Thermal Field And Fire Resistance Of Concrete Filled Shs Columns Under Two-adjacent-surface Fire Loading

Concrete filled steel tubular (CFST) columns have advantages of high load carrying capability, good anti-seismic capability, convenient in construction and cost economy. As the steel tube is exposed to fire directly, the fire resistance of CFST columns is a principal factor to determining the safety of the building in fire. Current criterions and specifications about fire safety design of buildings (including the CFST structure) are all centered on columns in uniform fire, while in actual structures the frame columns may be exposed to fire with single surface, two adjacent surfaces, two opposite surfaces or three surfaces because of the existing walls. Asymmetry thermal field results in asymmetry material degradation, which would lead to the additional eccentricity. Meanwhile, asymmetry thermal field results in asymmetry dilatational strain, which induces columns'additional deformation towards the surface exposed to fire. Corner columns of buildings are generally in two-adjacent-surface fire, which would yield biaxial additional eccentricity and biaxial additional deformation. Stress mechanism of columns in two-adjacent-surface fire is different from columns under other fire loading conditions then there's significant need to investigate fire performance of these columns. Thermal field, deformation, fire resistance of concrete-filled square hollow section (SHS) columns were analysed and a simple equation was developed for predicting the fire resistance. Three main parts of this paper are as following:(1) Thermal field analysis of concrete-filled SHS columns under two-adjacent-surface fire loading.A transient heat transfer FEM model was developed using ABAQUS to analyze the thermal field of concrete-filled SHS columns under fire, whose results were validated by the experimental results. The difference of thermal field of columns in two-adjacent-surface fire and four-surface fire was investigated. Parametric study was conducted to analyze their effect on the thermal field of concrete-filled SHS columns in two-adjacent-surface fire, including exposure time, the dimension of cross-section, steel ratio and the thickness of fire proof.(2) Mechanical performance analysis of concrete-filled SHS columns under two- adjacent -surface fire loading.Based on analysis of thermal field, the FEM model for calculating fire resistance of concrete-filled SHS columns in two-adjacent-surface fire was developed. The axial deformation versus time curves, the mid-height deflection versus time curves and fire resistance can be attained and validated by the experimental results. Finally, the stress mechanism of columns in two-adjacent-surface fire was studied.(3) Parametric studies on the fire resistance of concrete-filled SHS columns under two- adjacent -surface fire loading.Parametric studies was performed to explore their influence on the fire resistance of concrete-filled SHS columns under two-adjacent-surface fire, including load level, dimension of cross-section, slenderness ratio, load eccentricity, strength of steel and concrete, steel ratio and the thickness of fire proof. Then a simple equation was developed for predicting the fire resistance of concrete-filled SHS columns in two-adjacent-surface fire. The influence of fire boundary (four-surface, three-surface, two-opposite-surface, two-adjacent-surface or single-surface fire loading) on fire resistance was discussed.