FHWA launches new nationwide seismic bridge design training

Public Roads, Autumn, 1996 by James W. Keeley

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.

Development of Training

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.

First Training Feature

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.