Research On Static Behaviour And Design Approach For PC Pylon Anchorage Zone Of Cable-stayed Bridge

Due to its long-span capacity,convenient construction and aesthetic feature,the cable-stayed bridge has been widely developed in China during the past four decades.As the key part in cable-stayed bridge,the pylon anchorage zone characterized by complicated construction technology,stress concentration,design difficulty etc.,has attracted much interests among bridge engineers recently.However,the research in design approach for pylon anchorage zone is insufficient,and current design code lacks of specific instructions,resulting in that time-consuming and expensive model tests and repetitive FEM analysis are often required to verify the design in engineering practice.Based on strut-and-tie model(STM)theory,the static behavior of a new design of pylon anchorage zone with one-way prestressed tendons was investigated on the background of Xijiang Bridge.The economical and reasonable design approach of concrete pylon anchorage zone was also discussed.The main contents of this thesis are listed as the follows:1)An improved genetic evolutionary structural optimization(IGESO)method was proposed to cope with the problem that non-optimal solution may appear when the strut-and-tie model is automatically generated by the method of topology optimization.By adding punishment gene in the chromosome,the remove probability of some elements with large error in sensitivity was reduced,by which the non-optimal solution can be restrained.The IGESO method was programmed by MATLAB and its effectiveness was verified by numerical examples.2)A series of horizontal 2D optimal topological shapes of PC pylon were obtained by the proposed IGESO method.Then two horizontal 2D STMs with precise geometric parameters were proposed,named as “3 Ties-2 Struts-Model”(with the thickness-width ratio of front wall being as ?<0.6)and “3 Ties-3 Struts-Model”(with the thickness-width ratio of front wall being as ?? 0.6),respectively.Compared to other previous models,the two proposed models exhibit more accuracy in design calculation whereas only fewer design parameters are needed,suggesting its IV great potentialities in quantitative design of prestressed tendons and pylon section optimization.3)A full-scale model test was carried out to investigate the static behavior of a new design of pylon anchorage zone with one-way prestressed tendons.The prestress loss laws and construction control of short straight tendons were studied in the model test.Strains,cracks and displacements caused by three load cases(i.e.applying prestress only,operation state and 1.6 times overload state)were measured in the test.The results showed that the structure performed well in each load case,suggesting that the new design is reasonable and reliable.4)A refined FEM model was established with ABAQUS and analysis for the ultimate load capacity of the pylon anchorage zone was carried out.Through the analysis of the whole process for ultimate load,the ultimate bearing capacity and failure mode of the pylon anchorage zone with one-way prestressed tendons were obtained.The developments of the deformation,stresses as well as concrete cracking of the zone were also investigated.The results showed that: the safety factor is 2.840;concrete cracks would start from the inside corner of the front wall and spread through the area around cable hole on the outside front wall;rebars near the inside corner and the outside front wall would yield finally;under ultimate limit state,the front wall collapsed whereas the side wall had little damage,suggesting that the failure of structure depends on the front wall rather than the side wall.At the end of load process,the side wall maintains compression stress about-2.0MPa,and the straight one-way prestressed tendons have not yield yet.5)Since conventional ESO cannot tackle multiple load case with different magnitudes,three improvement strategies based on ESO were proposed,including: ?)weighting global criterion method;?)improved weighting method;?)envelope method.The applicability and availability of such three improvement strategies were demonstrated and verified through the comparison with numerical experiments.6)A design method based on STM under multiple load case was proposed and successfully applied for the reinforcement design in PC pylon with one-way prestressed tendons.The result of this design well matched the experimental phenomena in the full-scale model test,which showed that the proposed method was feasible and effectual.Some suggestions on the design of PC pylon with one-way prestressed tendons are also given at the end.