New Jersey, like many eastern and central U.S. states, is located in a low-to-moderate seismic hazard area. Bridge engineers currently use the AASHTO seismic design and retrofit requirements, which are based on data considered to be 10 to 20 years out of date.
With the publication of the Recommended LRFD Guidelines for the Seismic Design of Highway Bridges, designers now have an alternate resource, which incorporates new research results with experiences gained from around the world in recent earthquakes.
Even though the Guidelines have not yet been adopted by the AASHTO Subcommittee on Bridges and Structures, they are available for use. One of the benefits the Guidelines provide to bridge designers in low to moderate seismic risk areas is the “No Seismic Analysis Demand” method, by which seismic analysis is not required for most regular bridges as long as capacity design principles and minimum design details are provided. Other benefits include the treatment of new seismic load resistant systems, improved soil information, mandatory identification of the seismic load path by the designer, and providing a state of the art, performance based method of designing bridges for seismic loading.
In New Jersey, designers are encouraged to compare the results of the seismic criteria that is now contained in the AASHTO LRFD Bridge Design Specifications against the criteria contained in the NCHRP Report 472, Comprehensive Specification for the Seismic Design of Bridges1. If the comparative analysis reveals that the NCHRP 472 criteria results in a lesser demand to the seismic demands of a bridge design, the designer is directed to advise the department. Based on the department’s assessment of the comparative analysis, the designer may be directed to complete the design with detailing that is based on either the AASHTO LRFD results or the NCHRP results.
NJDOT compared two bridges originally designed under the seismic criteria contained in the AASHTO LRFD Bridge Design Specifications. Both were designed under a seismic Performance Level Category of “Critical,” and it was found that the impact to the required seismic construction detailing would be less when designed to the NCHRP 472 criteria. If the bridge structures had been assigned a seismic Importance Category of “Essential,” the opposite was observed. In both cases, costs were comparable, but the newer Specifications provide more explicit insight into structural behavior and give the designer more options and tools to ensure achievement of the desired performance levels.
NJDOT is currently assessing its soil characteristics to identify or establish soil site factors for the State. It is hoped that with this identification, the State will be a better position to decide on the adoption of the NCHRP 472 criteria.
Submitted by Jose A. Lopez, New Jersey DOT
1. The NCHRP 472 report summarizes the methodology used to develop new specifications, which are described in the four-part ATC-MCEER Joint Venture publication, “Recommended LRFD Guidelines for the Seismic Design of Highway Bridges.” The project was funded by the National Cooperative Highway Research Program under Project 12-49. The Guidelines are available for purchase from MCEER or ATC. The NCHRP report can be downloaded from http://trb.org/publications/nchrp/nchrp_rpt_472.pdf
The State of Alaska DOT is using some of the new techniques available in the Recommended LRFD Guidelines for the Seismic Design of Highway Bridges. Many of the advanced approaches offered in the Guidelines are particularly appropriate for design in highly seismic areas such as Alaska.
As recommended by the Guidelines, Alaska Department of Transportation and Public Facilities (DOT&PF) bridge engineers commonly use cracked section properties in considering the rigidity of bridge piers (contrary to the current AASHTO Division 1-A, which uses gross properties), and find the ability to use soil pressures to resist seismic loads at the abutments useful for certain types of projects. Pile bents with concrete filled tubes are commonly used in Alaska, and the LRFD Guidelines contain the first provisions for the use of these tubes.
Design issues such as soil-structure interaction, liquefaction, structure displacement verification, P-Delta limitations, and short-period structure response adjustments are also addressed by the Guidelines. These and other state-of-the-practice design issues are addressed in the LRFD Guidelines and should be considered by bridge engineers until AASHTO adopts new seismic design provisions.
Submiitted by Elmer E. Marx, P.E., Alaska DOT&PF