Research On Out-of-Plane Stability And Torsion Of Half-Through Truss Bridge

Half-through truss bridge has the structural characteristics of low girder height and open top.These characteristics make this bridge has the advantages of small space occupation,low elevation of bridge deck and unlimited height of vehicles.Because these advantages,half-through truss bridges are widely used in railway,port,military and urban landscape bridges.But there is no connection member between the top chords of this bridge,the lateral stiffness of the upper chords of is low,so the out-of-plane instability of the upper chords is easy to happen.The existing research and design codes are still imperfect.Aiming at the shortcomings and doubts of the stability of the upper chord of half-through truss bridge,this paper using theoretical method and numerical analysis did the research about four item:(1)Elastic Buckling Theory of Upper Chord,(2)Factors Affecting the Stability of Upper Chord,(3)Torsional Stiffness of Whole Bridge,(4)Nonlinear Stability Analysis.After these four studies,this paper mainly obtain conclusions as:(1)When calculating the buckling load of the upper chord,the existing research and design codes assume that the distribution state of axial force is parabolic or equal.Under certain conditions,the two simplified models have distinct deviation,and will influence the engineering application.The critical elastic buckling force of chords on half-through truss bridge is studied by this paper.In this paper,the axial force model of the upper chord is equivalent to step-shaped which is close to to the actual situation.The calculation results are well agreed with the FEM results.The calculation theory needs to consider the support effect of diagonal web members and the influence of crossbeam height.But the influence of bending and torsion deformation on the stability is not be considered,if the bridge is congenitally not equipped with a higher level of lateral bending stiffness or torsional stiffness,the critical buckling load can not be accurately calculated by the theory in this paper;(2)The dimension parameter which influence the stability of half-through truss bridge is width-span ratio because changing width-span ratio will change the lateral bending stiffness and torsional stiffness at the same time.Lateral bending stiffness and torsional stiffness will affect the stability of the upper chord,and lateral bending stiffness is the determining factor.The wind braces at the bottom can effectively contact two main trusses,and can effectively improve the stability and the lateral stiffness of the upper chord of half-through truss bridge by a large margin.When ahalf-through bridge is subjected to asymmetrically distributed loads,the lateral stiffness and torsional stiffness will also effect the axial force distribution of the upper chord,the lower the lateral stiffness and torsional stiffness is,the more unbalanced the distribution of axial force between left and right chords;(3)For calculating the torsional stiffness of half-through truss bridge,the main girder of half-through truss bridge is equivalent to a thin-walled member with an open groove section.Half-through truss bridge occupies small space volume,the chord and the main truss can resist spatial warping through bending,in this paper,the method to consider this gain effect is studied.Research results indicate that the gain effect is the main source of the torsional stiffness of half-through truss bridge.Installing diagonal braces in the height of deck can effectively improve the torsional stiffness of the main girder;(4)When analyzing the structural stability of half-through truss bridge,nonlinear of material and geometry should be considered,and initial imperfection has great influence on structural stability.When considering initial imperfection,should use the form of "One Half Wave Lateral Bending".The asymmetric distributed loads have a great influence on the nonlinear stability of the upper chord,at this point,one chord loses its stability before the other chord loses its stability and the bearing capacity drop sharply comparing with elastic result.For large-span half-through truss bridges,their torsional stiffness is inherently low,and the installation of diagonal braces in the height of deck has a good effect on improving their stability.The research contents of this paper can be used in the structural design of semi-open truss,and can also be used as a reference for damage assessment and reliability research.The related deficiencies need further study.