Design and Construction of Segmental Bridges in Areas of High Seismicity
Key design and construction challenges to providing robust seismic performance in precast and cast-in-place segmental concrete structures are illustrated in this article, using recent projects that incorporate the rigorous seismic design requirements first implemented by California in the 1990s as examples.
Key design and construction challenges to providing robust seismic performance in precast and cast-in-place segmental concrete structures are illustrated in this article, using recent projects that incorporate the rigorous seismic design requirements first implemented by California in the 1990s as examples.
Segmental structures have unique design and construction issues related to seismic reliability, whether they are cast-in-place or precast. Experience with performance-based design criteria for seismic loading conditions and recent California Department of Transportation/American Segmental Bridge Institute (Caltrans/ASBI)-sponsored testing of precast elements for seismic loads have provided us with design approaches for achieving this reliability while maintaining the cost and schedule benefits of segmental construction.
This article begins with a discussion of the basic principles of the current design philosophy for seismic design of bridges, namely the “capacity-based” approach, as a point of reference. Next, two projects are discussed to illustrate the most recent approaches to two of the three different design methods and construction of segmental bridges in high seismic zones. The third approach, cast-in-place segmental bridges, is discussed in another article, “Design of the Vietnam Veterans Memorial Bridge over the James River” by Taka Kimura, Victor Ryzhikov, and Joe Showers. Finally, the results of the most recent large scale tests of precast segmental structural elements conducted at the University of California at San Diego and sponsored by Caltrans and ASBI are summarized.
Implementation of Seismic Design
Capacity-Based Design Approach. Recently published state-of-the-practice seismic design codes, guidelines and criteria for bridges are based on a design approach that focuses on forcing a predetermined element to fuse. The fuse is typically a structural fuse in the column, although it can also be directed to other elements, such as bearings. For this article, attention is confined to cases with structural hinging in the column, commonly referred to as the “plastic hinge,” where the column undergoes controlled inelastic action at the base or top through multiple cycles and continues to maintain axial load carrying capacity.
As a result, other primary elements of the structure, such as the superstructure and foundation, are designed for loads that are based on the plastic capacity of the column. These elements, referred to as “capacity-protected,” must be designed with considerable overstrength to achieve the desired behavior in a reliable manner, thus preventing damage to elements except the column.
Following the load path that is established by the plastic action of the column under cyclic loading conditions, the superstructure must be designed to resist loads that otherwise would not be accounted for from the design of the structure for service loads (dead loads, live loads, etc.). For segmental structures, this results in consideration of the column-to-superstructure connection and the behavior of the superstructure girders, deck and joints when subjected to tension, positive moment at the connection, and opening of the joints between segmental sections.
Precast Segmental Bridges with Integral Connections: “Spaghetti Bowl”
In developing a design for concrete box girder structures, the Nevada Department of Transportation (NDOT) wished to follow the design configuration typically used by Caltrans on cast-in-place concrete structures, namely the continuous superstructure with integral connections to the column (Figure 1).
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| Figure 1: Spaghetti Bowl, Las Vegas, Nevada. |
This approach provides for a very reliable structure, but also requires a column-to-superstructure connection consistent with the capacity-based design approach. This project was the first to apply capacity-based design to precast segmental structure with integral column-to-superstructure connections. Use of the precast segmental structure type resulted in design issues common to all structures subject to seismic loads and also issues unique to this structure type. These issues included the following:
