Updating the Guidelines for the Seismic Design of Highway Bridges Status update for the Mid-America Ground Motion Workshop February 2003

Overview Current Provisions NCHRP Project Status Issues What’s Next?

Current Seismic Design Provisions Based on ATC-6 seismic design guidelines developed in the late 1970’s Based on 1988 national seismic hazard maps which are no longer considered adequate or correct Soil site factors which have been demonstrated in many recent earthquakes as being incorrect and inadequate

NCHRP Project Requested in 1997 by AASHTO More experience gained during recent earthquakes. More research completed during the previous 10 years Current LFD/LRFD provisions years out of date

Basic Research Tasks: Develop seismic design provisions that reflected: Latest design philosophies Latest design approaches New insight into ground motion and geotechnical effects Incorporate into LRFD Specification Focus: Designing new bridges rather than retrofitting existing ones. NCHRP Project 12-49

Brief History on NCHRP Requested in 1997 by AASHTO August 1998, NCHRP Project work began Final Report completed, November 2001 December 2000, 3 rd Draft Review,T-3 and NCHRP Panel decide to move “cut & paste” LRFD recommendations to a stand-alone Guide Specification format. April 2001, Distributed proposed stand-alone Guide Specification November 2001, Refined Guide Specification distributed to states. December 2001, Trial Design program started. Trial Designs completed, Feb/Mar 2002

Trial Designs – 13 states & FHWA (19 trial designs) Arkansas New Jersey Missouri Washington Alaska California Oregon South Carolina Tennessee Illinois Nevada Georgia New York FHWA-Federal Lands Hwy Div

Trial Designs Nationwide effort Broad range of seismic hazard Spans – 46 ft to 216 ft Lengths – 133 ft to 1320 ft

Brief History on NCHRP T-3 Committee and others involved in the trial designs met on April 28, 2002 to discuss the results.

Taken to the annual meeting of the AASHTO Subcommittee on Bridges and Structures as an agenda item. (May 2002) Brief History on NCHRP 12-49

Operational vs. Life Safety levels of performance Life Safety performance (MCE) –prevent collapse/loss of life –significant damage Operational performance (MCE) –immediate service –minimal damage Owner’s Discretion

T-3 Technical Committee for Seismic Design Agenda Item #3: Proposal to adopt NCHRP Project Recommendations as a stand-alone Guide Specification

AASHTO Subcommittee on Bridges & Structures Annual Meeting – May 2002 Agenda Item #3 Implement the results of NCHRP Project Adoption as a stand-alone Guide Specification for the Seismic Design of Highway Bridges

Outstanding Issues/Concerns May be forced to apply to existing bridges Return Period for Life Safety Event is too high (3% PE in 75 years) More design effort required

May be forced to apply to existing bridges Developed for new design Isn’t it better to use specifications based on the most current scientific and engineering knowledge for our new structures? More pressing system needs do not support the expenditure of funds on a seismic retrofit program in most states.

Return Period for Life Safety Event is too high (3% PE in 75 years) Earth Science Community: Return period captures the ground motions possible for rare but scientifically credible earthquakes Looking for low probability of collapse from rare, credible earthquakes. Life Safety:  Demand) <  (Capacity)

More design effort required As technology improves (enhanced methods for assessing demands, enhanced tools for developing capacity), design is becoming more complicated and is taking more effort. But in most states, seismic demands are adequately addressed by “no analysis required” provisions.

Proposed Guide Specifications 1996 USGS Maps Improved/validated soil site factors Best scientific and engineering approaches and technologies currently used worldwide Reviewed by broad cross-section of State bridge engineers and consultants, earthquake engineers, experts from various industries and technologies Comprehensive parameter study and trial design program produced bridge designs Provides a significantly higher level of performance

Adoption as a Guide Specification Supports implementation of AASHTO- sponsored research Supports fact that existing provisions are out of date Allows for guidelines to more effectively be improved and updated; states will be more serious about looking at the guidelines and providing input for changes As a Guide Specification, states will not be required to use it

AASHTO Subcommittee on Bridges & Structures Annual Meeting – May 2002 Agenda Item #3 The proposed guide specification was not adopted by the subcommittee; about 30% of the states voted for adoption.

Issues Return Period Complexity of the current draft guidelines Ground Motion Maps Area of Influence (more bridges to look at) Complexity

What’s Next Ground motion workshop Address issues of states clearly not in favor of adoption as is. Future research efforts needed?