| In recent years, lots of bridges have been built with the rapid development of high speed railway construction in China. The bridge is required to be low building height if limited by clearance under bridge and elevation at bridgehead when crossing over a river or a road. So steel through truss composite bridge is a good choice. The one with multi-crossbeam and concrete slab composite floor system has lower building height and better mechanical behavior than that with other type of composite floor system.This paper is supported by a scientific research project of ministry of railways. Taking a 96m high speed railway bridge as the background, the paper presents an in-depth and systematic study on the calculation theory and mechanical behavior of steel through truss bridge with multi-crossbeam and concrete slab composite floor system. Main innovation achievements are as follows:Firstly, the analytical solution and simplified formula for the level of floor system acting together with main truss are put forward. Utilizing the compatibility condition between floor system and bottom chord, the deflection expression of crossbeam is deduced and the analytical solution for the level of floor system acting together with main truss is reached. The influences of cross section area of bottom chord, width and thick of concrete slab, out-plane bending stiffness of crossbeam, and other factors are investigated. The simplified formula is reached by multiple non-linear regression analysis and reasonable simplification. The model test results show that both the analytical solution and simplified formula have good precision.Secondly, the analytical solution for the second system force of bridge floor is put forward. The concrete slab supported on crossbeams is taken to be the elastic supporting continuous slab. The effects of vertical flexibility and torsion of crossbeams are taken into consideration, "Five-moment Equation" and "Five-force Equation" under uniform distributed load and general load are deduced, and then the analytical solution is reached. The solution is verified by finite element results. Thirdly, the simplified calculation formula for the force-delivering ratio of two load transfer path and each crossbeam is deduced, and the bearing mode of main truss is put forward based on the research results in subparagraph 2. Several factors influencing the force-delivering ratio are investigated, such as the vertical-stiffness ratio of large crossbeam (node crossbeam) to small crossbeam (internode crossbeam), the ratio of the distance between crossbeams to the width of concrete slab, and so on. The conclusion of the bearing mode of main truss can be used in guiding the preliminary design of bridge.Fourthly, the 1:6 reduced-scale model tests of a 96m steel through truss bridge with multi-crossbeam and concrete slab composite floor system for high speed railway has been completed. The stress characteristic and the level of floor system acting together with main truss are researched, according to comparative tests by several loading cases for pure steel model, semi-composite model, and full-composite model. The model tests and finite element results reveal the fine accuracy of the calculation methods and formulas in this paper.Finally, the design and calculation method of stud in composite crossbeam is put forward, and the calculation formulas of stud shear force in the first and second force system are deduced respectively. Refined finite element analysis for the whole bridge is completed, simulating the relative slip between crossbeam and concrete slab. The stud force conditions in several layout schemes are investigated, which show that the design and calculation method of stud is highly practical and applicable.The research results in the paper provide evidence for design of steel through truss composite bridge and have been applied in bridge construction for high speed railway with remarkable economic returns and social benefits.
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