Experimental And Analytical Study On Eccentric Compression And Seismic Behavior Of Recycled Aggregate Concrete Filled Square Steel Tube Long Columns

The mechanical properties and durability of recycled aggregate concrete?RAC?are lower than that of natural aggregate concrete,which restricts the application of RAC in the building structure.Pouring recycled concrete into steel tube to form RAC filled steel tube,the problem of poor durability of RAC is improved due to the airtight of the steel tube.The restraining effect of the steel tube on RAC makes its mechanical performance improved.The same superior performance as the concrete-filled steel tube structure has been obtained by RAC filled steel tube with the interaction of steel tube and core recycled concrete,which provid a new way for the application of waste concrete to the engineering structure.At present,scholars at home and abroad have less research on the mechanical properties of RAC filled square steel tubular long columns,It is necessary to study the eccentric compression and seismic performance of the RAC filled square steel tubular long columns in order to promote its application in practical engineering project.The eccentric compression tests were carried out on 15 specimens of recycled concrete filled square steel tubes,to study the influence of different factors on the eccentric compression performance of the specimens.On the basis of experimental research,the theoretical analysis and finite element simulation were conducted,and the following conclusions are obtained.?1?The failure of the specimens is divided into two types,one is the local buckling failure at the end of the specimen and the other is the overall bending failure,and the specimens of two types of failure have experienced the elastic stage,the elastoplastic stage and the decline stage of the bearing capacity.?2?The bearing capacity of the specimen decreases slightly with the increase of the replacement rate of the recycled coarse aggregate,decreases obviously with the increase of slenderness ratio and eccentricity,and increases obviously with the increase of steel tube wall thickness.Adding some expansion agent into RAC,the restraining effect can be produced in the expansion of core concrete,and the increase of the vertical bearing capacity of the specimen is about 14%.The bearing capacity of the specimen after adding fly ash has a tendency to increase,but the result has a certain discreteness.?3?The axial and lateral stiffness of the specimen appear a decreasing tendency with the increase of the replacement rate of recycled aggregate,and decrease with the increase of eccentricity and slenderness ratio.The influence of the slenderness ratio on the axial stiffness of the specimen is related to eccentricity,and the initial axial stiffness decreases significantly with the increase of eccentricity.With the increase of the wall thickness of the square steel tube,the axial stiffness of the column increases obviously,the lateral stiffness increases slightly,and the deformation ability increases significantly.?4?The lateral deflection curve obtained by the test shows that the deflection curve coincides with the sine half-wave curve when the eccentric compression specimen is subjected to the overall bending failure,and the sinusoidal half wave curve can be assumed in the theoretical calculation.When the eccentric compression specimen is subjected to the local buckling failure of the end of the specimen,the deflection curve in the elastic stage conforms to the sinusoidal half wave curve,but the specimen enters the stage of elastic-plastic and the decline of bearing capacity,the deflection curve is asymmetrical and the assumption of sinusoidal half wave curve has some approximationin the theoretical calculation.?5?The axial load-longitudinal and lateral strain relationship curves exist some difference for two different types of failure.For the specimens with overall bending failure,the longitudinal compressive strain and transverse tensile strain of the upper end and middle span of specimens are all higher than the yield strain values,square steel tube plays a very good restraining effect on the core RAC.For the specimens with the compression failure at the end of specimens,the variation of the longitudinal strain at the upper end of the specimen during the loading process is more significant than the lower end and middle span of specimens,and the transverse strain at the upper end of the specimen exceeds yield strain value.The square steel tube wall is restrained after adding ribbed stiffeners at the lower end of specimens,and the longitudinal and lateral strain values of the tube wall are relatively small,they do not yield until the specimen are destroyed.?6?Numerical simulation analysis on the mechanical performance of the recycled concrete-filled square steel tubular single eccentric long columns are carried out by the finite element software ABAQUS,the mechanical performance of the eccentric compression long column is fully analyzed.The influence of various parameters such as RAC strength,steel strength,eccentricity and steel rate on the performance of eccentrically loaded column were studied.The sensitivity method is used to determine the influence degree of each factor.Analysis shows that the vertical bearing capacity of the single bias long column is the most sensitive to eccentricity and steel ratio,is relatively less sensitive to the strength of steel and RAC.?7?The theoretical calculation formula of RAC filled square steel tube single eccentric compression column is derived from the limit equilibrium method.Strength reduction coefficient?₁ is quoted to consider the mechanical properties of RAC is poorer than natural aggregate concrete.With reference to the results of this test,the coefficient?₁ can be taken as 0.95 when the replacement rate of recycled coarse aggregate is 100%,and the coefficient?