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This magazine is an archived publication and may contain dated technical, contact, and link information.
|Publication Number: Date: Fall 1996|
Issue No: Vol. 60 No. 2
Date: Fall 1996
Bridge designers have had difficulty applying the American Association of State Highway and Transportation Officials (AASHTO) bridge seismic design specification since its adoption in 1983 because it requires an understanding of dynamic analysis, seismic hazard concepts, elastic and inelastic structural response, soil-structure interaction, and structure ductility, among other things.
The difficulty of applying AASHTO's seismic specification is not well understood nationwide and has been reflected in inappropriate project scoping, budgets, and schedules. Guidelines to help bridge designers apply the specification to actual common bridge types around the United States have been lacking. The lack of these guidelines has resulted in both nonconservative and overly conservative designs around the United States.
Application guidelines have also been lacking for geotechnical engineers who play a very important role on the design team. The successful application of AASHTO's bridge seismic design specification requires strong teamwork between structural and geotechnical engineers.
Earthquake design is a national requirement, not just a West Coast one as many would think. The current AASHTO specification emphasizes the national importance of earthquake design by showing areas of moderate to high seismic design requirements in the Northeast, South Carolina, Puerto Rico, Missouri, Arkansas, Tennessee, Kentucky, California, Arizona, Utah, Montana, Idaho, Oregon, Washington, Alaska, and Hawaii.
There are about 575,000 bridges in the United States. About 60 percent of these were constructed before 1970 with little or no consideration given to seismic resistance. The 1971 San Fernando earthquake was a major turning point in the development of seismic criteria for bridges in the United States. The subsequent AASHTO seismic design specification for bridges was adopted in 1983. It includes the relationship of the bridge site to earthquake faults, the seismic response of the soils at the site, and the dynamic response characteristics of the bridge.A
This project has been a five-year effort by bridge/geotechnical engineers for bridge/geotechnical engineers. It focused on the needs of the customer -- practicing bridge/geotechnical engineers -- through several means.
First, a steering group was formed with practicing bridge engineers from around the United States to guide the development of the training. They participated in the scope-of-work and selection process to obtain a private engineering company to develop and present the training. The quality-based selection procedures of the Brooks Act were followed to select a company that had demonstrated both experience with seismic design and the capability to teach seismic design to practitioners. BERGER/ABAM of Seattle, Wash., was selected in November 1992.
Second, a survey was conducted of federal, state, and private engineers to obtain any existing seismic design guidelines or procedures to help define the current state of the art.
Third, several national seismic bridge and geotechnical experts were added to the steering group to provide technical strength and input from national leaders in the field.
The steering group was active throughout the development of seven seismic design examples. They helped guide the production of the examples and provided comments on various drafts of the examples. They also helped develop content for two national, satellite training seminars.
The project required a cooperative effort between several groups within the Federal Highway Administration (FHWA). The Office of Technology Assistance (OTA) and the Office of Engineering Research and Development (OERD) provided the majority of the funds. The National Highway Institute (NHI) provided the coordination for two national, satellite training seminars. The Central Federal Lands Highway Division Office (CFLHD) provided the technical direction for the project and supplied the contract administration for BERGER/ABAM services.
The first feature of the project is seven seismic design examples that illustrate the application of AASHTO's seismic analysis and design requirements on different bridge types across the United States. Each provides a complete set of "designer's notes" covering the seismic analysis, design, and details for a particular bridge, including flow charts, references to applicable AASHTO requirements, and thorough commentary that explains each step. In addition, each example highlights separate issues such as skew effects, wall piers, elastomeric bearings, and pile foundations.
The first example is a 74-meter (m), reinforced concrete box girder, two-span overcrossing with spread footing foundations. The bridge is located in the northern Rocky Mountains region. It is designed for an acceleration of 0.28 g (gravity force). It is an AASHTO Seismic Performance Category C bridge. The example begins with a full analysis and design using the single-mode spectral method with basic (fixed) foundation supports. The next section illustrates analysis and design using the single-mode spectral method with foundation spring supports and abutment springs. Other sections illustrate analysis using the uniform load method and the multimode spectral methods for both basic and spring supports.
The second example is a 122-m, three-span, skewed steel plate girder bridge over a river in New England in AASHTO Seismic Performance Category B with a design acceleration of 0.15 g. The superstructure rests on steel-reinforced elastomeric bearings and wall piers with spread footings on rock. This example highlights the modeling of elastomeric bearings, skew effects on girder systems, varying cross sections, and wall pier design.
