Fier Performance Of Square And Rectangular Tubed-Reinforced-Concrete Columns With End Restraints

The tubed-reinforced-concrete(TRC)column,also known as steel tube confined reinforced concrete(STCRC)column,possesses some advantages,e.g.high bearing capacity,good ductility,excellent seismic performance and ease of connection to RC beams.In recent years,TRC columns are gaining usage in high-rise buildings and large-span structures.However,the fire performance of square and rectangular TRC columns has not yet been studied.Furthermore,the fire performance of a column in a frame is inevitably affected by the adjacent beams and columns and beam-column joints,causing its fire performance is highly coupled with the end restraints and different from that of the pin-ended column.Motivated by the above-mentioned issues,this thesis intends to investigate the fire performance of square and rectangular TRC columns that pin-ended,with axial and rotational end restraints,and integrated in a single storey-single span frame and to develop structural fire design methods for TRC columns with different end conditions.The research methods of this thesis include experiment,finite element modelling and theoretical analysis.The main contents are listed as follows:(1)Seven square and rectangular TRC columns were tested to failure,under the combined actions of ISO 834 standard fire and concentric or eccentric loading.The evolution of cross-sectional temperature,axial deformation,end rotation and mid-height lateral deformation over time of the tested specimens were measured.The failure modes and fire resistance of the tested columns were obtained.Typical heat transfer and deformation behaviour of TRC columns in fire were revealed.Based on the test results,the influences of load ratio,load eccentricity and sectional aspect ratio on the fire resistance of square and rectangular TRC columns were discussed.(2)A sequentially-coupled thermo-mechanical finite element analysis(FEA)model was developed using ABAQUS for the fire performance analysis of pin-ended square and rectangular TRC columns,which was validated against the fire tests conducted in this thesis and the collected test data of concrete-filled steel tubular(CFST)and RC columns.The load redistribution in the TRC columns during heating was analysed.The fire performance of TRC columns was compared with that of CFST columns with the same steel amount.Through parametric studies,the influences of sectional dimension,load ratio,slenderness ratio,load eccentricity ratio,sectional aspect ratio and fire protection thickness on the fire resistance of pin-ended columns were analysed.Simplified formulas were proposed to calculate the temperatures of square and rectangular TRC columns without and with sprayed fire protection.Practical design methods were developed to determine the high-temperature concentrically-loaded and eccentrically-loaded buckling resistance of pin-ended square and rectangular TRC columns,which remained consistent with the ambient-temperature design.(3)A FEA model was developed to study the fire performance of square and rectangular TRC columns with end restraints.The axial and rotational restraint levels at column ends in non-sway frames were determined and these restraints were modelled effectively using the spring elements.The influences of either only axial restraint or only rotational restraint or axial and rotational restraints on the fire performance of TRC columns were investigated.Simplified design methods were developed to determine the axial force and buckling length ratio of TRC columns at failure and then to determine the fire resistance of TRC columns with constant end restraints.The proposed design methods were of clear physical meaning and consistent with the design methods of pin-ended columns.Methods to determine the temperature-dependent restraint stiffnesses were proposed.The influences of temperature-dependent restraints on the column fire resistance were investigated and recommendations for the fire resistance design of TRC columns with temperature-dependent end restraints were proposed.(4)Considering the overall effect in frame structures,a FEA model was developed to study the fire performance of single-storey and single-span TRC frames.The sectional temperatures,overall deformations and internal forces and moments in the frame and its failure mode were studied.The influences of column load ratio and beam load ratio on the fire behaviour of the frame were revealed.The fire resistance of the frame column was compared to that of the non-frame column with either pin-ended boundary conditions or end restraints,after which practical recommendations were given for the fire resistance design of frame columns.Fire design methods were developed for the TRC columns within three levels,i.e.pin-ended,constant/temperature-dependent restrained,and restrained in a frame.