overview current provisions nchrp project 12-49 status issues what’s next?

T-3 Technical Committee for Seismic DesignT-3 Technical Committee for Seismic Design

AASHTOAASHTOSubcommittee on Bridges and StructuresSubcommittee on Bridges and Structures


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 12-49• 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 12-49

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• Current LFD/LRFD provisions 10-20 years out of date


Basic Research Tasks: Develop seismic design provisions that reflected:

Latest design philosophiesLatest design approachesNew insight into ground motion and geotechnical effectsIncorporate into LRFD Specification

Focus: Designing new bridges rather than retrofitting existing ones.

NCHRP Project 12-49


Brief History on NCHRP 12-49

• 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)

• 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



• 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

• Life Safety performance (MCE)– prevent collapse/loss of life– significant damage

• Operational performance (MCE)– immediate service–minimal damage

Owner’sDiscretion

AASHTOAASHTOSubcommittee on Bridges and StructuresSubcommittee on Bridges and Structures


T-3 Technical Committee for Seismic Design

Agenda Item #3:

Proposal to adopt NCHRP Project 12-49 Recommendations as a stand-alone

Guide Specification


AASHTO Subcommittee on Bridges & Structures

Annual Meeting – May 2002Agenda 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

• 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 engineeringknowledge for our new structures?

More pressing system needs do not support the expenditure of funds on a seismic retrofitprogram 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 complicatedand 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 2002Agenda 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?

overview current provisions nchrp project 12-49 status issues what’s next?

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