Earthquake Response And Research On Reliability Of Long-Span Cable-Stayed Bridge Based On Pile Soil And Water Coupling

With technical improvement and enhance of economic ability, there is an increasing number of long-span cable-stayed bridges across rivers or channels; meanwhile, anti-seismic problems for these bridges have became hot spots in engineering. In anti-seismic analyses and researches for these bridge projects, structural coupling is a precondition for accurately analyzing dynamic responses of these structures; and of course, the accuracy of structural dynamic responses depends on characteristics of vibration and input forms. In this article, structural coupling are researched by combined with the random vibration theory applied to cable-stayed bridges. The main subjects have been done as follows:First, domestic and foreign methods and cutting-edge technologies for pile-soil-water-structure coupling and dynamic reliability analysis for cable-stayed bridge were summarized. Based on the summary, several types of spatial variation effects were suggested for long-span cable-stayed bridge in consideration of pile-soil-water-structure coupling and seismic ground motions. And practical significant of reliability analysis with the ultimate goal of calculating the probability of failure was expounded.Second, a viscoelastic artificial boundary dealt with soil boundary in pile-soil-structure was derived, and’M’method and pile-soil interface method from standards were expounded, then the reliability of the viscoelastic artificial boundary was verified through numerical examples. Long-span Su Tong Bridge was taken as a calculation example, applyed the viscoelastic artificial boundary and comparatively analyzed three engineering conditions which contained the consideration of piles and structures, the consideration of pile-soil-structure coupling and another one without consideration of pile effects. Hence the effects of pile-soil-structure coupling were obtained as minimizing the natural frequency of bridge, lengthening cycle, significantly reducing the pile reaction force and also markedly decreasing axial forces and bending moments of main beam, axial forces of cables, axial forces and bending moments of the tower. These effects showed that this cable-stayed bridge was an especially flexible structure, static internal force calculation of which was safe to use the bottom consolidation mode, but the value of this calculation was relatively large.Third, in derivation, added mass calculation method was applied for deposing water-structure coupling; and this method was comparatively analyzed with the Morison method and the standardized calculating method to find out the differences among them. The calculating principle of added mass calculation method and standardized method was the same, but Morison equation could consider the movement coupling of water and structure and the damping. The following conclusions were obtained by using the Morison equation to calculate the different diameters and depths of cellular pier under various conditions:the smaller the diameter of the pier, the more significant the effect was; under influences of waves and earthquake, the maximum displacement response results in view of fluid-solid interaction was greater than those not taken the fluid-solid interaction into account; effects of fluid added mass and the fluid-solid interaction were influencing responses of piles and columns, but the impacts of trends and extent were different. Long-span Su Tong Bridge was taken as a calculation example, applying the added mass method and comparatively analyzed two engineering conditions which were the one in consideration of piles, soil and structures and another one in view of piles, soil, water and structures. Changes were large in dynamic characteristics of the hydrodynamic structure; and with the increasing of water depth, the frequency was gradually minimising, and the drop trend was dramatic; moreover, it was remarkably impacting on the internal force of main beams and towers. Therefore, the conclusion was the role of deepwater should be taken into account in the analysis of long-span cable-stayed bridge.Fourth, the approaches for structure responding process calculated by pseudo excitation method, power spectral density function of the seismic non-stationary random process, travelling wave effect, partially coherent effects and local site responses were described. Site characters of Su Tong Bridge were calculated by Ruiz and Penzie modelling; and Ruiz and Penzie models are gained. Composition of pseudo excitation method was programmed by applying ANSYS platform, and was simulated on large-span Su Tong Bridge. It also was compared with the time history analysis method and response spectrum analysis, so that the suitability of this method was found out, which was regarded as a useful discussion for analyzing seismic excitation structure.Fifth, the differences of dynamic response on pile-soil-structure and pile-soil-water-structure models for large-span Su Tong Bridge were researched. By considering the pile-soil-structure interaction, the stiffness of the overall structure was significantly smaller than the value obtained when cap was consolidation; natural period of vibration became longer; and the structural displacement was obviously greater than that without interaction with the maximum value reached about25%. Pile-soil interaction reduced the internal force of the upper structure about15%less than the value gained when cap was consolidation. Displacement response of the pile-soil-water-structure was increased about10%than the value of the pile-soil-structure, the internal forces increased in the range of about50%and the displacement increased by about20%. Based on the pile-soil-water-structure model, the travelling wave effects and nonlinear effects were analyzed, and drew a conclusion that influences on the structures by the different speeds of travelling waves were inconsistent; furthermore, nonlinear effects were deeply impacting on the internal force and displacement along the bridge direction, increased around10%.Sixth, traditional responding surface methodology, standardized response spectrum method and Monte Carlo numerical simulation method were combined to construct a combining response surfaces method for calculating structural random reliability based on quadratic polynomial. By fitting a polynomial function to approximately replace and represent the performance function of the interactive relationship between structural random input and output variables; meanwhile, reliability was analyzed according to the criteria for structural damage and limit state equation. The five models involved pile, soil, water and structure of large-span Su Tong Bridge was taken as cases, applying quadratic polynomial and JC method to analyzing the reliability under random seismic loads. Because reliability indexes calculated by pile-soil-water-structure model is smaller than those calculated by the consolidated cap model, pile structural model, pile-soil-structure model and pile-water-structure model, but these are still safe enough, and the reliability indexes of critical sections are similar, the bridge design is very reasonable. Moreover, although the two methods computed dynamic reliability indexes are different, the results are close to each other with the maximum difference about5%.