Behaviors Of Axially Loaded Tubed Steel Reinforced Concrete Stub Columns After Exposure To Fire

Tubed steel reinforced concrete(TSRC)column is a commonly-used kind of tubed concrete(TC)columns.The steel tube of a TSRC column mainly provides confinement to the inner concrete core without sustaining the axial load directly.Due to the constraint of concrete,the local buckling of the web and flange of the section steel can be avoided.The shear force in a TSRC column is mainly born by the section steel and thus the shear failure of the concrete can be prevented.Thanks to the composite action between steel and concrete,TSRC columns possess the advantages of high bearing capacity,good ductility,and ease of construction.TSRC columns are widely used in the bottom columns of high-rise buildings,where the axial load is heavy and the seismic performance is essential.Up to now,only the ambient-temperature and fire behavior of TSRC stub columns has been studied,whereas the post-fire behavior of TSRC columns has not yet been reported.The axial compression performance of stub columns is the most fundamental research for TSRC columns.It is necessary to investigate the residual bearing capacity and residual compressive stiffness of TSRC stub columns after fire exposure.The main contents and research results of this paper are as follows:(1)Finite element(FE)models to predict the post-fire behavior of circular and square TSRC stub columns were developed using the software ABAQUS,including a heat transfer model and a stress analysis model.The accuracy and effectivity of the FE models were validated by the collected test data in the relevant references.(2)The influences of seven key parameters,i.e.ISO-834 fire heating time,sectional dimension,steel tube yield strength,section steel yield strength,concrete compressive strength,sectional steel tube area ratio as well as sectional section steel ratio,on the residual bearing capacity and residual compressive stiffness of the circular TSRC stub columns after fire exposure were analyzed and discussed.The residual bearing capacity and residual compressive stiffness decrease with the increase of fire heating time and increase with the increase of sectional dimension.The increase of steel tube yield strength,sectional steel tube area ratio,concrete compressive strength,section steel yield strength as well as sectional section steel ratio can enhance the residual bearing capacity of circular TSRC stub columns significantly.In addition,with the increase of sectional section steel ratio and concrete compressive strength,the residual compressive stiffness of circular TSRC stub column increases obviously.Based on the ambient-temperature design method that recommended by the current design standard,practical formulae for determining the residual bearing capacity and residual compressive stiffness ofcircular TSRC stub column after fire exposure were proposed with the consideration of material degradation.The proposed formulae can predict the bearing capacity and residual compressive stiffness of circular TSRC stub columns well.(3)The constraint mechanism of steel tube to concrete core within the circular and square TSRC stub columns was compared.The influence of cross-section shape on the ductility index of TSRC stub columns was analyzed.The working mechanism of axially loaded TSRC stub columns was studied.In the whole process of axial compression,the load bearing ratio of concrete first decreases,then increases and finally decreases again.The change trend of the load bearing ratio of section steel is contrary to that of concrete.Through the parametric studies,the effects of various parameters on the post-fire performance of square TSRC stub columns were investigated.ISO-834 fire heating time and sectional dimension had obvious influence on the residual bearing capacity and residual compressive stiffness of square TSRC stub columns.Formulae for calculating the residual bearing capacity and compressive stiffness of square TSRC stub columns after fire exposure were proposed.The proposed formulae can predict the bearing capacity and residual compressive stiffness of square TSRC stub columns well.