Research On Bending Resistance Capacity Of Concrete Filled Steel Tubular Truss Girder

Concrete filled steel tubular (CFST) truss girder, in which CFST chord members and circular hollow section (CHS) steel web members are directly welded together, is a new type of CFST structure subject to bending moment. Recently, CFST truss girder is widely used as stiffening girder for cable-stayed bridge or continuous rigid-frame bridge because of its merits like light selfweight, remarkable flexural rigidity, high bearing capacity, excellent seismic behavior, construction convenience and so on. Although a lot of research about CFST truss girder’s direct welding joint can be tracked in literatures, research about CFST truss girder’s overall mechanical characteristics under bending and structural design method is rather rare.This paper is devoted to model test investigation and finite element method (FEM) analysis of failure mode, bending resistance and structural design method of CFST truss girder, with main research content, output and results given as below:(1) Bending test of CFST truss girder under concentrated load at mid-span was conducted. Some important experimental results were observed in the following. Main failure mode of CFST truss girder is fracture of bottom chord in tension. Concrete filled in both chord tubes exerts notable influence on overall ultimate bending moment capacity of CFST truss girder but hardly changes chord’s failure mode. Bottom CFST chords would demonstrate typical mechanical characteristics of CFST members under tension once failure occurs.(2) Test results of CFST truss girder were modeled by FEM with general purpose software ANSYS, and main parametric study about failure boundary classification as well as influence limit were carried out. When concrete whatever filled in top or bottom chord was simulated with three-dimensional constitutive model, and steel with five-broken-line model, overall load-deformation curve and sectional strain distribution by FEM agree quite well with those from test. FEM parametric study reveals that shear-span ratio, chord sectional steel ratio, and web-to-chord strength ratio play great roles on critical failure modes and concerned bearing capacities, while web arrangement type and concrete strength grade slightly affect bearing capacities and don’t change failure mode.(3) With experimental investigation, FEM parametric study and practical example verification, design formulae of chord fracture as well as formula applying conditions for CFST truss girder were established on the basis of existing bearing capacity formulae for axially loaded CFST members.