Hysteretic Behavior Of Recycled Aggregate Concrete Filled Square Steel Tubular Stub Columns Based On OpenSees

Due to the urbanization and the improvement of rural housing,the consumption of concrete has been at a constantly high level for years.This needs a large amount of aggregate which results in permanent destruction of the surrounding environment in the source area.Meanwhile,due to urban reformation,a large number of old buildings and roads were demolished,among which 40% was waste concrete.For the purpose of environmental protection and waste recycling,many experimental research efforts were devoted to reuse the waste concrete and produce recycled aggregate concrete(RAC).Encasing recycled concrete inside steel tube to form the recycled aggregate concrete filled steel tube(RACFST),significantly improve the side effects of the inclusion of recycled aggregates on the concrete,including low strength,low elastic modulus and high scatter in mechanical properties.To date,extensive research work has been conducted on the static,seismic and long-term behavior of RACFST with circular cross-section.Yet limited research has been carried out on the RACFST with square cross-section,even fewer references are focused on their seismic behavior,even though the square cross-section are more commonly used in buildings.In this context,this thesis presents the finite element analysis on the seismic behavior of the RACFSST columns.The main research work includes:(1)Comparative analysis was carried out to specify proper uniaxial stress-strain relationship of the concrete core and steel tube for square RACFST columns used in OpenSees,and values for the key coefficients were suggested based on the analysis results.A finite element analysis model was then built to investigate the seismic performance of the RACFSST columns.The reliability of the finite element results ws benchmarked against the available experimental results.(2)Parametric analysis was conducted to investigate the seismic behavior of square RACFST columns.Considered parameters include the RA replacement ratio,compressive strength of concrete core,yield strength of steel tube,the ratio of the steel area over the concrete area at the cross-section,the axial load ratio and slenderness,et al.It was shown that the axial load ratio,slenderness ratio and the ratio of the steel area over the concrete area at the cross-section are key factors influencing the seismic resistance of square RACFST columns.The RAC replacement ratio has relatively significant influence on the ductility of this kind of composite columns.(3)Based on the finite element analysis results,the restoring-force model for square RACFST columns was proposed by modifying the peak value of lateral force and the corresponding displacement,the deterioration stiffness in the available restoring-force for square CFST columns using natural concrete.The formula for predicting the ductility coefficient of the square RACFST columns was then established based on the proposed restoring-force model,which is in good agreement with the finite element results and the experimental results.·The analysis results indicate that within the parameter ranges that may be used in real application,the square RACFST columns has excellent ductility with the ductility coefficients all above 3.0.