Research On Caternary Action Of The Restrained Steel Beams In Fire

Steel structures have advantages such as light weight, super anti-seismic properties, attractive appearance, convenient construction and recyclability, so it's getting more and more widely used in building structures. At present, the design theory and design method of architectural steel structures at room temperature are mature. However the building fires occurred frequently in recent years have brought serious damage on the structural safety, which caused plenty of casualties and property losses. The fire-resistant research of steel structures has become the major direction of the fire-resistant research of architectural structures. Now the fire-resistant research of steel structures mainly focuses on the fire resistance of steel components or structures in the early stage and the heating process., the catenary action of steel beams, membrane effect of slabs and progressive collapse of steel frames in the later stage of the fire haven't been thoroughly studied. Based on the above research background, experimental study and numerical simulation on the caternary action of the restrained steel beams in fire are carried out in this paper; the failure modes, mechanical properties and the forming process of the caternary action of the restrained steel beams in fire are revealed; the factors which affect the caternary action of the restrained steel beams in fire are investigated, and the decision criteria of the caternary action of the restrained steel beams are proposed.Based on the summary of existing mechanical properties of steel at both room and elevated temperatures in standards of various countries, the material mechanical property tests on Q235 steel at both room and elevated temperatures are carried out, and the steady tests at elevated temperature are conducted to investigate material mechanical properties of steel. The mechanical properties of steel such as elastic modulus, yield strength, tensile strength and the stress-strain curves at both room and elevated temperatures are obtained through tests, which provide important initial data for the following experimental study and numerical simulation on the catemary action of the restrained steel beams in fire.Based on the material mechanical property tests on Q235 steel at both room and elevated temperatures, fire experiments on 5 restrained H-section steel beams are performed. The failure modes, mechanical properties and the forming process of the catemary action of the restrained steel beams in fire are revealed; the furnace temperature-time curves corresponding to each specimen, temperature-time curves of specimen and deformation-time curves of specimen at mid-span are obtained; the effects of load ratio n and axial restraining stiffness of supports Kx on the caternary action of the restrained steel beams in fire are investigated. The test results show that the load ratio n and axial restraining stiffness of supports Kx are the key factors on the caternary action of the restrained steel beams in fire. When the load ratio n is constant, as Kx increases, the large deformation of the specimen occurs later; when the axial restraining stiffness of supports Kx is constant, as the load ratio n increases, the large deformation of the specimen occurs earlier.Accurate finite element models of every specimen in the fire tests are set up using the finite element software ABAQUS; the heating transfer analysis and mechanical analysis are performed on the finite element models of every specimen using fully coupled thermal analysis method; the temperature-time curves of specimen and deflection-time curves of specimen at mid-span are obtained; the comparison show that although there exist errors, the overall trends of numerical simulation results agree with tests results, which verify the accuracy and feasibility of the finite element models.Based on the accuracy of the finite element models, parametric analysis of the factors that affect the caternary action of the restrained steel beams in fire are carried out, focusing on the axial restraining stiffness of supports Kx, rotating restraining stiffness in the supporting plane KzR, load ratio n, load type, span-depth ratio l/h and load distribution. The simulation results show that:? the axial restraining stiffness of supports Kx, rotating restraining stiffness in the supporting plane KzR and load ratio n are the key factors for the caternary action of the restrained steel beams in fire; ? different load types have little effect on the caternary action of the restrained steel beams in fire; ? the span-depth ratio l/h and load distribution have certain effect on the caternary action of the restrained steel beams in fire.Based on the parametric analysis, the decision criteria of the caternary action of the restrained steel beams in fire are investigated in this paper. The research results show that:for the steel beams with merely reliable axial restraints(0.3kb<Kx<1.0kb) or weak rotating restraints in the fire similar to the standard fire curve of ISO-834, when the load ratio n is small(0.1<n?0.5), the catemary action occurs when the mid-span deflection of the restrained steel beams reaches 1/15 of the span; the failure occurs when the mid-span deflection of the restrained steel beams reaches 1/11 of the span. When the load ratio n is large (0.5<n?1.0), the caternary action occurs when the mid-span deflection of the restrained steel beams reaches 1/18 of the span; the failure occurs when the mid-span deflection of the restrained steel beams reaches 1/12 of the span.