Research On The Mechanical Behavior Of The Multiplanar Joints Of Concrete Filled Steel Tube Structure

Compared with the hollow steel tube,the concrete filled steel tube structure is better featured in high bearing capacity,superior plasticity and toughness,construction convenience,strong fire resistance and good economic effect.As the key part of the concrete filled steel tubular structure,the joints' stress condition is complex and it is the most vulnerable part of all.So the bearing capacity of the concrete filled steel tube structure is often decided by the joint's load capacity.This paper mainly concerned with the mechanical behavior of the multiplanar joints of concrete filled steel tubular(CFST)by theoretical analysis and Finite element analysis.The detailed research works in this thesis mainly include the following four aspects:(1)Summarize the theoretical analysis method of the bearing capacity of concrete filled steel tubular joints and the derivation of the formula for calculating the bearing capacity from the basic theory of elastic-plastic theory.In this paper,the typical failure modes of concrete filled steel tubular joints are analyzed,and the theoretical calculation based on the classification of failure modes is used to calculate the joints' bearing capacity.(2)The Finite element models of concrete filled steel tube Multiplanar joints were carried out with the ABAQUS software platform based on reasonable selection of constitutive model of steel pipe and concrete,element type,interface model and grid technology.Through finite element analysis under different loading modes,the bearing capacity,failure mode and load displacement curve of the joints were analyzed.(3)The parametric analysis was performed on concrete filled steel tube multiplanar joints and the influence to the bearing capacity of the joints was obtained.Compared with the finite element analysis and the theoretical calculation of the bearing capacity of concrete filled steel tubular multiplanar joints,the finite element method is verified to be correct and the reasons for the difference between the finite element outcome and the theoretical one is obtained.(4)In practical engineering,the phenomenon of grout hole on the branch pipe and the concrete shortage is common.Based on finite element analysis,the influence to the bearing capacity of the phenomenon of grout hole on the branch pipe and the concrete shortage is analyzed.The influence of the size of grouting hole on the branch pipe can be analyzed through change the parameters,and reasonable treatment of grout hole on the branch pipe and the concrete shortage methods are proposed.The results indicate that filling concrete in the chord pipe and filling concrete both in the chord pipe and branch pipe can improve the bearing capacity of concrete filled steel tube multiplanar joints.The theoretical calculation based on the classification of failure modes can be used to judge the joints' failure mode and calculate bearing capacity.Changing of the parameters has an influence on the bearing capacity of the concrete filled steel tube multiplanar joints.But the influence must be analyzed based on the interaction of parameters rather than simply considering a single variable.The grout hole on the branch pipe and the concrete shortage will reduce the carrying capacity of the joints,which need to be dealt with in the actual project.