Post-fire Behavior Analysis On Square Concrete Filled Steel Column-Steel Beam Connections

The properties of structural material are greatly varied in high temperature caused by fire, which lead the damage or even collapse of structures. With the increased use of concrete-filled steel tubular (CFST) columns in structures, it is becoming more and more important to study their behaviour after exposure to fire in order to give an assessment and repair the fire-damaged structure. And the behavior of CFST beam-column connections is needed to be studied in order to analyze the performace of the whole structure. The paper presents a finite element analysis on square CFST column-steel beam connections by use of finite element methods based on ABAQUS. The main achievements can be summarized as follows:First, based on the determinations of material thermal properties and concerning about the effect of water vapor, a finite element model was established to calculate the temperature field of square CFST column-steel beam connections during fire. Some testing results, including tests of CFST colums, steel connections and CFST column-reinforced conrete beam connections, were used to verify the rationality and feasibility of the theoretical modeling. Using this model, the temperature field during fire including the cooling stage of square CFST column-steel beam connections was discussed and a parametric study was conducted. The parameters include: fire duration time, sectional dimention, thickness of the concrete slab and thickness of the column fireproof material.Furthermore, after choosing proper material constitutive relations, model element type and the calculation method, a finite element model was established to calculate the load versus deformation relations of square CFST columns-steel beam connections after exposure to fire. The modle also concerns about the constraint factors and the interface model between the concrete and the steel tube. Some testing results with defferent environmental temperature and loading path, were used to verify the effectiveness and feasibility of the theoretical modeling. The tests includes the CFST column at ambient fire, in fire, after constant fire and after exposure to fire, as well as the CFST column- steel beam connections at amient fire and after exposture to fire. The calculated results using the FE model are on the whole in good agreement with the experimental results, which proved the validity of the model.Finally, with the finite element models, condisdering the heating and loading history, the mechanism of square CFST column-steel beam connections working in entire stage of fire was discussed, including: loading at ambient temperature, heating, cooling to the ambient temperature and post-fire loading to failure. The load versus deformation relations and the bending moment versus beam rotation relations were analyzed. The effects of the entire stage of fire were studied. The axial stress distribution of the column section, the deformation and stress of the connections as well as the interaction between concrete and tube of the column were also discussed. Finally, a parametric study was conduted. The parameters includes: axial load level, time of fire heating and the beam-column stiffness ratio, strenth of concrete and steel.