Fire Resistance Of Concrete Filled Rhs Columns Under Three-surface Fire Loading

Concrete filled steel tubular (CFST) columns have gained wide applicatioin in multi-storey and high-rise buildings for their advantages of high load carrying capacity, good ductility, flexibility of construction and high fire resistance. As one form of CFST columns, Rectangular Hollow Section (RHS) columns acquire more attention for their high flexural rigidity, simple construction of joint besides advantages metioned above. Fire resistance is one of the most important factors affecting building safety, current criterions and specifications in operation, CECS159:2004 and DBJ13-51-2003, etc. assume CFST columns being exposed to fire uniformly. But in fact, because of the barrier effect of adjacent walls, columns maybe under three-surface fire loading, which would result in asymmetry thermal field and then form additional eccentricity owning to offset of section material strength center. In addition, asymmetry thermal field causes dilatational strain of surface exposed to fire is greater than that not exposed to fire, which arouses additional dilatational strain. It's therefore significant need to investigate RHS columns under three-surface fire loading based on previous research on those under four-surface fire loading. Experimental research and finite element analysis are carried out for RHS columns under three-surface fire loading, includeing:(1) Experimental research of RHS columns under three-surface fire loadingThree full-scale RHS columns, including one column subjected to four-surface fire loading and the others three-surface fire loading, were tested, thermal field of cross section, axial deformation and lateral deformation were measured. The failure mode of the columns was obtained and the affection of load level and fire boundaries to fire resistance was analysed.(2) Finite element analysis of RHS columns under three-surface fire loadingThree-dimensional FEM model was developed using ABAQUS to analyze thermal field and fire resisrance of RHS columns and the results were validated by the experimental results. Then the mechanism of RHS columns under three surface fire loading and sustained load was researched, including thermal field, failure mode, distribution of stress, strain and evolution, etc.(3) Parametric studies of RHS columns under three-surface fire loadingParameters were studied to investigate their influence on thermal field and fire resistance, including the influence of exposure time, dimension of cross-secition, steel ratio, thickness of fire proof and depth to width ratio on thermal field; the influence of load level, dimension of cross-section, slenderness ratio, steel ratio, strength of steel and concrete, load eccentricity, depth to width ratio and thickness of fire proof on fire resistance, summarize the rules of their affection. A simple equation was put forward for predictinig the fire resistance of RHS columns under three-surface fire loading, which may provide a reference for engineering applications.