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Office of Highway Safety
I-35W Bridge CollapseMinneapolis, MN
I-35W Bridge CollapseMinneapolis, MN
Mark Bagnard
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Investigation IssuesInvestigation Issues
• Inadequate load capacity due to adesign error
• Why the collapse occurred
• Why the design error went undetectedfor 40 years
• Gusset plate issues related todistortion and corrosion
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Summary of Collapse EventsSummary of Collapse Events
2 closed insidesouthbound traffic lanes
2 closed outsidenorthbound traffic lanes
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Summary of Collapse EventsSummary of Collapse Events
Construction equipment andvehicles
Piles of aggregate
U10 west node
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Summary of Collapse Events
Source: MPD
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Deck truss
Southapproach
Northapproach
Summary of Collapse Events
North
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Initial Investigation ActivitiesInitial Investigation Activities
• Pre-collapse condition of bridge
– Cracks
– Corrosion
• History of bridge
– Construction
– Inspection
– Fatigue evaluations
– Prior maintenance projects
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Source: FHWA
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Initial Investigation FindingsInitial Investigation Findings
• Most of the structure was in goodcondition
• Generally well maintained
• Significant attention given to fatigueissues
• Inspected more frequently thanrequired
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Initial Investigation FindingsInitial Investigation Findings
• Two previous construction projectshad significantly increased the deadload
• Construction activities on day ofcollapse had concentrated weightover node U10 west
• Physical evidence indicated thatfailure at U10 was initiating event
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Initiating EventInitiating Event
• Analysis of surveillance cameracollapse video
• Inspection of bridge components
– Superstructure
– Bearings
– Piers
• Additional analyses corroboratedinspection findings
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Fracture and DeformationFracture and Deformation
North
Lowerchords
Upperchord
Upperchord
U10E
U10W
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Initialcompressionfailure
Initial tensionfracture
Fracture and DeformationFracture and Deformation
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Initial Tension FractureInitial Tension Fracture
North
UpL9/U10W
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Gusset Plate ThicknessGusset Plate Thickness
U2 U4 U6 U8 U10 U12 U14
L1L3 L5
L7 L9L11 L13
U0
1/2” thick gusset plate (50 ksi) 10 of 29 gusset plates
5/8” thick gusset plate (50 ksi) 4 of 29 gusset plates
1” thick gusset plate (50 ksi) 13 of 29 gusset plates
1 3/8” thick gusset plate (100 ksi) 2 of 29 gusset plates
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Gusset Plate Shear AnalysisGusset Plate Shear Analysis
L1 U2 L3 U4 L5 U6 L7 U8 L9 U10 L11 U12 L13 U14
Gusse
tP
late
D/C
ratio
0.0
0.5
1.0
1.5
2.0
2.5
U2 U4 U6 U8 U10 U12 U14
L3 L5L7 L9
L11 L13L1
U4 U10 L11
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Finite Element ModelingFinite Element Modeling
Compressiondiagonal
Tensiondiagonal
Orange and red shading:exceeds yield stress
Stress
Yieldstress
0
Allowable
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Factors That Did Not ContributeFactors That Did Not Contribute
• Corrosion damage
• Fracture of a floor truss
• Pre-existing cracking
• Bearings and piers
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• 1977 Modification
– Deck thickness increased
– Added over 3 million pounds
• 1998 Modification
– Barriers / deicing system
– Added 1.2 million pounds
• August 1, 2007
– Construction materials concentratedabove U10
Increases in Dead LoadIncreases in Dead Load
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Concentrated Construction LoadsConcentrated Construction Loads
Constructionmaterials383,000 lbs
Constructionequipment195,500 lbs
Center span combinedConstruction load578,500 lbs
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Increasing Loads on U10W Gusset PlateIncreasing Loads on U10W Gusset Plate
Dead Load of Original Bridge Design
Construction Materials and Vehicles
Traffic
1977 Added Deck(Less Milled-off Lanes)
Cri
ticalL
oad
on
U10W
GussetP
late
s
1998 Modified Barriers
Total loadat collapse
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Increasing Loads on U10W Gusset PlateIncreasing Loads on U10W Gusset Plate
Dead Load of Original Bridge Design
1998 Modified Barriers
Construction Materials and Vehicles
Traffic
1977 Added Deck(Less Milled-off Lanes)
Cri
ticalL
oad
on
U10W
Gusset
Pla
tes
Expected capacityof gusset plates
for proper AASHOdesign
Total loadat collapse
Missing reserve capacityfor proper design
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Accident Loads on ½-inch-Thick Gusset PlatesAccident Loads on ½-inch-Thick Gusset Plates
Compressiondiagonal
Tensiondiagonal
Orange and red shading:exceeds yield stress
Stress
Yieldstress
0
Allowable
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Accident Loads on 1-Inch-Thick Gusset PlatesAccident Loads on 1-Inch-Thick Gusset Plates
Compressiondiagonal
Tensiondiagonal
Stress
Yieldstress
0
Allowable
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• Error not discovered during originalchecks and reviews by:
– Design firm
– State and federal transportation officials
• Other opportunities for detection
– Load ratings
– Annual inspections
Detection of Design ErrorDetection of Design Error
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Load RatingsLoad Ratings
• Required when significant changeoccurs that affects load-carryingcapacity
• First load rating performed in 1979
– Pavement overlay project to increasethickness of bridge deck
• Additional load ratings performed in1995 and 1997
– Modifications to barrier system
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Load RatingsLoad Ratings
• Load rating programs do not includeor consider gusset plate strength
• If gusset plates had been includedin load ratings– Should have revealed improperly
designed gusset plates
– May have determined that improperlydesigned gusset plates werecontrolling members
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Bridge InspectionsBridge Inspections
• Bridge was inspected at a frequencygreater than required by NBIS
• Condition ratings
– Deck
– Superstructure
– Substructure
• Evaluate condition, not designadequacy
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2003 Photo
Compressiondiagonal
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Gusset Plate DistortionGusset Plate Distortion
• Bowed gusset plates not addressedthrough inspections
• At least one inspector had observedbowing but did not report it
• Lack of specific training referencesto bowing could cause bridgeinspectors to give inadequateattention to this condition
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Board Meeting HWY07MH024Board Meeting HWY07MH024
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Board Meeting HWY07MH024Board Meeting HWY07MH024Grand River Bridge, source: ODOT
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Board Meeting HWY07MH024Board Meeting HWY07MH024Failed Gusset Plate, source: ODOT
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Gusset Plate CorrosionGusset Plate Corrosion
• Corrosion not a factor in I-35Wbridge collapse
• Visual inspections alone areinadequate to detect or quantifygusset plate corrosion
• NDE can greatly enhance accuracyof inspections
• Use NDE when appropriate toevaluate gusset plates
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Safety RecommendationsSafety Recommendations
• The design error was not initiallydetected during
– Reviews by the design consultant
– Reviews by Federal or Statetransportation agencies
• The design error remainedundetected
– Through subsequent load ratings
– Through annual bridge inspections
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REPORTS AVAILABLE AT:REPORTS AVAILABLE AT:www.ntsb.gov/publictn/h_acc.htm