# Fatigue Performance Test and Numerical Analysis of Composite Girders with CSW-CFST Truss Chords
## 1 Introduction  
Due to the composite bridge, corrugated steel webs and concrete-filled steel tubular (CSW-CFST) chord (Figure 0(a)) have less torsion stiffness and a narrow space between the corrugated steel web and the CFST chord which is adverse to welding [1]. As a result, a new type of composite structure has been proposed [2], referred to as the composite girder with CSW-CFST truss chords (Figure 0(b)). The composite girder with CSW-CFST truss chords, which replace the concrete bottom slab and steel truss webs, has many advantages, including reducing the dead load, improving fatigue crack resistance [3], avoiding localized joint damage [4], and preventing buckling of steel chords subject to hogging moment by concrete-filling the chords [5]. Recently, the composite girder with CSW-CFST truss chords has been successfully applied in some bridges, as shown in Figure 2.  
The research community has provided valuable and interesting results on the relevant static performance of composite girders with CSW-CFST truss chords. The shear lag effect in composite girders with CSW-CFST truss chords was investigated. The results showed that concrete-filling in the chord had little influence on the shear-lag effect [6]. A model test and numerical analysis of the torsion capacity of composite girders with CSW-CFST truss chords was carried out, and a simplified calculation formula for their torsion rigidity and resistance was presented [7]. The flexural capacity of continuous composite girders with CSW-CFST truss chords was tested [8]. The study analyzed the influences of the main structural parameters on flexural capacity, and found that the ultimate capacity of thecomposite girder with concrete-filling in the chords was 1.22 times that without concrete-filling in the chords. The dynamic impact coefficient and live load correction coefficient of composite girders with CSW-CFST truss chords was researched [9]. The failure modes of composite girders with CSW-CFST truss chords under the combined action of bending and torsion stresses was investigated [10].  
While there is relatively mature theory relating to the statics of composite girders with CSW-CFST truss chords, practical engineering applications have not considered the fatigue performance, and no fatigue performance evaluation methods have been defined in design specifications. Currently, information about fatigue model tests and numerical analyses of composite girders with CSW-ST truss chords can only be found in [11], in which hot-spot stress distribution, fatigue failure modes, and fatigue life have been explored.  
The research results above have demonstrated that concrete-filling in the chord could effectively improve the static properties, and existing studies [12; 13; 14; 15; 16] show that concrete-filling in the chord can improve the fatigue performance of the joints. However, previous fatigue testing has not involved full model testing but only testing of joints. Thus, to investigate the influence of concrete-filling in the chord on the fatigue performance of composite girders with corrugated steel webs and steel tubular (CSW-ST) truss chords, a fatigue performance test and numerical analysis were conducted in this study. The study focused on the extrapolation method and the distribution of hot-spot stress, the mechanism by which concrete-filling in the chord influences the fatigue performance, and fatigue failure modes. A fatigue life evaluation method for composite girders with CSW-CFST truss chords is proposed and discussed using the fatigue performance test results.