Study On Load Transverse Distribution Coefficient Of Three-Main-Truss Structure

In order to meet the requirements of railway transportation, traffic capacity of new railway bridge has been developed from the previous two-lane railway to the four-line railway. The location of some bridges is very important, and there are a number of railways or highways passing from the area. Therefore, combined railway and highway bridge project began to increase gradually, which sets higher requirements for the bearing and running ability. If the two-main-truss structure is adopted, then there are a few problems. For example, axial force of the main girder is large, stress of the beam is unreasonable, the height of the beam is too high, stress amplitude of vertical member is high, the integrity of the truss is bad, and the members are hard to make and fix. With the problems mentioned above, two-main-truss bridges are difficult to meet the requirement of the load, and it is not economical to build two bridges. As a result, three-main-truss structure is one of the best choices. Compared with the two-main-truss structure, the transverse distribution of deck load between three main trusses is more complicated. The deck load transverse distribution of the three-main-truss structure is studied on this thesis through the following aspects:Based on the classical calculation theory of load transverse distribution coefficient, the calculation formula of the elastic supported continuous beam method which was derived by the initial parameter method was introduced. And modified calculation formula of the elastic supported continuous beam method was derived by considering the longitudinal torsional stiffness of three-main truss structures. Equivalent simply supported beam method which was used to calculate the load transverse distribution coefficient for continuous beam bridge was introduced. The formula for calculating the conversion coefficient of the moment of inertia of the three-span continuous beam was derived. The formulas were verified by examples.The calculation method of the load transverse distribution coefficient of simple supported beam and continuous beam in different position of longitudinal axle was discussed. The bending stiffness parameter a was introduced. Conversion coefficient of the moment of inertia Cw in different position of three-span continuous beam bridge was derived. The formulas were verified by examples.Equivalent plane model of cable stayed bridge and suspension bridge with three cable planes and three main trusses was built. And results from the plane model by multiplying the load transverse distribution coefficient were compared with the spatial model results.The above analysis and results of the corresponding numerical examples show that: Modified elastic support continuous beam method in the calculation of the load transverse distribution coefficient of simply supported beam and continuous beam can achieve the same accuracy requirements with the modified eccentric pressure method. The formulas of the bending stiffness parameter a and conversion coefficient of the moment of inertia Cw are proved to be correct by examples. The error of the plane model results by multiplying the load transverse distribution coefficient is small compared with the error of the spatial model results. Thus the method can be used in the preliminary design calculation of three-main-truss stracture. In the cable stayed bridge and suspension bridge with three cable planes and three main trusses, the ratio of the cable force in the middle cable and the side cable is about 1.1 under the dead load and specified symmetrical live load, in which dead load occupies the main part. It proved that the cable stayed bridge and suspension bridge with three cable planes and three main trusses are reasonable structures.