The third example is a skewed, 21-m, single-span, prestressed concrete girder bridge with tall, closed, seat-type abutments on spread footings. This bridge is in the Mississippi Valley with a design acceleration of 0.36 g. It illustrates AASHTO's requirements for tall abutments and the Mononobe-Okabe method of determining seismic earth pressure forces.
The fourth example is a 98-m, reinforced concrete box girder, three-span, skewed bridge in the western United States with a design acceleration of 0.30 g. It has two column bents with pinned connections between the columns and spread footing foundations. It illustrates skew effects, foundation springs for spread footings, two-column behavior, and pinned base column design.
The fifth example is a 454-m, steel plate girder bridge in the inland Pacific Northwest with a design acceleration of 0.15 g. It has nine spans and consists of two units -- a straight four-span Unit 1 and a curved five-span Unit 2 with a 396-m radius curve. The superstructure is composed of four steel plate girders with a composite cast-in-place concrete deck. The substructure elements, seat-type abutments, and single-column intermediate piers are all cast-in-place concrete supported on steel H-piles. All substructure elements are oriented to the centerline of the bridge. This example illustrates preliminary seismic design, multiple-unit behavior, deck force transfer through steel cross frames, seismic performance category B effects on single-column piers, and steel pile design.
The sixth example is an 88-m, sharply curved (104 degrees), three-span, concrete box girder bridge in the northwestern United States with a design acceleration of 0.20 g. The substructure is composed of steel pipe piles in monolithic end-wall-type abutments and single-column flared piers founded on drilled shafts. This example illustrates the effect of large curvature, drilled shafts, integral abutments with piles, and rectangular flared column to circular drilled shaft detailing.
The seventh example is a 219-m, 10-span, prestressed girder bridge with open pile bents and a design acceleration of 0.10 g. The superstructure consists of three continuous-span bridges arranged in a 3-4-3 span series. This example illustrates preliminary seismic design for six different options for the pile piers -- three versions of concrete piles and three versions of steel piles. The three versions of both the concrete and steel piles include one plumb pile and two batter piles. For each option, the seismic analysis is done using hand calculations to illustrate how various pile options can be quickly evaluated without using a computer.
Copies of the design examples will be sent to participants of the two satellite seminars described as the second training feature.
The second feature is instruction in seismic design application, provided through two, one-day, national, satellite seminars broadcast from the University of Maryland. Bob Mast and Dr. Lee Marsh of BERGER/ABAM are the instructors. They participated in authoring the seven design examples, and they developed the seminar materials. The seminars are listed as NHI's Course No. 13063 -- "Seismic Bridge Design Applications."
The first seminar was aired on April 25, 1996. It included seven, 50-minute sessions that covered seismic design philosophy, structural dynamics concepts, example analysis and design of a two-span bridge, modeling guidelines, multimode dynamic analysis, and column and pier design. Jim Roberts, California's chief bridge engineer and chairman of AASHTO's T-3 Seismic Technical Committee, made introductory comments supporting the need for national seismic training. Each participant received a course workbook containing copies of the nearly 300 high-quality graphic slides used in the seminar. Nearly a half dozen working models were used by the instructors to illustrate the dynamic response of bridges. Two homework problems were given to the seminar participants to work prior to the second seminar. They are based on the first and second design examples and are intended to help engineers "learn by doing" actual analysis and design variations.
Video tapes of the first satellite seminar are available for loan and copying from NHI. Copies of the course workbook for the first seminar are available from the FHWA Research and Technology (R & T) Report Center, 9701 Philadelphia Court, Unit Q, Lanham, MD 20706. The telephone number for the R & T Report Center is (301) 577-0906.
The second satellite seminar was conducted on July 25.
A satellite seminar is a very efficient and cost-effective way to reach a large audience at one time. For example, the first broadcast reached approximately 600 participants at 20 different sites around the United States, from the East Coast to Alaska, at a fraction of the cost and time required to present the seminar at 20 different locations. However, in satellite seminars, the opportunities for interaction between the participants and the instructors are very limited. Instructor-participant interaction is very important for seismic design training, so FHWA is developing a third feature of this project.
OTA, OERD, NHI, and CFLHD are working together to develop this new feature. It involves a "help desk" service that will provide three levels of training assistance. The first level will provide answers to questions about the seismic design examples and the two seminars. The second level will provide seismic technical assistance on actual bridge design projects under development. The third level will provide the production and delivery of interactive training for state bridge engineers at their office to meet their specific seismic training needs.
James W. Keeley is the technical services engineer for the Central Federal Lands Highway Division Office of FHWA in Denver, Colo. Keeley has more than 25 years of experience with actual design and construction of bridges in the western United States. He is a licensed professional engineer in Oregon.