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1
Cause MappingCause MappingCause MappingCause MappingProblem Solving • Incident Investigation • Root Cause Analysis • Risk Mitigation
Bridge design extremely narrow to width, weather conditions very windy
Tacoma, Washington
Tacoma Narrows Bridge
Spanning Puget Sound
Death of dog; potential for serious injuries
Bridge remains in water
Loss of primary travel route to Kitsap
$1.6 MLoss of revenue from tolls during outage
Complete loss of bridge (Insurance $4M)
Construction of replacement bridge
Salvage, removal of damaged bridge
1st major suspension bridge collapse in US
$22 M
$6.0 M
$14.0 M
$0.4 M
Safety Goal
Impacted
Potential for serious injuries
Bridge collapsed
Suspender cables broke
Analysis – Bridge Collapse
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Suspension Bridge
Analysis – Bridge Collapse
Suspender cables broke
Stress > strength
High stress on suspender
cables
Evidence: Eyewitnesses, video
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Analysis – Bridge Collapse
High stress on suspender
cables
Bridge load, weight
Deck hangs from main cable by suspender
cables
AND
Design of bridge
Evidence: Calculated in stress analysis
Evidence: Calculated in design
Large twisting motions of deck
AND
Analysis – Bridge Collapse
Large twisting motions of deck
Bridge captured significant wind
energy
Bridge deck weak in torsion
Wind force on bridge
AND
Evidence: Eyewitnesses, video
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Analysis – Bridge Collapse
Wind force on bridge
42 mile per hour wind
Wind acted on greater surface
area
AND
Solid trusses in deck
Bridge constructed as
designed
Evidence: Weather data
Analysis – Bridge Collapse
Large twisting motions of deck
Bridge captured significant wind
energy
Bridge deck weak in torsion
Wind force on bridge
AND
Evidence: Eyewitnesses, video
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Analysis – Bridge Collapse
Bridge deck weak in torsion
Deck narrow to relative length
Length of crossing
Light traffic
AND
Flexible roadbed
AND
Bridge constructed as
designed
Evidence: Calculated in stress analysis
Analysis – Bridge Collapse
Bridge constructed as
designed
Design decisions
Money savings ($3M cheaper)
Aesthetics
AND
Design review process
ineffective
AND
Late changes to design
Respected, experienced
designer
AND
Evidence: Testimony
Evidence: Testimony
Evidence: Testimony
Evidence: Testimony
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Warning signs were missed.
Lessons Learned®
State Highway Department Engineers protested bridge design as “fundamentally unsound”
Workers had issues with bridge stability
Study regarding bridge “bouncing” found that twisting motion would destroy bridge; recommended cutting holes in solid girders to allow wind to pass through or deflect the wind with steel fairings (contract being drafted on day of failure)
Investigation board blamed “the entire engineering profession”. Said engineers needed to test all suspension bridge designs thoroughly using models in a wind tunnel.
Design pushed beyond boundaries of engineering practice but met the requirements of accepted theory at the time.
Aerodynamic forces were little understood. “There seemed to be almost no recognition that wind created vertical movement at all.”
Lessons Learned®
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BUT:It took 10 years (completed October 1950)Cost $14 millionResulted in 3 deaths (broken boom cable, fell off bridge, failure of temporary section)
Lessons Learned®
The replacement bridge was a “landmark of aerodynamic bridge engineering.
Cause MappingCause MappingCause MappingCause MappingProblem Solving • Incident Investigation • Root Cause Analysis • Risk Mitigation