Post-fire Behaviors Of Slender Reinforced Concrete Columns Confined By Square Steel Tubes Under Compressive And Flexural Load

Steel tube confined reinforced concrete(STCRC)columns is a new kind of composite member,which is fabricated by filling concrete into steel tube.But steel tube is terminated at beam-column connections,therefore no axial load is applied on steel tube directly and the steel tube mainly provides confinement to concrete.This kind of member has been widely used in practice for its high load bearing capacity and ease in construction of the beam-column connections.Untill now,extensive researches have been conducted on behaviors of STCRC columns at room temperature whereas few studies have been reported on post-fire performance of STCRC columns.The avilable studies on post-fire behaviours focus on circular columns and square stub columns.Due to the difference of stress mechanism between columns in circular and square section and difference of failure mode between stub columns and slender columns,it's necessary to conduct research on the post-fire behavior of STCRC slender columns.The main contents and achievements are listed as follows:(1)Twelve eccentrically loaded square STCRC columns were tested,including eight columns after exposure to ISO 834 standard fire and four columns at room temperature.Three parameters are considered,including heating time(45min,90min),eccentric ratio(0.25,0.5)and column height to cross-section dimension ratio(3,10).The experiments consist of two parts,which are heating and cooling phases and mechnical tests.The temperatures in specimenswere recorded during the heating and cooling phases and then the load-displacement relationships and strains of the steel tube were measured during the subsequent compressive tests.The test results show that the residual load capacity decreases obviously with the increase of these three investigated parameters.(2)A finite element(FE)model including heat transfer analysis and stress analysis was developed using the program ABAQUS to investigate the post-fire behavior of STCRC columns.The Finite element model was validated by comparing FEresults and test data.(3)Based on the FE model,parametric studies were conducted to identify their influences on the buckling capacity.These parameters include heating time,slenderness ratio,cross-section dimension,material strengths,steel tube to concrete area ratio,reinforcement ratio and load eccentric ratio.The results show that slenderness ratio,load eccentric ratio and cross-section dimension are the main factors affecting the buckling capacity and compressive-flexural capacity.The heating time have slight influence on stability coefficient.The buckling capacity and compressive-flexural capacity increase with the increase of cross-section dimension,material strengths,steel tube to concrete area ratio and reinforcement ratio.(4)Based on the post-fire cross-sectional capacity formulas of STCRC columns,formulas of buckling capacity and compressive-flexural capacity were proposed by inducing effects of global buckling and compressive-flexural effect.