Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Civil and Environmental Engineering

Supervisor

El Damatty, Ashraf

2nd Supervisor

Pfeil, Michele S.

Affiliation

Federal University of Rio de Janeiro, Brazil

Joint Supervisor

Abstract

Cable-stayed bridges are complex structures with several advantages such as aesthetical appeal, economic use of materials, and efficient construction method. Due to these advantages and the extensive knowledge gained from projects over the years, longer cable-stayed bridges are being constructed. As span lengths increase, structures become more flexible, which makes the accurate evaluation of wind loads critically important in the design of cable-stayed bridges. A large number of variables are involved in the design of cable-stayed bridges. Those include overall geometric dimensions, cross-sectional dimensions, number of stay-cables and pre-tensioning forces to be applied to the cables. Taking all variables into account, and considering the need to conduct multiple moving load analyses and to calculate accurately aerodynamic wind forces, a design optimization process for such bridges becomes challenging. In this thesis, a numerical model capable of achieving this design optimization task is developed. The numerical model uses a structural system in which the deck is composite steel-concrete with two I main girder. The developed numerical model is based on the Finite Element Method (FEM), the Real Coded Genetic Algorithm (RCGA), and the Discrete-Phases Design Approach. The latter classifies variables into two categories: (i) main variables: number of stay-cables, I-girder inertia, concrete slab thickness, tower cross-section external dimensions, tower height above the deck; (ii) secondary variables: I-girder dimensions, stay-cable areas and pre-tensioning forces. The main variables are design variables optimized directly by the RCGA, while the secondary variables are indirectly optimized by the discrete phases. Buffeting wind loads are considered as equivalent static forces, which were validated through a theoretical-experimental correlation. This powerful tool is used to assess the importance of considering truck versus lane loads, as well as wind buffeting loads and various aeroelastic instabilities in the design optimization process. Results show that the most critical load combination include the wind effect, and that the critical wind velocities of aeroelastic phenomena play a significant role for high values of basic wind speeds.

Summary for Lay Audience

Cable-stayed bridges are composed by three main elements: (i) deck, (ii) tower(s), and (iii) cables. The deck is responsible for providing the road surface. The inclined cables are attached to the deck and to the towers, this way, helping to support the deck along the span (river, valley, etc.). Cable-stayed bridges are complex structures because the great number of stay-cables increases significantly the number of parameters to be determined during the design. In order to improve the design process, many studies have been dedicated to optimizing the size and characteristics of cable-stayed bridge elements. The loads considered in these analyses included the weight of the bridge elements and the weight of vehicles.

By not considering the wind in the optimization process of cable-stayed bridges, the design project may require significant modification when the wind is evaluated. This way, in this study, the following question will be answered: what is the influence of considering wind loads during the optimization process of cable-stayed bridges? To answer this question, a computational program was developed. In this program, the design process is divided into phases in order to reduce the analysis computational time. Each phase is responsible for one stage of the design, including the weight of elements, weight of vehicles at different positions of the bridge, and the action of wind.

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