Research On Fatigue Behaviour And Calculation Method Of Joints In Concrete-filled Rectangular Hollow Section Truss Bridge

Supported by the National Natural Science Foundation and National Key R&D Program of China,the fatigue behavior of joints in the concrete-filled rectangular hollow section(CFRHS)truss bridge(a novel bridge type)was studied in this paper.The key technical problem of fatigue calculation method was theoretically and practically solved.The research work can offer evidences for the relevant coming specifications.The main research work and outcomes are summarized as follows:(1)As the first series of fatigue tests on CFRHS joints,a total of 26 CFRHS joints was tested,which included stress concentration factor(SCF)test and fatigue test.The SCF test included 11 Y-joints and 2 truss K-joints,where the influences of angle between braces and chord(?),brace-to-chord width(?),width-to-wall thickness ratio of the chord(2?),brace-tochord thickness ratio(?),concrete filling and PBL stiffeners on SCFs were investigated.The results indicated that the concrete filling can effectively restrict the outward deflection of the chord,thereby reduced the SCF at weld toe.Based on the CFRHS chord,installing the PBL ribs can further reduce the SCF at weld toe.In addition,the value of the SCF was related to the ?,?,2? and ?.The fatigue test included 11 Y-joints,where typical crack modes and fatigue lives under different nominal stress ranges were measured.Then S-N curve based on hot spot stress range was derived.The results showed that the uniform S-N curve based on hot spot stress range can be used for rectangular hollow section joints,CFRHS joints and CFRHS joints stiffened with PBLs.(2)The finite element model of CFRHS joints to predict SCFs was developed.In the finite element method,the element type,integral scheme,mesh size,contact relationship and weld simulation method were determined.A good agreement was found between finite element and test results.(3)The parametric analysis on the SCF of CFRHS T-,X-and K-joints with and without PBL ribs was carried out by finite element method.Correspondingly,the SCF parametric equations were proposed.A comparison on the SCF between CFRHS joints(with or without PBL ribs)and rectangular hollow section joints was made based on the proposed SCF parametric equations.The decreasing degree of the SCF on the CFRHS joints(with or without PBL ribs)to rectangular hollow section joints was quantificationally given.(4)Based on experimental and theoretical outcomes,the fatigue assessment method for the joints in CFRHS truss bridges was built up,where the SCF parametric equations and S-N curve based on hot spot stress range were included.In terms of SCF and S-N curve,the fatigue behaviour between CFRHS joints and the other tubular joints were compared.The results indicated that CFRHS joints had excellent fatigue behaviour among these joints.(5)A typical rectangular concrete-filled steel tubular truss bridge was selected as a calculation case.The entire bridge model was established and then the fatigue behaviour of its joint was assessed using the proposed hot spot stress approach.Finally,the anti-fatigue structure optimization of the original joint was carried out.The proposed approach to calculate the fatigue behaviour of the CFRHS joints is easy to understand in physical concept and accurate in the result.