₁ can be taken as 1.0 when the replacement rate of recycled coarse aggregate is 0%,when the replacement rate of recycled coarse aggregate is between 0%and 100%,the insertion method can be adopted.The calculated values are in good agreement with the experimental values,and the discreteness of calculation results is small.The low cyclic-reversed loading tests on 6 specimens of RAC filled square steel tubes under constant axial load were carried out,with the changing of factors such as slenderness ratio,axial compression ratio and steel ratio.The seismic behaviors of the specimens were discussed such as the failure modes,mechanical performance,hysteretic property,ductility ductility ability,energy dissipation capacity,bearing capacity and stiffness degradation.At the basis of the experimental research,the theoretical analysis and finite element simulation were carried out,and the following conclusions are obtained.?1?Under constant axial load and cyclic lateral load,RAC filled square steel tube long columns have experienced the elastic stage,elastoplastic rising stage and the elastoplastic decline stage.When the specimen is destroyed,the plastic hinge,which can be rotated along the direction of loading,is formed at the bottom of the column,where RAC is crushed and tube wall is buckling,this failure is a typical bending one.With the change of steel tube wall thickness,axial compression ratio and slenderness ratio,there are some differences in the process of destruction and the characteristics of destruction.?2?The hysteresis curves of the specimens are more full,which shows that the specimens have good ductility and energy dissipation capacity.The hysteresis curve of the specimen with smaller wall thickness is slightly poor;the hysteresis curve of the specimen with large axial compression ratio is slightly plump;The hysteresis curve of the specimen with long slenderness ratio appears pinch phenomenon.The descending section of the skeleton curve of the specimens is relatively gentle,which shows that the later deformation ability and ductility of the specimen are better.With the increase of wall thickness,lateral stiffness increases slightly,and horizontal bearing capacity increases obviously.The lateral stiffness of the specimen with long slenderness ratio is smaller,and its horizontal bearing capacity decreases;with the increase of the axial compression ratio,the horizontal bearing capacity of the specimen increases slightly,and the descending segment of the skeleton curve is steepening.?3?The ductility factors of the specimens are between 2.61³.46,and the specimens can still maintain a slow decline in bearing capacity and experience greater deformation after reaching the above ductility factor.The yield drift of the specimens are between1/86¹/64,and the damage drift are between 1/29¹/20,which show that they have good anti collapse ability.With the increase of the steel tube wall thickness and slendness ratio,the ductility factors of the specimens are increasing;with the increase of the axial compression ratio,the ductility factor of the specimen decreases.?4?With the increase of cyclic displacement,the equivalent viscous damping coefficient?_eq of the specimen increases gradually.Coefficient?_eq is between 0.141 and0.157 when the specimen reaches the yield load,coefficient?_eq is between 0.221 and0.240 at peak load,and coefficient?_eq is between 0.340 and 0.394 at limit load,which shows that the RAC filled square steel tube column has good energy dissipation capacity.In the condition of the same displacement,the coefficient?_eq of specimen with big wall thickness is larger than which of small wall thickness;The coefficient?_eq of the specimen with big slendness ratio is smaller;the coefficient?_eq of the specimen with large axial compression ratio is larger.?5?The finite element software ABAQUS was used to simulate the hysteresis curve and skeleton curve of the specimen,and the calculated value is in good agreement with the test value.It is proved that the analysis model is reasonable and feasible for the aseismic analysis of RAC filled square steel tube column.According to sensitivity analysis theory,the influences of RAC strength grade,steel grade,steel tube wall thickness,axial compression ratio and slenderness ratio on the horizontal bearing capacity of the RAC filled square steel tube column were studied.The horizontal bearing capacity of the RAC filled square steel tube column is the most sensitive to slenderness ratio,followed by steel ratio,yield strength,axial compression ratio and compressive strength of RAC.?6?The influence of the second-order effects?P-??on the bearing capacity and deformation of the specimen was analyzed on the basis of experimental data.Adopting the national standard GB50936-2014 to calculate the compression-bending capacity of RAC filled square steel tube column,the calculating values are in good agreement with the test values.It is suggested that the reduction factor?₂ of elastic modulus of RAC should be introduced to the calculation of the lateral stiffness of columns,its calculation results are in good agreement with the measured elastic stiffness.?7?Three linear restoring force model of RAC filled square steel tube column is established by test fitting method.The curvilinear skeleton curve is divided into three linear segments:from the origin point to the yield point,from the yield point to the peak point and from the peak point to the failure point.The load and displacement of each specimen at the feature point are dimensionless treated on the basis of the measured data,and a simplified three linear skeleton curve model,which can be used to analyse the elastoplastic reaction of RAC filled square steel tube structure is given.?8?A variety of seismic damage assessment models were used to calculate the damage index D of RAC filled square steel tubular long columns at the different development process.According to quantitative index of storey drift in grade division of earthquake damage of buildings standard,and combined with the failure form of the specimen in the test process,on the basis of comprehensive analysis of the calculation results of various seismic damage assessment models,the damage quantification index of RAC filled square steel tubular long columns is given in accordance with aseismic performance level.