Refined Numerical Simulation And Seismic Performance Analysis Of Low-Ductility Reinforced Concrete Structures

China is a country susceptible to earthquakes characterized by dense urban population and wealth concentration.The coexistence of newly-built and old buildings as well as their distinct seismic capacities highlight the potential threat that engineering structures in urban areas are faced with in China.The weakness and key point of anti-seismic issues for urban engineering structures primarily focus on the existing RC structures that have been in service over years characterized by low seismic precautionary intensity and insufficient construction measures.Based on the researching background as stated above,this doctoral dissertation redefines the concept of "low-ductility structure" as well as its design features by conducting literature review,earthquake reconnaissance and seismic code study.Taking the existing low-ductility RC structures and their members as the main research objects,this research is primarily conducted by means of refined numerical modeling and analysis.The contents of Chapter 2,Chapter 3 and Chapter 4 are developed around the two main lines which are finally combined together using numerical substructure method to achieve the refined analysis on the seismic performance of low-ductility RC structures.The primary research content of this dissertation comprises five aspects as follows:(1)The refined modeling approaches for fundamental RC members are studied and developed.The calculation of equivalent principal strain and expressions of tensile damage index for bar-based combined element are proposed.After that the refined modeling approaches of fundamental RC members including beams,columns,shear walls and slabs are further developed using bar-based combined element method to capture the strong nonlinear behavior,reflect the damage evolution process and predict seismic failure mode of the structural members.The effectiveness of this method is verified with pseudo-static and shaking table test results.The applicability and applicable scope for three typical bar-based combined elements as truss-based model,frame-based model and beam-truss model are explored to study the key influencing factors.The typical shear wall test,such as the pseudo-static test of coupled shear wall that considers the coupling effect between wall panels and coupling beams is introduced to conduct the model verification and parametric analysis.(2)Two categories of novel refined analytical models for RC beam-column joint are proposed.The first category of joint combined element considers both shear deformation of joint panel as well as bond slip of beam reinforcement at the joint core area.The shear stress-strain relationship for the joint core area can be determined based on the modified strut-tie model.The range of parameter determination at the joint core can be expanded and further hierarchically verified with a series of member and structure test results.In order to avoid the predetermination of performance parameters at the joint core area and meanwhile achieve the real-time update during the analysis,the second category of detailed modeling approach for the beam-column joint based on the truss-based model is proposed.This model considers adopting new concrete compressive strength degradation theory and concrete constitutive relationship according to the mechanical behavior of the joint region,and is correspondingly verified with test results.(3)The seismic performance of low-ductility RC frame structures is analyzed and evaluated based on the proposed joint combined element.Combined with Wenchuan earthquake reconnaissance and the development history of codes for structure seismic design,the influence factors for the seismic performance of low-ductility RC structures are sumarized.The key design parameters for typical low-ductility RC frames can be determined by use of parameter sensitivity analysis.A method to build detailed finite element numerical model for low-ductility RC frame structure is developed based on the proposed joint element.The seismic performance of low-ductility RC frame structure is evaluated with the use of nonlinear static analysis method,incremental dynamic analysis method and the proposed mainshock-aftershock earthquake sequences method.The seismic failure mode,seismic collapse margin and seismic performance margin under mainshock-aftershock earthquake sequences for the two categories of structures are systematically investigated.(4)The elasto-plastic numerical substructure method is developed to study the failure mode of seismic-damaged low-ductility multi-story RC frame structures in Dujiangyan City.By means of the numerical substructure method,the developed analytical model of columns and proposed two categories of refined joint elements are applied to analyze the seismic behavior of a severely-damaged low-ductility multi-story RC frame during Wenchuan earthquake.Four refined numerical substructure analytical models of different scales based on this prototype frame structure are established and nonlinear seismic analyses are performed.The seismic performance and failure mode of the structure under strong ground motions are investigated to reveal the damage evolution process and disaster mechanism of critical structure members under earthquakes.(5)The refined modeling and analysis of collapsed low-ductility medium rise bearing wall buildings in Chile are conducted.The pre and post processing programs for the proposed bar-based combined element is developed.A method to build three-dimensional refined numerical model for low-ductility RC shear wall structure is presented based on the proposed bar-based combined element.An as-built low-ductility medium rise RC bearing wall building that collapsed during the 2010 Chile earthquake was selected for modeling.The three-dimensional refined numerical analytical model of the as-built structure based on the proposed bar-based combined element was established.In this model,a series of influence factors are considered such as the strength and stiffness degradation behavior,real-time degradation of concrete compressive strength,wall-to-slab coupling effects and size effect of mesh partition,etc.The possible precipitating factors explaining the overturning-type collapse of the prototype building during earthquake are explored via nonlinear static analysis and measured-ground-motion-based dynamic time-history analysis.