Modeling the nonlinear seismic behavior of cable-stayed bridges with passive control bearings

Abstract

A three-dimensional nonlinear finite element model is developed for cable-stayed bridges under static and dynamic loadings based on the total Lagrangian formulation. The model can account for the large displacements that are usually associated with extended in plane contemporary cable-supported structures. A four-node isoparametric cable element is introduced and proposed for the idealization of cables. The element can predict both in-plane and out-of-plane responses. In addition, the cable element takes into consideration the effect of pretension which is one of the features of cable-stayed bridges. Four-node isoparametric beam elements are proposed to model the deck and towers. The element is formulated for general symmetric sections including multi-vent box sections, plate sections and cut-off corner tower sections. The introduced beam element handles large displacements, shear deformations and curved configurations. However, the main advantage of the proposed element is the reduction of degrees of freedom associated with some deck types such as the box sections where one beam element can represent the main girder. Moreover, the internal nodes for both beam and cable elements can be eliminated during the solution of equilibrium equations; however the response can be retrieved at their locations without approximations or loss of accuracy. Furthermore, the eccentric nature of cable-deck and cable-tower connections can be considered by a proper transformation. The control and improvement of the vibrations of cable-stayed bridges necessitates the inclusion of energy dissipation devices at certain locations. The problem becomes difficult to solve with the very large number of degrees of freedom associated with accurate modeling of the bearings and the bridge components. A simplified model is proposed for the dissipation devices in which a two-node element is introduced for a stiffness-type analysis approach. The element is capable of withstanding axial and shear forces. The parameters that control the hysteretic behavior of the element can be estimated out of sophisticated analytical models and/or experimental results. In addition, guidelines based on simplified assumptions are presented for approximating the force-displacement curve of some devices for preliminary analysis and design. The investigation of an appropriate passive control system started with exploring the ideology of seismic isolation by considering a two-dimensional model for a single-plane harp-type cable-stayed bridge. In order to have more realistic insight into the usefulness of passive energy dissipation devices, two different cable-stayed bridges models with double-plane harp and fan-type cables are presented for the numerical analysis. Significant reduction in earthquake induced forces along the bridge can be achieved with the energy dissipation devices as compared to the case of using conventional connections.