Hoan Bridge Failure Analysis Hoan Bridge Failure Analysis Wisconsin Department of Transportation City of Milwaukee, WI December 13, 2001 City of Milwaukee, WI December 13, 2001 LEHIGH FHWA LICHTENSTEIN

OUTLINE  Introduction  Issues at the Beginning  Overview of the forensic investigation  Detailed Analysis  Retrofitting of the structure to remain  Conclusion

ISSUES  Secure the area under the failed span  Traffic Control and Management  What caused the failure  Open southbound bridge  Repair/Replace existing bridges  Follow-up on National Issues

CAUSES OF FAILURE?  Factors Investigated:  Structural Design of Lateral Brace System  Details of Shelf Plate Connection Assembly (Constraint)  Thermal Forces  Live Load Forces (Fatigue)  Material Properties  Weld - Fabrication Quality  Could this have been caught in inspection?  Vulnerability of other bridges

OVERVIEW OF HOAN BRIDGE FORENSIC INVESTIGATION  Visual Examination Of Fractures Before Demolition  Remove Critical Components After Demolition  Evaluate Material Properties  Global And Local Stress Analysis Of Detail  Fractographic And Metallographic Studies  Model Crack And Geometric Condition  Assess Crack Instability And Arrest

Location F-7 View from West

Location E-7 View from West

SHELF PLATE DETAIL LOCATION E-28

EXTERIOR GIRDER 108G1 SECTION F-F

Girder E P.P. 28 Shelf Plate Weld Fracture Origin Crack Origin

Girder E P.P. 28 Gusset Plate Crack Origin Crack Bifurcation Crack Origin

Girder E P.P. 28 Crack Origin Cleavage Fracture

Girder E P.P. 28 Shelf Plate Fracture at Hole Repair

Fatigue StriationsDuctile Fracture Surface Abrasion

Girder E P.P.28 Bottom Flange Fracture

Girder D P.P. 28 Crack Origins D 28 Crack Origin Bottom Flange Crack Arrest

Girder D P.P. 28 Crack Origins Cleavage Fracture at Web EdgeCorrosion Pitted Cleavage Fracture

Girder D P.P.28 Bottom Flange SEM Crack Arrest in Weld HAZ

Girder D P.P. 28 Crack Arrest in Bottom Flange Crack Arrest Boundary Cleavage Fracture at Crack TipCleavage/Ductile Fracture at Crack Tip Weld Flange Base Metal

Girder B P.P. 26 Girder B P.P. 26 Fracture Origins

Girder B P.P. 26 Cleavage/Ductile Fracture at Origin Corrosion Product Cleavage Fracture Near Origin

Girder B P.P. 26 Shelf Plate Web Thumbnail Defect

3D Computer Model Of Failed Span/Unit S2a

Finite Element Model Of Joint Assembly

FRACTURE MODEL

EQUIVALENT PENNY SHAPED CRACK

CRACK ARREST MODEL

Bracings Disconnected From Shelf Plate

Weigh-in-motion Testing, E. Lincoln Ave. Viaduct

FINDINGS OF ANALYSIS  A Crack-like Geometric Condition Existed At Intersection Of Shelf Plate And Transverse Connection Plate  Geometry Resulted in High Constraint Stresses From Weight loads And Weld Shrinkage That Were 36% Greater Than Yield Strength Of Material  The Second Retrofit Hole In Center Girder E Increased Stress By 10% At Critical Point  The Bridge Strain Rate Fracture Toughness Of Girder Webs Was 120ksi -, Typical Of A36 Steel Plate

FINDINGS OF ANALYSIS (continued)  Fracture Was Predicted For The Webs Of All Three Girders; Girder E Started The Failure  Only Girder D Was Capable Of Arresting The Dynamic Crack That Extended To The Flange  Flaws in the Detail Is Not Visually Detected

FATIGUE-FRACTURE FAILURE OF BRIDGES Typical progression of failure: Micro-discontinuities are present in almost all large fabricated structures, usually in the welds With time (and traffic loads), discontinuities could develop into larger fatigue cracks It takes many years for fatigue cracks to grow the first few inches ---- normally can be detected visually during field inspections. When fatigue cracks reach a critical size, brittle fracture could result (usually on a cold night).

EFFECT OF CONSTRAINT ON FRACTURE TOUGHNESS As constraint is increased, fracture toughness decreases even though the inherent metallurgical characteristics of the steel are not changed A triaxial state of stress occurs ahead of the crack which restricts yielding and ductility in the member. Constraint (triaxial) played a major role in the fracture at the joints Fracture mechanics helps us understand and explain this failure

Retrofit of The Hoan Bridge Approach Spans Wisconsin Department of Transportation City of Milwaukee, WI

HOAN BRIDGE RETROFIT Removed all lateral bracings Removed shelf plates and grind welds smooth and flush with girder webs Provided positive attachment between connection plate and tension flange Strengthened pier diaphragms for wind forces Reconstructed demolished span

HOAN BRIDGE RETROFIT HIGHLIGHTS  3D Analysis & field test proved bridge can transfer wind loads without lateral bracings.  Performance of retrofit has been field tested  Eliminated fatigue/fracture prone details, driving forces, avoids progressive failure  Future maintenance inspection needs no higher than that for similar steel bridges  Lower cost/completed by year-end  Minimal disruption

RETROFITTED SECTION AT INTERIOR (All Lateral Bracings And Shelf Plates removed)

RETROFITTED SECTION AT PIER (Shows strengthening of end diaphragms for wind)

RECONSTRUCTION OF DEMOLISHED SPAN Maintained same superstructure configuration Omitted lateral bracings Improved girder design to ease fabrication and erection Replaced 151 ft section of new girder --- from existing splices

Conclusions  Cause of failure determined  Replaced demolished span  Removed welded shelf plates  Removed lateral bracings  Strengthening of diaphragms Triaxial Constrained in a critical weld detail is the cause of failure.

Conclusions Facility service restored and no more problem reported Continued inspections to monitor potential problems Promote lessons learned

National Implications  Predictive Models Have Been Developed  Implemented Cost Effective Rehabilitation Strategies  Heightened Awareness Of Need For Inspections  Education On Use Of The Welded Shelf Plate Detail

CHALLENGES  How can we improve on the detail (triaxial)  Are there better tools to monitor bridges with this type of detail  What would you do differently if you were called in?  What other improvements can be made to the improved materials (HPS) or fabrication