Study On Characteristics Of Multi-span Concrete Extradosed Bridges

Extradosed cable-stayed bridges are transitional structure between girder bridges and cable-stayed bridges.The first prestressed extradosed bridge is built in Japan in 1994:Odawara Blueway Bridge.Ever since this new typology has become an attractive structural type around the world because these bridges have many strongpoints such as rational structure system,Suitable for many kinds of span,convenience construction,economy and comeliness shape.In China,the first cable-stayed bridge is built in 2000;it is still a new bridge type.In order to popularize and apply it in China,many questions are worthy of further investigation,such as the optimization method of stay cables,bridge construction monitoring,the analysis of the cable saddle,the analysis of maximum load bearing capacity and so on.A lot of research methods were used in the research,such as the theoretical study,a finite element analysis,a loading test.Four key technologies in design and construction of extradosed cable-stayed bridges were studied in the thesis.1.After a careful review of the existing theory of stay cables optimization,and an optimization solution was proposed based on genetic algorithm(GA)and multi-objective optimization theory.Based on the GA and Pareto solution,the paper develops a program by mixed programming of MATLAB and ANSYS to improve the cable forces distribution of the bridge.Ningjiang Songhua River Bridge,a multi-span extradosed bridge is cited as an example and is analyzed by the program,and the effectiveness of this method is proved by the analysis of inner force state before and after optimization.2.Impossible destructive modalities and secure reserve of the bridge can be acquired by the study of the ultimate bearing capacity for a bridge structre.Using the full-range analysis method based on nonlinear continuum mechanics,considering geometrical nonlinearity and material nonlinearity,the maximum load bearing capacity of Ningjiang Songhua River Bridge is analyzed.By using general-purpose finite element program ANSYS,two three-dimensional solid element models of the Ningjiang Songhua River Bridge are constructed respectively based on the five-parameter Willam-Warnke yield criteria and the Drucker-Prager yield criteria.Then,the entire load range of the bridge,which is from a design load to a failure load,is analyzed respectively by using the two models.Moreover,the corresponding ultimate bending bearing capacities for the two models under three kinds of conditions are obtained and are compared with the calculated values based on the current bridge code(JTG D62-2004).The average errors between the ultimate bearing capacities of the two finite element models and the bridge code results are respectively 2.7%and 6.5%,and the corresponding maximum errors are 5.9%and 8.6%,which means that the results based on the two finite element models accord well with the bridge code results.The structure works at the elastic state when the ratio of the load to the designed live load is less than 3.The structure with the load on its midspan is of a minimum surplus capacity factor of 2.08.3.Saddle is a very important member in extradosed bridges,its spatial stress analysis is one of the most difficult and pivotal problems in design.By means of the ABAQUS,the solid element models for the pylon and saddle were established and the numerical analysis of the saddle was conducted,respectively using the extended finite element method and the concrete plastic damage constitutive model.The stress distribution and crack condition of cable saddle using parallel steel pipes is calculated by numerical simulation.Under the action ofthe cable force of the parallel steel pipe,the cable saddles would generate a splitting crack which would run through the entire channel,and the crack had a depth of 25cm.The maximum principal compressive stress of the cable saddle zone occurred in the corner of the cable saddle reached a numerical value of 23.8 MPa,caused by stress concentration in the corner.However,since the value did not reach the standard value of the compressive strength of the concrete(fck=32.4MPa),the concrete would not reach compression failure.4.Ningjiang Songhua River Bridge adopted a continuous structure with 4 pylons and 5 spans,high redundancy is the characteristic of the bridge,construction process wound directly affect the finished state of the bridge.Cantilever placing cast method was used in construction,the structural stress and deformation changed and need to be monitored in construction.The results of the theoretical calculations are found to be well in accord with those of the measured values in construction monitoring of bridge.The results also satisfied the requirement of the code for design of highway reinforced concrete and prestressed concrete bridges and culverts(JTG D62-2004).Field test is an effective method for understanding the behavior and fundamental characteristics of a bridge structure.The analytical models have shown a good agreement with the experimental results under the loading conditions.Both the results of field static and dynamic load test and analytical results have indicated the bridge with adequate load-carrying capacity.