Study On Secondary Stress And Numerical Analysis Of Parallel Chord Steel Truss Bridge

The trusses are normal structures in steel construction. Trusses can be used to be the tensile bars, compression bars, flexural components, torsional components. When the truss was used to be flexural component, it is called bridge truss. The forcing characters of the truss are reasonable, easy to calculate, convenient construction, strong adjustability, and there no lateral forces are to bearing support. When the span of a bridge is longer than 40 meters, the truss structures are reasonable. Simply supported truss bridges are the most common use in the bridges of the railroad.The research method of the thesis is focusing on a truss bridge to the theoretical study and numerical calculation about the secondary stress of parallel chord steel truss bridge basing on others study production. The situations are complicated, when the trusses are forced. We must ignore subordinate factors for predigestional calculation, and reflect main forcing factors. We use the ideal pin-connected model, instead of using the real truss model, when calculating the internal forces of the truss bridge. We adopt basic assumptions as follows:(1) The nodes are pin-connected; (2) The axis of the poles are straight and intersect at the center of node; (3) the action line of the load passes the nodal point of truss. If the truss bridges accord with the above-mentioned assumptions totally, it would just have axial forces in the truss members. The stresses which caused by axial internal forces of the chords are called direct stress. But in the factual structures, we usually adopt gusset plat and high strength bolt to joint the bars. In fact, the nodal points of truss bridges are stiff connected. When it was bending evidently, the truss members would be caused end moment. Furthermore, the center of gravity do not intersect at the center of node, it will cause eccentricity moment because of eccentricity. If we are careful in designing and making, the secondary stress can be avoided. So in the thesis I ignore the eccentricity of truss members in nodes.The thesis set up space model of a through parallel chord truss bridge by MIDAS. I divide the models to pin-connected and rigid-connected two types. Through rigid-connected model I set up semi-rigid-connected infinite model by releasing the girder end restriction. By calculating, I get the numerical stress of truss members with semi-rigid-connected, and contrast with the detection stress. The real joints of nodes are rigidity, but the calculated models are pin-connected. So it causes differences of stresses. According to the result of the calculation, the secondary stresses in upper chords and web member are very small, so it can be ignored in designing. In designing we can think that the nodes are pin-connected. The stresses in lower chords are smaller, so we must calculate the secondary stresses in designing. By releasing beam end restrictions step by step, the stresses in lower chords match with detection stress under the situation of releasing 25% restriction, so in designing we can release the girder end restrictions to 25%. In the dynamic calculations, we can validate the results are matched with the results of the dead loads calculations.