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Office of Research and Engineering
Gusset Plate Inadequacy
Carl R. Schultheisz
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Board Meeting HWY07MH024
Presentations
1. Bridge description and collapse2. Construction activities on bridge at
time of collapse3. Gusset plate inadequacy
4. Finite element analysis5. Design and review process6. Bridge load rating and bridge load
analysis7. Bridge inspections8. Gusset plate inspections
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Introduction
January 15, 2008, interim safetyrecommendation to the Federal HighwayAdministration Require bridge owners to evaluate gusset
plates when modifications were beingconsidered
Based on Initial on-scene findings
Federal Highway Administration analysis
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Overview
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 Basics
Design began in 1962 Governed by AASHO specifications
1961 AASHO Standard Specifications forHighway Bridges
1961/1962 AASHO Interim Specifications
1964 Minnesota Highway DepartmentStandard Specifications for Highway
Construction
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Bridge Design Basics
Allowable Stress Designmethodology
Stress= Force/Area
Truss member forces counteract theweight of structure and traffic
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Bridge Design Allowable Stress
AASHO allowable stress no morethan 55 percent of the yield stress
Yield stress defines limit of usable
load-carrying capacity of a material The allowable stress level ensures
a substantial margin of safety
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Bridge 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
B id D i G l
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Bridge Design
GeneralReview Design documents obtained from Mn/DOT
and Jacobs Engineering
Included drawings and computation sheets
Complete and detailed documentation,
except for main truss gusset plates Design was also evaluated using finite
element models
Appropriate design methodology No significant deficiencies, other than in
main truss gusset plates
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Bridge Design Gusset Plates
AASHO specifications stated that gussetplates shall be of ample thickness toresist shear, direct stress, and flexure
No documents showing main truss
gusset plate design Documents did show design
methodology for welded floor trussgusset plates
Analysis guided by gusset plate designfor welded floor trusses
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Compression
diagonal2,288,000 lbsTension
diagonal1,975,000 lbs
Vertical540,000 lbs
tension
Upper chord2,147,000 lbs
tension
Upper chord924,000 lbscompression
U10 - Member Forces Transferred by the Gusset Plate
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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 Calculations
Shear stress = Shear forceArea
3,071,000 pounds100 square inches
=
= 30,710 pounds persquare inch
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Bridge 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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R lt G t Pl t Sh
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Results - Gusset Plate ShearAnalysis
L1 U2 L3 U4 L5 U6 L7 U8 L9 U10 L11 U12 L13 U14
Gusse
tPlateD/C
ratio
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 Summary
Analysis showed that the U10gusset plates had clearly
inadequate capacity
Other gusset plates had similarinadequate capacity
No records were found for design of
main truss gusset plates No other design deficiencies found
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Result
January 15, 2008
Interim safetyrecommendation to the Federal
Highway Administration
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