A bridge design method consists of a design process coupled with an appropriate analysis model. Current design methods for bridge substructures and superstructures for special events are not well defined in existing design specifications and technical references. As a result, designs vary widely from very conservative and costly to extreme and unsafe, with AASHTO specifications providing little guidance for special design event loading combinations or for the load magnitudes when combinations are applied.

The current state of the practice for design of bridges subjected to extreme events such as vessel impact, earthquakes, and flooding is often unreasonably simplistic. Current design methods often fail to consider three-dimensional interaction between the bridge elements: pilesoil, pilesoilpile interaction between piles within a group, pilegroup structure, and proper substructure-superstructure interaction and response. This simplistic approach may result in significant increases in bridge costs. In addition, current design processes are not interdisciplinary or interactive. The design process is partially phased and segmented between structural, geotechnical, and hydraulics engineers. Each discipline develops their design segment with little interdisciplinary input, and often with little regard for their impact on other disciplines. This lack of interaction between disciplines and the use of simplistic analysis models has very often resulted in overly conservative and expensive designs.

This situation has been exacerbated by recent requirements to focus on special design events such as ship impact, seismic, and scour effects. The development of a new rational design methodology (design process and analysis model) is needed to permit rational and cost-effective designs of bridge substructures and foundations.

This project is a multistage effort to improve the state of the practice for designing bridges for extreme events, such as ship impacts, scour, and seismic events. Project Activities are as follows:

• Identify and evaluate contemporary design methods; select a currently recommended design method and publish an FHWA Technical Advisory describing the method.

• Outline a comprehensive R&D program to develop an improved design method.

• Conduct a national conference to disseminate technical information on the best current method(s) and status of the R&D program.

• Initiate further technology transfer activities to implement the new methods.

The 2nd Technical Working Group meeting was held September 26-29, 1994, in San Francisco, California, to select best current methods, outline critical R&D needs, and define the format and objectives of the national conference scheduled for November 1996 in Atlanta, Georgia.

The primary focus of this meeting was current and future procedures for the design of highway bridges for seismic events. Presentations where made by California DOT, Oregon DOT, Washington DOT, and a variety of West Coast consulting engineers. In addition, presentations on future methodologies were made by Lawrence Livermore Labs, PMB Engineering (Offshore Oil), and the University of California at Berkeley. In response to need areas identified at the first two TWG meetings, three small Purchase Order projects are currently underway:

1. Synthesis Report on Loading Combinations of Extreme Events. This is a jumpstart activity by the FHWA to define the problem, identify potential solutions, and develop strategies for a much larger NCHRP project. This project was voted a top-five research priority by the AASHTO Bridge Committee ant both their 1994 & 1995 meetings.

2. Guidelines for the Structural Design of Highway Bridges Subject to Vessel Impact. A consensus was reached at the first TWG meeting that the Florida DOT (FDOT) procedures constitute the best current design method for highway bridges subjected to vessel impact. In accordance with the TWG, FHWA funded a P.O. to develop guidelines that provide a detailed step-by-step delineation for the FDOT procedure. A complete draft of guidelines was presented to the TWG at the San Francisco meeting for review and comment. The TWG indicated that the guidelines needed greater detailing on how a coupled-foundation-to-structure analysis is performed. The third Purchase Order was developed to address this need.

3. Guidelines for the Design of Lateral Loaded Foundations will provide a detailed step-by-step procedure for a coupled-foundation-to-structure analysis. This will include the equivalent pile length method, equivalent spring method, and equivalent foundation stiffness matrix method. The Guidelines will be a standalone FWHA document that will be feathered into other FHWA documents such as "Guidelines for the Structural Design of Highway Bridges Subject to Vessel Impact."

Not available.

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U.S. Department of Transportation
Federal Highway Administration