Evaluation of Design Level Earthquakes of NCHRP Project 12-49 for Seismic Design of Bridges in New Jersey and the Northeast United States

Abstract

NCHRP Project 12-49 is intended to develop comprehensive specifications for seismic design of bridges considering all aspects of the design process including: (1) design philosophy and performance criteria, (2) seismic loads and site effects, (3) analysis and modeling, (4) design requirements, and (5) detailing. These new specifications will be nationally applicable with provisions for all seismic zones. Performance levels for life safety and operational are defined for the rare earthquake event (MCE) and for the Expected earthquake event. While the ratio of the accelerations from the two events is about 1.5 to 2 in the Western United States, this ratio is about 3 to 4 in the Eastern United States. The maximum considered earthquakes (MCE) in New Jersey based on the proposed NCHRP 12-49 provisions are significantly higher than current design level earthquakes based on current AASHTO LRFD criteria. Maximum accelerations for relatively stiff bridges in soft soils located in North New Jersey from NCHRP 12-49 are approximately twice those from current LRFD accelerations. The current guidelines for seismic design of bridges in New Jersey are based on the 1998 LRFD Specifications (w/interims 1999-2003). The specs also require Safety Evaluation performance level as well as Functional Evaluation level for “critical” bridges including bridges carrying Turnpike traffic. Because of the several significant changes in the design earthquake and design criteria provided in the new provisions, there are questions on how these new provisions will affect the design and performance of bridges in states nationwide as well as the retrofit of existing bridges and the associated costs; in particular, the return period and soil factors. The current recommended return period for the MCE in NCHRP 12-49 is 2500 years. This return period seems conservative for the Northeastern US especially for “non critical” bridges. The soil factors were based on seismic records from the Western US and were higher for soft soils. Hence, there is a need to evaluate the impact of the maximum accelerations in new seismic design provisions proposed in NCHRP Report 12-49 on the seismic design and detailing of bridges in New Jersey and the Northeastern US and whether the use of such high accelerations and high soil factors in areas of low seismic risk such as New Jersey is justified.