Fire Resistance Behaviour And Design Of Concrete-filled Shs In Non-uniform Fires

By combining the mechanical advantages of steel and concrete materials, concrete filled square hollow steel (SHS) columns have been gaining their popularity in high-rise buildings recently due to the excellent moment bearing capacity, convenience in construction, and high fire resistance. In concrete-filled SHS applications, the fire resistance design is one of the crucial issues as the steel tubes are exposed when the structures are against fire. However, most current codes and researches focus on the situation when columns are under four-side fire, and can not be adopted to predict the bearing capacity of SHS columns under three-side, opposite-two-side or single-side fire which also occur in real applications. Concrete-filled SHS columns under the other three conditions may have lower overall temperature and less severe damage in materials than those under four-side fire. The absence of exposing to fire in some sides may also cause additional lateral temperature deformation and increase the bending moment on columns, making the static behaviour of such columns different from those ones under four-side fire. In this contest, experimental and theoretical studies were carried out to investigate the fire resistant behaviour and the ultimate capacity of concrete-filled SHS columns under high temperatures, including the following four main parts.(1) Experimental study on the fire resistant behaviour of concrete-filled SHS columns under three-side and single-side fire.Six full-scale SHS columns subjected to sustained loading with different loading levels or loading eccentricities were tested to collapse, among which four were under three-side fire and the other two were under single-side fire, to investigate the deformation-loading curves and the failure mode of the columns. The measurement includes both thermal and mechanical parameters, like the temperatures at the representative points along the cross-section, the axial deformation, the lateral deflection at the mid-hight of the column, and the ultimate fire resistance capacity, et al.(2) Cross-sectional temperature distribution of concrete-filled SHS columns and parametric analysisBased on the thermal properties of material components like steel, concrete and fire coating, finite element models were built by ANSYS to investigate the typical temperature field and temperature distribution along the cross-section of concrete-filled SHS columns under three-side, opposite-two-side and single side fires with the consideration of the contact thermal resistance. The models were verified by test results from papers published to date. Parameteric analysis was then conducted to discuss the influence of the exposure time, the area of steel over concrete, the dimension of cross-sections, and the thickness of fire coats on the temperature distribution along the cross-section of concrete-filled SHS columns.(3) Theoretical analysis on the fire resistant behaviour of concrete-filled SHS columns at high temperaturesTheoretical analysis were conducted using Fortran 90, considering the path of constant loading under elevating temperatures, to investigate the fire resistance of concrete-filled SHS columns under three-side, opposite-two-side and single side fires with the material thermal-mechanical parameters determined according to the collected studies on the thermal expanding coefficients, the strength, the elastic modulus and the stress-strain relationship for steel and concrete under high temperatures. Using the test results in this thesis and those from published papers as benchmark, the analysis method was proved to be reliable to predict the fire resistant behaviour of concrete-filled SHS columns. The axial deformation versus time curves and the mid-height deflection versus time relations were investigated for the concrete-filled SHS columns under single-side, opposite-two-side, and three-side fires, respectively, to clarify the mechanism of the columns under different fire conditions and the influence of the different fire boundary conditions.(4) Parameteric analysis on the fire resistance of concrete-filled SHS columns under non-uniform fires and practical designParameteric analysis was carried out to investigate the influence of the loading level, the cross-sectional dimension, the slenderness ratio, the loading eccentricity, the steel and concrete strength, the ratio of steel area over concrete and the thickness of the fire coats on the fire resistant capacity of concrete-filled SHS columns under single-side, opposite-two-side and three-side side fires. Based on the analysis results, equations were brought up to predict the residual strength of concrete-filled SHS columns with parameters whithin the range of real concrete-filled SHS applications when subjected to single-side, opposite-two-side and three-side side fires. Designing recommendations for the thichness of spray insulation were suggested for concrete-filled SHS columns under different fire resistant levels.