4 v 7 Gusset Plate Inadequacy

8/6/2019 4 v 7 Gusset Plate Inadequacy

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Office of Research and Engineering

Gusset Plate InadequacyGusset Plate Inadequacy

Carl R. Schultheisz


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Board Meeting HWY07MH024Board Meeting HWY07MH024

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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tension


Compressiondiagonal

Horizontalcomponent

1,361,000 lbs

Tension


1,363,000 lbs

SHEAR


tension


Compressiondiagonal

Horizontalcomponent

1,361,000 lbs

Tension


1,363,000 lbs


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SHEAR


tension


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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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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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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