Overall Conceptual Seismic Design and Local Seismic Capacity Design for Components of Bridges

Introduction

To give clear and correct directions for seismic design optimization of bridges, the “overall conceptual seismic design” and the “local seismic capacity design” methods are proposed to obtain uniform and rational seismic demands and improved seismic capacities of structural components in seismic design of bridges.

There are many methods to realize seismic design optimization so as to improve the seismic performance of structures (Gong, 2003; Li, 1997; Liu, 2003; Vagelis, 2009; Zou, 2002). However, the structural seismic design optimization will be in wrong direction and invalid unless the initial structural seismic design itself is appropriate and rational. It is hoped that the strategies and methods proposed in the chapter are able to give clear and correct directions for seismic design optimization of bridges.

There are two paths to improve the seismic design of bridges so as to make the seismic demands more uniform and rational along structural components. One is to reduce the seismic demands as much as possible, and the other is to increase the seismic capacities. Correspondingly there are two design methods, the overall linear seismic conceptual design and the local nonlinear seismic capacity design. From overall to local design strategies, some new ideas and strategies of seismic design which can effectively improve seismic performance of bridges are proposed separately in this chapter.

Overall conceptual seismic design is a conceptual design method applicable for the whole structure based on the linear seismic analysis. When the components of bridges work in an elastic state during the earthquakes, the conceptual design will be an effective and efficient strategy. From the perspective of overall conceptual seismic design, four conceptual seismic design strategies are proposed focusing on the whole structure.

• 1.

  Taking a long span continuous girder bridge as an example, an optimal design for the layout and detail of the bridge components, including geometry of piers, arrangement of piers, location of expansion joint or braking pier, is carried out so as to reduce the seismic demands in the transverse direction as much as possible. The seismic performance of the bridges with different adjustments in different site conditions is calculated respectively. Based on the comparison of the results, the forms of some piers and the locations of expansion joints are required to be adjusted to improve the bending stiffness distribution for the bridge.

• 2.

  Taking a long span cable-stayed bridge as an instance, the original design proposal makes use of the inverted Y shape bridge tower, while the seismic design dominated by the first longitudinal vibration mode may lead to overlarge relative displacement between the girder and the tower. In order to solve the critical problem, a new-type spatial bridge tower is proposed by integrated analyses of the structural dynamic characteristics, design displacement and seismic responses. Compared with the original tower, the new spatial tower improves the seismic performance of the bridge significantly.

• 3.

  With respect to the elastic cable seismic isolation device installed between the girder and the lower horizontal beam of the tower to mitigate excessive seismic effects, the influences of the elastic cable stiffness on the dynamic characteristics and the seismic demands are investigated by parametric finite element analyses of a real cable-stayed bridge. The seismic isolation mechanism of the elastic cables is discussed.

• 4.

  Girders of the SCC bridge are composed of the structural steel and the concrete. Compared with a conventional concrete girder bridge with the same span, the superstructure height and weight are generally less. Therefore the substructure can be made of hollow or more flexible piers under earthquakes so that the total cost can be reduced greatly. Response spectrum analyses are carried out for two design proposals of a typical four-span continuous bridge, the “reinforced concrete bridge with solid column piers” and the “SCC bridge with hollow column piers”. Seismic responses of the two design proposals are compared with each other to discuss the seismic potential of the SCC bridge. The seismic performance of a long span SCC arch bridge is also studied by response spectrum analyses.