Segmental Bridges

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). 

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:

• Confining “plastic hinging” to visible, repairable substructure elements (by proper detailing of the column and using the capacity-based approach to designing adjacent structural elements)
• Maintaining elastic behavior of superstructure
• Ensuring the performance of precast segment-to-segment joints under cyclic loads
• Confining integral superstructure substructure elements.

Precast Segment to Column Joints. The most difficult design issue we addressed was development of the column-to-superstructure connection. Special measures were taken to assure a fixed connection that was protected from damage when plastic hinging occurred in the column. First, a unique precast segment was designed that would fit over the column (Figure 2), and be rigidly connected to the column by a combination of closure pour and multi-directional post-tensioning.

Figure 2: Computer renderings of reinforcement required at the pier cap section to provide a reliable integral connection between the column and superstructure.

Post-tensioning by Dywidag bars and strand in the longitudinal direction and additional strand in the transverse direction provided a joint that remains in compression when subjected to the plastic column loads. The stages of reinforcement and post-tensioning fabrication were developed in multiple drawings in order to develop a section that could be constructed (Figure 3).

Figure 3: (Top) Computer renderings of reinforcement and post-tensionin integration at the pier cap section. (Bottom) Constructed pier cap section being lowered onto the column.

Performance of Precast Segment-to-Segment Joints. There is a consideration in the design for the performance of the joints between segments under cyclic loads. For the span-by-span construction method used on these structures, additional post tensioning was added as “continuity” PT in both the deck and soffit of the box girder (Figure 4). Analysis conducted for the design demonstrated that the stress levels were low enough under plastic loading conditions to expect that no damage would occur during cyclic loading brought on by the design event earthquake. Subsequent large-scale physical testing at the University of California at San Diego confirmed this.

Figure 4: Schematic of longitudinal post-tensioning in the superstructure. Note that the PT is internal to the box, but external to the girders.

Precast Cantilever Construction: Paksey Bridge, Bangladesh

This 1786-m (5,894-foot) -long precast concrete box-girder bridge is not located in an area of especially high seismicity, but the design of its superstructure and the substructure were affected by the requirements of the seismic loads. For the super- structure, the seismic design approach to lock up the superstructure with the substructure had to be balanced against the need for the continuous concrete box-girder to move with temperature changes. This approach resulted in the need for “lock-up devices” (LUDs) at the top of the pier between the superstructure and the pier.

When combined with the effects on the design of the unusually deep scour and the scour pit, the loads that the LUDs were required to resist were approximately 2200 kips (24 MN) at each pier. This was considerably larger than the loads estimated by the original contract document prepared by other consultants. Prior to the design of this project, the largest LUD in the world was capable of resisting an 18-MN load.

The design allowed for two devices to be used at each pier to reduce the size required to just under 12 MN each and, thereby, provide a device that met the load requirements but also was available to contractors. Even so, once potential suppliers and certification processes were reviewed, the primary supplier, the American company TechStar, decided to form a joint venture with an Italian company, ALGA, so that the devices could be supplied within the contract schedule. One main consideration in this decision was that the only testing facility in the world that was large enough to handle the 12-MN device was at ALGA’s site.

Since the scour “pit” was capable of moving from abutment to abutment, analysis for the site required that the scour potential be considered along the length of the bridge. The depth of the pit could reach 50 m (165 feet), resulting in a structure configuration where pier heights could vary from 3 m to 53 m (10 feet to 175 feet). Such a variation in stiffness caused large loads to be concentrated at the short piers.

Testing Program

The objectives of the recent testing of precast segmental structural elements, which was sponsored by Caltrans and ASBI and conducted by the University of California at San Diego, were to:

• Evaluate performance of joints under fully reversed cyclic loads
• Evaluate performance of integral superstructure-to-substructure connections under seismic loads
• Evaluate system performance under longitudinal seismic loads (effective width, column hinging, superstructure ductility)
• Validate current AASHTO requirements that no more than 50 percent of post-tensioning steel should be external to overcome the lack of information on seismic performance of precast segmental bridges.

Two specific areas were identified as key to drawing conclusions — the mid-span (Phase I testing) and the pier segment area or support area (Phase II testing). The conclusions of the testing were as follows:

• Phase I (High Moment & Low Shear)
  – Segment-to-segment joints can undergo significant repeated opening and closing without failure.
  – The combination of internal and external post-tensioning results in the lowest ductility and deformation capacity.
  – Joints are more ductile with 100-percent external tendons, and permanent displacements and joint openings are reduced.
  – The use of cast-in-place deck closure joints reduces superstructure post-earthquake residual displacements and improves the energy dissipation capability.
• Phase II (High Moment & High Shear)
  – Segment-to-segment joints can undergo significant opening and closing without failure. Significant vertical slip between segments occurred only after compression failure of the test units.

The conclusions of this testing program were instrumental to the design of the replacement of the skyway portion of the eastern spans for the San Francisco-Oakland Bay Bridge currently under construction. In addition, the principal researchers and authors of the published test reports — Prof. Frieder Seible, Rafael Manzanane from TY Lin, Ray McCabe from HNTB and Juan Murillo from PB, have been asked by ASBI to develop a proposal to the American Association of State Highway and Transportation Officials (AASHTO) to change current design specifications to reflect the findings of the testing programs that have proven that the current guidelines are in opposition to the goal of accomplishing more ductile seismic responses for the safe seismic design of bridges.