Long-Term Behavior Of Axially Loaded Circular Tubed Reinforced Concrete Stub Columns

As a new type of composite members,tubed reinforced concrete(TRC)components have been widely used in the support structures of super high-rise and long-span spatial structures due to their high bearing capacity and good seismic performance.Long-term deformation will aggravate the deformation of the vertical members.Due to the difference in the deformation of the vertical members and the gradual accumulation of the differences,the additional stress of the horizontal members may be caused,which may even cause the building to tilt and affect the normal use function.Due to the tri-axial stress state of core concrete in this kind of component,stress redistribution between steel and core concrete,no vertical load carried by steel tube,and creep Poisson's ratio effect which would induce the development of constraint with time,the time-dependent behaviour of circular tubed reinforced concrete is relatively special,which makes it impossible to use the analysis method of concrete filled steel tube or reinforced concrete columns.Therefore,this paper conducts experiments and theoretical research on the time-dependent behaviour of circular tubed(reinforced)concrete stub columns subjected to axial compression.The main work is presented as follows.(1)Carried test and comparative analysis of three measurement methods and measured shrinkage deformation in three kinds of sealing method.Determined the mature long-term behaviour test method for tubed reinforced concrete.The bolt was pre-embedded in the concrete,and the measuring bracket was welded by bolt to measure the axial deformation of the member.Used a waterproof aluminum foil tape to seal the seam to achieve a good sealing quality.(2)The long-term tests of 8 sets of tubed reinforced concrete stub columns under axial compression were conducted to investigate the time-dependent behaviour of this kind of component.The concrete strength grade was C50 and the initial stress level was 0.4.The effects of loading age(7d,28 d,90d,180d),reinforcement ratio(0%,2.2%),the steel ratio(3.7%,7.7%)and bonding friction(brushing oil,no brushing)on the time-dependent behaviour of CTRC components were studied.The test results show that after 60 days of loading,the long-term deformation reached 30%~90% of the elastic deformation at the loading time.The loading age and the reinforcement ratio have a great influence,and the steel content and the bonding friction have little effect.(3)The long-term deformation numerical analysis method of tubed reinforced concrete members considering the constraint time-dependent and the vertical stress redistribution of the section was established,and the test results were verified.Furthermore,a parameter analysis was carried out.Concrete compressive strength,concrete age at first loading,reinforcement ratio,and ratio of steel area over concrete area were used to investigate the long-term deformation characteristics of CTRC.The simplified calculation method of creep(effective modulus method,mean stress method and age-adjusted effective modulus method)for the long-term deformation calculation of CTRC was proposed.Based on the step by step integration method,the comparative analysis of the effective modulus meth od,the average stress method,and the effective modulus method of age adjustment was carried out on the accuracy of the long-term deformation analysis of circular tubed reinforced concrete.It was suggested that the mean stress method could be used in practical use.