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Unknown Foundations

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FHWA Unknown Foundations Summit

Other Strategies for Dealing with Unknown Bridge Foundations
Stuart Stein, GKY and Associates

Project objectives

Motivation
Large population of unknown foundation structures
Impacts associated with bridge failures "What should bridge owners do to manage the risk"

Research team
Lot of discussion on investigation, but what are economic impacts and consequences

Literature review
Risk-based methodologies
HYRISK/extension (software)—look at scour, cost-benefit methods
Geotechnical methods
Design methods—performance vs. design guidelines; what is performance;

Literature review—Non-Destructive Evaluation
NCHRP 21-05 "Determination of Unknown Depth of Foundations"

Literature review—Economic Impacts
Time/running/rebuilding costs—time associated with bridge out of use, commuting costs
Loss of life
Elasticity—detours raise travel cost; what go happen to average daily travel (ADT) time if bridge out of use

Main Survey
Developed with Jorge Pagan
Sent to AASHTO
17 respondents from 15 states—not great but what expected
Follow up calls/emails
Meetings with Virginia DOT & Maryland State Highway Administration

Rebuilding Time Survey
Sent survey to AASHTO distribution list—to get rebuilding time, no consensus, political pressure, ADT
26 respondents

Scour Related Bridge failure survey
How many failures due to scour
Phone calls

Characterizing Bridge Population
1,200 UF bridges built in last 5 years
40 arterial bridge roads with UF in last 5 years—we not doing our job

Scour failures survey
Half million bridges over water in US
More than 100 scour failures
Probability is 1 in 5,000—that's pretty good performance
Need look beyond population of failures
Don't have lot of failures

HYRISK failure probability
Varies from 0.002 to 0.73
Used to prioritize
If good condition or countermeasures—100-500
Implies 30,000 failures per year

Findings of Interest
Can we come up with assumptions during which era bridge was built—no general assumptions made
Older structures built before 1960 build on timber pilings
Not lot spent on bridge to be replaced in 5 years
Don't want spend lot of money if nothing will happen anyway

Scour Risk guidelines Look for plans—we aren't looking hard enough
Improve data management
High priority structure—do some reconnaissance

High Priority Structure

Scour Risk guidelines Screening analysis

Probability Failures
Survey total = 77 per year
HYRISK = 33,000 per year

Probability of Scour failures
1 in 100

Minimum Performance level
Not associated with design standard
1 in 500 for local road, lowest priority

Justifying automated scour monitoring
Install if death cost if lower than risk

Scour risk guidelines
Maryland said can study bridge, calculate scour, cost $50,000—can just fix without that
for $15,000 Don't do analysis if action is less than studying

Potential actions Look for plans,
Do analysis
Develop countermeasures

Questions

Q: Be careful about failure definition; shouldn't be can't carry traffic, should be lying on ground. You said 100 bridges is a low number; I say it's appalling. I think presenting in the wrong way

SS: I look at from cost-perspective. From risk perspective, not significant. Studying and fixing problems go cost more than the costs associated with failure. If any structural repair is required, that's a failure.

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More Information

Contacts

Silas Nichols
Office of Bridges and Structures
202-366-1554
E-mail Silas

Joe Krolak
Office of Bridges and Structures
202-366-4611
E-mail Joe

 
 
Updated: 04/07/2011
 

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United States Department of Transportation - Federal Highway Administration