Study On The Deck System And Stiffness Of Four-lines High-speed Railway Cable-stayed Bridge With Two Main Trusses

Since the beginning of21th century more than10extra large span four-line high-speed railway steel bridges have been completed or under construction in China with the rapid developing of the construction of high-speed railway. The structure types of these bridges are diverse. Most of these bridges have three main trusses or arches. When only two main trusses are used, middle suspenders or side suspenders are often arranged to decrease the width of the deck system. BJ Bridge which is under construction now is a Four-lines High-speed railway Cable-stayed Bridge (FHCB) with the spans arranged as (57.5+109.25+230+109.25+57.5)m. Restricted by the construction conditions on site, only two main trusses can be arranged, and no space available on the deck to arrange middle suspenders or side suspenders. Cable-stayed bridges are flexible structures. The characteristics of FHCB with two main trusses are flexible, wide deck and large live load. To guarantee the safety and comfort of the service of high-speed trains, the stiffness of the whole bridge and the deck system is the key problem and should be pay more attention.Take BJ Bridge as background, the following problems are studied: the structure type of the deck system and their influence to the stiffness of the whole bridge, the stiffness and optimization of the deck system, the simulation method of the model test of deck system, and the calculation method of the final cable forces and the tension cable forces of the final tensioning. Main research work and innovative achievements are listed below.1. Aimed at the characteristics of FHCB, two deck system plans are presented. One is24m total wide monolithic deck system (A1), another is wide reduced monolithic deck system with side stringers and horizontal K-shaped braces (A2). A2is a innovative deck system type and has not applied before.2. The features of A1, A2and their influence to the stiffness of the whole bridge are studied. A calculation method of the level of deck system working together with the main trusses is proposed. The results show that the influence of the monolithic deck system to the stiffness of the whole FHCB is considerable. Under the action of the four-line worst-case live load,58%and62%of the cross section area of A1and A2work together with main trusses respectively,12%and14%of the deflection of midspan are cut down from that of the bare main truss structures (Ao) respectively,9%and10%of the maximum cable forces are decreased respectively.3. The local deformations of A1and A2deck systems under the worst-case live load are studied and compared. The physical significance and calculation method of the relative deformation of two trucks of one line are presented. The short wave and transversal wave of deformation of deck system, the absolute and relative displacements, and the transfer approach between them are also presented and expounded. The following conclusions are obtained. Compared with A1, the relative displacement of the deck system at the center of one panel of A2are decreased about6.7%to12.6%, the maximum curvature of the short wave and transversal wave of A2are decreased49%and9.2%respectively, and the relative deformation between the two trucks of the outer line and inner line within three meters of A2are decreased38%and48%respectively. A2is recommended to BJ Bridge and adopted.4. A Multi-variable multi-Objective Parallel Overlay Optimization method (MOPOO) is proposed. The computational formulas are developed and the calculating flow is presented. Using this method, the structure of A2is optimized based on the local stiffness of the deck system. The overlay better area of the relative displacement wc at the center of one panel and the relative deformation Δ1,2of the two trucks of outer line within three meters are obtained. In this area wc and Δ,2are all under better state while the steel consumption remains the same.5. Simulating A2deck system of BJ Bridge, a three panels segment model of deck system with the reduced scale1:6is designed and manufactured. The simulation methods of the axial load of lower chords and the elastic supports of the joints of main trusses from the cables are proposed. The tests of about ten load cases are completed. The deformations, stress states and the effect of horizontal K-shaped braces are investigated. The results show that the stress state of the lower chord is simple and clear because of the arrangeem.at of the side stringers. Most of the loads are transferred to the side area of the crossbeams by the side stringers, thus the crossbeams are under the best state, and the deformation and stress state of the deck system are also improved. The model tests verify the idea of proposal of A2.6. A conforming, equilibratory and optimized calculation method to calculate the final cable forces in the final tensioning is proposed. A double-combine incremental finite element simulation method to calculate the tension cable forces in the final tensioning is also proposed. The double-combine means the combination of finite element model with and without cable elements, and the combination of external forces and cable forces. The computational formulas and calculating flow are presented. In the construction the final cable forces can be obtained by only one round tension process using the final tension forces and tension sequence which calculated by this method, and the tension work is reduced and the safety of the bridge during tensioning can be guaranteed.The research results have already been used in the design and construction of BJ Bridge, and provide references to the study, design and construction of other bridges.