Office of Highway Safety
I-35W Bridge CollapseMinneapolis, MN
I-35W Bridge CollapseMinneapolis, MN
Mark Bagnard
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
Summary of Collapse EventsSummary of Collapse Events
2 closed insidesouthbound traffic lanes
2 closed outsidenorthbound traffic lanes
Summary of Collapse EventsSummary of Collapse Events
Construction equipment andvehicles
Piles of aggregate
U10 west node
Summary of Collapse Events
Source: MPD
Deck truss
Southapproach
Northapproach
Summary of Collapse Events
North
Initial Investigation ActivitiesInitial Investigation Activities
• Pre-collapse condition of bridge
– Cracks
– Corrosion
• History of bridge
– Construction
– Inspection
– Fatigue evaluations
– Prior maintenance projects
Source: FHWA
Initial Investigation FindingsInitial Investigation Findings
• Most of the structure was in goodcondition
• Generally well maintained
• Significant attention given to fatigueissues
• Inspected more frequently thanrequired
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
Initiating EventInitiating Event
• Analysis of surveillance cameracollapse video
• Inspection of bridge components
– Superstructure
– Bearings
– Piers
• Additional analyses corroboratedinspection findings
Fracture and DeformationFracture and Deformation
North
Lowerchords
Upperchord
Upperchord
U10E
U10W
Initialcompressionfailure
Initial tensionfracture
Fracture and DeformationFracture and Deformation
Initial Tension FractureInitial Tension Fracture
North
UpL9/U10W
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
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
Finite Element ModelingFinite Element Modeling
Compressiondiagonal
Tensiondiagonal
Orange and red shading:exceeds yield stress
Stress
Yieldstress
0
Allowable
Factors That Did Not ContributeFactors That Did Not Contribute
• Corrosion damage
• Fracture of a floor truss
• Pre-existing cracking
• Bearings and piers
• 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
Concentrated Construction LoadsConcentrated Construction Loads
Constructionmaterials383,000 lbs
Constructionequipment195,500 lbs
Center span combinedConstruction load578,500 lbs
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
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
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
Accident Loads on 1-Inch-Thick Gusset PlatesAccident Loads on 1-Inch-Thick Gusset Plates
Compressiondiagonal
Tensiondiagonal
Stress
Yieldstress
0
Allowable
• 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
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
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
Bridge InspectionsBridge Inspections
• Bridge was inspected at a frequencygreater than required by NBIS
• Condition ratings
– Deck
– Superstructure
– Substructure
• Evaluate condition, not designadequacy
2003 Photo
Compressiondiagonal
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
Board Meeting HWY07MH024Board Meeting HWY07MH024
Board Meeting HWY07MH024Board Meeting HWY07MH024Grand River Bridge, source: ODOT
Board Meeting HWY07MH024Board Meeting HWY07MH024Failed Gusset Plate, source: ODOT
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
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
REPORTS AVAILABLE AT:REPORTS AVAILABLE AT:www.ntsb.gov/publictn/h_acc.htm