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