Updating the Guidelines for the Seismic Design of Highway Bridges
Updating the Guidelines for the Seismic Design of Highway Bridges (slide 1)
Status update for the
Mid-America Ground Motion Workshop
Overview (slide 2)
- Current Provisions
- NCHRP Project 12-49
- What's Next?
Current Seismic Design Provisions (slide 3)
- 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 12-49 (slide 4)
Requested in 1997 by AASHTO
- More experience gained during recent earthquakes
- More research completed during the previous 10 years
- Current LFD/LRFD provisions 10-20 years out of date
NCHRP Project 12-49 (slide 5)
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.
Brief History on NCHRP 12-49 (slide 6)
- Requested in 1997 by AASHTO
- August 1998, NCHRP Project 12-49 work began
- Final Report completed, November 2001
- December 2000, 3rd Draft Review,T-3 and NCHRP 12-49 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) (slide 7)
- New Jersey
- South Carolina
- New York
- FHWA-Federal Lands Hwy Div
Trial Designs (slide 8)
- Nationwide effort
- Broad range of seismic hazard
- Spans - 46 ft to 216 ft
- Lengths - 133 ft to 1320 ft
Brief History on NCHRP 12-49 (slides 9-10)
- 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)
Operational vs. Life Safety levels of performance (slide 11)
T-3 Technical Committee for Seismic Design (slide 12)
Agenda Item #3:
Proposal to adopt NCHRP Project 12-49 Recommendations as a stand-alone Guide Specification
AASHTO Subcommittee on Bridges & Structures (slide 13)
Annual Meeting - May 2002
Agenda Item #3
- Implement the results of NCHRP Project 12-49
- Adoption as a stand-alone Guide Specification for the Seismic Design of Highway Bridges
Outstanding Issues/Concerns (slide 14)
- 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 (slide 15)
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) (slide 16)
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: α (Demand) < β (Capacity)
More design effort required (slide 17)
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 (slide 18)
- 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 (slide 19)
- 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 (slide 20)
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 (slide 21)
- Return Period
- Complexity of the current draft guidelines
- Ground Motion Maps
- Area of influence (more bridge to look at)
What's Next (slide 22)
- Ground motion workshop
- Address issues of states clearly not in favor of adoption as is.
- Future research efforts needed?