Research On Formulas For Bearing Capacity And Axial Stiffness Of Joints In Concrete-filled Rectangular Hollow Section Truss Bridge

Composite truss bridge with concrete-filled rectangular hollow section（CFRHS）has the advantages of light weight,high strength,good plasticity and tenacity.At the same time,it can be well popularized because of overcoming the construction disadvantages of the circular section.However,the joint of composite truss bridge with CFRHS is its weak part,and there is no obvious sign of deformation when the joint is damaged,so this kind of damage should be avoided in engineering.In view of this,supported by the National Key R&D Program of China（2016YFC0701202）,the bearing capacity and axial stiffness of rectangular hollow section（RHS）joints and CFRHS joints were studied in this paper.The main research work was carried out as follows:（1）The classification methods,detail requirements,and failure modes of RHS joints were systematically summarized.The graphical charts for Chinese engineers to design the joints were drawn according to the joints bearing capacity formulas of Standards for Design of Steel Structures.Based on the test data of 78 joints,the joints bearing capacity formulas in Standards for Design of Steel Structures,CIDECT 3 specification and Eurocode 3 specification were compared and analyzed.（2）The deformation differences between RHS joints and CFRHS joints were analyzed.For the bearing capacity of CFRHS compression joints,the calculation formulas proposed by Packer,Liu Yong-jian and Li were analyzed.For the bearing capacity of CFRHS tension joints,12 parameter models were established by the finite element analysis（FEA）method.In addition,the formulas for the effective distribution width failure of brace was modified.（3）Based on joints deformation,the simplified axial stiffness models of RHS and CFRHS T-joints were proposed,including a frame model,a plane strut-and-tie model,and a fixed end beam model.Then,by analyzing the simplified model,the theoretical formulas of joints axial stiffness were obtained.Furthermore,the calculation method of joints axial stiffness was obtained through discussing the applicability of the formulas.（4）The effective length of joint domain was the only unknown variable in the axial stiffness formulas of RHS joints and CFRHS joints.By FEA method,a total of 64 orthogonal FEA models were established to analyze the degree of influence of chord width（b₀）,brace-to-chord width ratio（β）,width-to-wall thickness ratio of chord（2γ）,height-to-width ratio of chord（α）,height-to-width ratio of brace（η）on the joints domain effective length.Based on this,a full-parameter analysis was carried out,and a total of 240 parameter models were established.According to the results,the formulas for calculating the joints domain effective length were obtained.Moreover,the accuracy of the formulas was verified.（5）Based on the calculation method of bearing capacity and axial stiffness of joints,the structural calculation of a CFRHS truss bridge was carried out.