4 v 7 gusset plate inadequacy
TRANSCRIPT
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Office of Research and Engineering
Gusset Plate InadequacyGusset Plate Inadequacy
Carl R. Schultheisz
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PresentationsPresentations
1. Bridge description and collapse2. Construction activities on bridge at
time of collapse3. Gusset plate inadequacy4. Finite element analysis
5. Design and review process6. Bridge load rating and bridge load
analysis7. Bridge inspections8. Gusset plate inspections
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IntroductionIntroduction
January 15, 2008, interim safetyrecommendation to the Federal Highway
AdministrationRequire bridge owners to evaluate gussetplates when modifications were being
considered Based on
Initial on-scene findings
Federal Highway Administration analysis
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OverviewOverview
Bridge design basics
Allowable Stress Design methodology
Design forces
I-35W design
Review of design documents
Evaluation of gusset plates
Clearly inadequate capacity of gussetplates at U10 and other nodes
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Bridge Design BasicsBridge Design Basics
Design began in 1962
Governed by AASHO specifications
1961 AASHO Standard Specifications forHighway Bridges
1961/1962 AASHO Interim Specifications 1964 Minnesota Highway Department
Standard Specifications for Highway
Construction
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Bridge Design BasicsBridge Design Basics
Allowable Stress Designmethodology
Stress = Force/Area
Truss member forces counteract theweight of structure and traffic
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Bridge Design Allowable StressBridge Design Allowable Stress
AASHO allowable stress no morethan 55 percent of the yield stress
Yield stress defines limit of usableload-carrying capacity of a material
The allowable stress level ensuresa substantial margin of safety
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Bridge Design - ForcesBridge Design - Forces
Design force: calculated in accordancewith AASHO specifications
Design force calculated from dead loadplus live load plus impact
Live load = AASHO-specified lane loads AASHO-specified lane load
Represents unusual, heavy traffic load
Provides additional margin of safety
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Bridge Design General ReviewBridge Design General Review
Design documents obtained from Mn/DOTand Jacobs Engineering
Included drawings and computation sheets Complete and detailed documentation,
except for main truss gusset plates
Design was also evaluated using finiteelement models
Appropriate design methodology No significant deficiencies, other than in
main truss gusset plates
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Bridge Design Gusset PlatesBridge Design Gusset Plates
AASHO specifications stated that gussetplates shall be of ample thickness toresist shear, direct stress, and flexure
No documents showing main trussgusset plate design
Documents did show designmethodology for welded floor trussgusset plates
Analysis guided by gusset plate designfor welded floor trusses
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Compressiondiagonal
Horizontalcomponent
1,361,000 lbs
Tension
diagonalHorizontalcomponent
1,363,000 lbs
U10 - Design for Shear
Upper chord2,147,000 lbs
tension
Upper chord924,000 lbscompression
A A
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U10 - Design for Shear
Upper chord2,147,000 lbs
tension
Upper chord924,000 lbscompression
Compressiondiagonal
Horizontalcomponent
1,361,000 lbs
Tension
diagonalHorizontalcomponent
1,363,000 lbs
SHEAR
Upper chord2,147,000 lbs
tension
Upper chord924,000 lbscompression
Compressiondiagonal
Horizontalcomponent
1,361,000 lbs
Tension
diagonalHorizontalcomponent
1,363,000 lbs
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U10 - Design for Shear
SHEAR
Upper chord2,147,000 lbs
tension
Upper chord924,000 lbscompression
Compressiondiagonal
horizontalcomponent
1,361,000 lbs
Tension
diagonalhorizontal
component1,363,000 lbs
Total = 3,071,000 lbs
Total = 2,724,000 lbs
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Bridge Design U10 CalculationsBridge Design U10 Calculations
Shear stress =Shear force
Area3,071,000 pounds
100 square inches=
= 30,710 pounds persquare inch
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Bridge Design U10 CalculationsBridge Design U10 Calculations
Demand
= Shear stress from member design
forces= 30,071 pounds per square inch
Capacity= AASHO specified allowable stress
= 15,000 pounds per square inch
Demand-to-Capacity ratio = 2.05
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Bridge Design U10 CalculationsBridge Design U10 Calculations
The Demand should be less than theCapacity
The Demand-to-Capacity ratio shouldbe less than 1
U10 Demand-to-Capacity ratio = 2.05 To make the Demand-to-Capacity ratio
equal to 1, the U10 gusset platethickness would have to increase from inch to slightly more than 1 inch
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Results - Gusset Plate Shear AnalysisResults - Gusset Plate Shear Analysis
L1 U2 L3 U4 L5 U6 L7 U8 L9 U10 L11 U12 L13 U14
GussetPlateD
/Cr
atio
0.0
0.5
1.0
1.5
2.0
2.5
U2 U4 U6 U8 U10 U12 U14
L3L5
L7 L9L11 L13
L1
U4 U10 L11
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Gusset Plate Analysis SummaryGusset Plate Analysis Summary
Analysis showed that the U10gusset plates had clearly
inadequate capacity Other gusset plates had similar
inadequate capacity No records were found for design of
main truss gusset plates No other design deficiencies found
R l
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ResultResult
January 15, 2008 Interim safetyrecommendation to the Federal
Highway Administration
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