Study On Mechanical Properties Of Hybrid Truss Bridges

The concrete composite girder bridge with steel truss webs, also known as hybrid truss bridge (HTB), is a new type of bridge structure with good development prospects, and this bridge type is still in the preliminary stage of development in China. This paper takes HTB as the research object, and the calculation theory of deflection, the shear lag effect of cross-section and the eccentric load effect are researched.The finite element method is adopted to do analysis and research on the basic mechanical properties of the prestressed concrete composite girder with steel truss webs, which provides a basis for further structural analysis and calculation. The results show that under load, deformation of steel truss web section does not meet the plane cross-section assumption; cross-section bending moment of the prestressed concrete composite girder with steel truss webs is mainly undertaken by the top and bottom concrete plates, and shear force is mainly undertaken by the vertical component force of steel truss webs.It was verified that the classical beam theory is not appropriate for the deflection calculation of HTB. Based on the mechanical properties of the prestressed concrete composite girder with steel truss webs, combined with the thin-walled box-girder conversion method, the deformation control differential equations for the prestressed concrete composite girder with steel truss webs are derived, and the theoretical formulas for deformation calculation of the prestressed concrete composite girder with steel truss webs under typical boundary conditions and load cases are given. The effect of the width-span ratio, the width-height ratio and structural geometric parameters on the accuracy of theoretical formulas was analyzed. The range of application for theoretical formulas was given, and the accuracy and stability of theoretical formulas within the given range were verified.The shear lag effect of simply supported concrete girder with steel truss webs under three typical loads, i.e. the mid-span concentrated load, the two points symmetrical load and the uniform load are analyzed. comparing The shear lag effect of the prestressed concrete composite girder with steel truss webs for different structural systems was compared. The results indicate that for continuously supported prestressed concrete composite girder with steel truss webs under uniform load, the negative shear lag effect is quite obvious near the cross-section of mid-support, which needs to attach great importance in the design. Based on the analysis results of shear lag effect, the effective distribution width of the prestressed concrete composite girder with steel truss webs was calculated. It was found that the effective width of simply supported prestressed concrete composite girder with steel truss webs calculated by the method of specification is conservative. The influence of the width-span ratio, the width-height ratio and structural geometric parameters on shear lag effect of the prestressed concrete composite girder with steel truss webs was analyzed. The results showed that the width-span ratio has greater influence on shear lag effect of the prestressed concrete composite girder with steel truss webs, while the width-height ratio and other parameters have less influence on shear lag effect.The normal stress eccentric coefficient of the prestressed concrete composite girder with steel truss webs under live load was discussed and compared with that of the ordinary box-girder. The results show that the eccentric load effect of the prestressed concrete composite girder with steel truss webs is significant, and the normal stress eccentric coefficient empirical value should be amplified appropriately for the torsional design of this type of girder. Furthermore, the eccentric coefficient of the prestressed concrete composite girder with steel truss webs was analyzed using equivalent truss webs method, and the formula for calculating the equivalent thickness of webs was given, which can be taken as a reference for the eccentric load effect research of the prestressed concrete composite girder with steel truss webs.