasi-brittle materials,then a sophisticated numerical simulation method using the improved XFEM is proposed for masonry-infilled RC frames.A parametric study is also conducted to consider the effect of different parameters on the behavior of masonry-infilled RC frames,which reveals the interaction mechanism.Finally,a simplified analysis model for masonry-infilled RC frame structures is proposed by considering th e actual state of all parts of the structure at peak load.The main content includes following four parts:(1)Extended finite element method is one of the most powerful method for dealing with the discontinuity problems.To properly modeling the fracture behavior of quasi-brittle materials such as concrete and masonry in infilled frames,a sophisticited plasticity-based rigid interface constitutive model coupling cohesion softening,contact,friction and shear dilatation,is proposed and efficiently incorporated into the XFEM formulation.An effective numerical integration scheme for accurately assembling the contribution to the weak form on both sides of the discontinuity is also introduced.The effectiveness of the proposed method has been assessed by simulating three well-known experimental tests in MATLAB.It is concluded that the numerical method can successfully capture the crack paths and accurately predict the fracture behavior of concrete structures.The influence of mode-II parameters on the mixed-mode fracture behavior is further investigated to better determine these parameters.(2)The current numerical models for masonry-infilled RC frames are struggling with the mesh-dependent problem,and failure patterns can not be successfully captured.On the basis of the further improvement on extended finite element method,with compressive yielding and cracking of the concrete and masonry units and multiple distributed cracks considered,a XFEM-based finite element modeling for masonry-infilled RC frames is implemented in the Finite element code FEAP,and it concerns three user defined elements and two material constitutive models and a macro command.In this modelling,the XFEM is adopted to model the cracking behavior and the compressive failure of con crete in RC frame members and masonry units in infill panels,and the discrete interface element is employed to simulate the behavior of mortar joints between masonry units and the behavior of the frame-to-infill interface.Preprocessing and postprocessing for the modelling are implemented in MATLAB.The models of several masonry-infilled RC frame specimens have been constructed in the FEAP software to demonstrate the applicability of the proposed finite element model.The numerical results have shown that the model can capture the damage patterns of the frame specimens.The results are also in good agreement with the load-displacement curves obtained from the experiments.(3)With the verified finite element modeling for masonry-infilled RC frames,a parametric study has been systematically conducted to consider the effect of different material properties,geometrical dimensions and design parameters on the behavior of masonry-infilled RC frames,which concerns aspect ratio of the infill panels,vertical loads applied on the specimens,the ratio of vertical loads between the frame columns and beam,longitudinal reinforcement and transverse reinforcement of the frame columns,sectional dimension of the frame columns,thickness of the infill panels and height of the frame beam.The results of the study indicate that the influence extent and general tendency of these parameters on the structural bearing capacity and failure patterns,reveal the interaction mechanism between the inflls and the bounding frames under the lateral loads.It provides a powerful data support for establish a simplified bearing capacity and numerical model for masonry-infilled RC frame structures.(4)Analyzing the results obtained from the simulation of the experiment and the parametric study,it found that in most cases considered here,the failure involves a dominant shear-sliding crack in the infill and diagonal shear cracks near the top of the windward column and the bottom of the leeward column.However,these three components have not reach their peak capacity simultaneously at the instant of peak capacity.Hence,the load carried by the structures is less than the load corresponding to failure of the three at the same instant.Finally,the results from the above study have been used to develop a simplified procedure for the behavior of a masonry-infilled RC frame.The vertical load distribution on the infill and the simplified lateral force-displacement curve is studied by numerical fitting analysis.Then,a diagonal strut model is proposed.The proposed simplified approach has been validated with the test results used above.The comparison indicates that the results of this method are very encouraging as it successfully captures the important features of the lateral force-vs.-drift response.This method can be further used to model multistory infilled frames as braced frames.