HOAN BRIDGE FAILURE

INTRODUCTION

• The Hoan bridge is 1.9m long.• It is located in the City of Milwaukee.

• It carries 36,000 vehicles daily over the Milwaukee River.

• It has a vertical clearance of 120 feet over the navigable waterway.

• It has a clear span of 600 feet.

THE PROBLEM

• The roadway began to visibly sag.

• The cracks were detected in the steel girders supporting one of the northbound spans.

• Two of the three girders had full depth fractures, leaving the span near collapse.

• The entire roadway was immediately closed to traffic.

• The most critically damaged section of the northbound roadway was removed by explosive demolition .

Damaged span being removed by explosive demolition

South approach Unit S2A showing location of fractured span.

Typical cross-section.

Visible fractures of center girder E-28 and east girder F-28. Note diagonal lateral bracing.

Cracked web of girder D-28.

INVESTIGATION AND CONCLUSIONS

• The cracking originated in the lower shelf plate joint assembly and traveled the length of a vertical 10-foot steel “I” girders in a single cycle.

• The joint assembly at this point was highly constrained.

• Task I (Emergency Retrofit) -

• Existing defects were identified in steel members and details of the southbound spans through visual inspection and non-destructive testing methods.

• Hole drilling was performed at select locations and short-term repairs were made to ensure the safety of the southbound bridge

• Task II (Forensic Investigation) -

• A failure analysis was conducted on the failed unit to ascertain causes and modes of failure and recommend future action for all similar spans on the bridge

• Task III (Long-term Retrofit) -

• Contract plans, specifications and estimates were developed for the structural steel retrofit and reconstruction of the demolished span of the Hoan Bridge in accordance with recommendations of the failure investigation and retrofit study.

SIGNIFICANT FINDINGS FROM THE INVESTIGATION

• Crack surfaces were examined under high magnification using a scanning electron microscope

• The fractures initiated in the web plate, most likely the interior girder, at the joint where the lower lateral bracing system framed into the web.

• The initiation site was located in the gap between the gusset plate and the transverse connection/stiffener plate.

Joint assembly where the lateral brace system frames into the girder web.

• Structural analysis and field-testing showed that type of live load applied to the Hoan Bridge would have produced a relatively low live load stress range.

• The stress due to the sum of all loads (DL+LL+WL+Thermal) was probably within acceptable design limits for the bridge

• A narrow gap between the gusset plate and the transverse connection/stiffener plate created a local triaxial constraint condition and increased the stiffness in the web gap region at the fracture initiation site.

• This constraint prevented yielding and redistribution of the local stress concentrations occurring in this region.

• As a result, the local stress state in the web gap was forced well beyond the yield strength of the material.

• Under triaxial constraint, the apparent fracture toughness of the material is reduced and brittle fracture can occur under service conditions where ductile behavior is normally expected.

• The first fracture probably initiated in the interior girder in the narrow web gap formed by the detail.

• The dynamic toughness of the interior girder flange was insufficient to arrest a high rate fracture initiating in the web.

• The web fracture continued to propagate through both girder flanges and completely severed the girder.

• This set off a chain reaction that caused fractures to initiate in the web gaps of the two exterior girders.

• The fracture continued through the flanges in the east exterior girder, but arrested in the flanges of the west exterior girder.

• The fracture of the exterior girder flanges is high enough that crack arrest is possible depending on load level.

• The reason arrest occurred in only one of the exterior girders can be explained by unequal re-distribution of loads during the failure sequence.

• The investigation has since determined these prior cracks were fractures similar to the ones resulting in failure.

LONG-TERM RETROFIT DESIGN

• The investigation recommended the shelf plate assemblies and lateral bracings should be removed to restore ductility and prevent a recurrence of such fractures.

Underside of bridge after lateral bracing has been removed.

Major items of work included:

• Removal of bottom lateral bracings and shelf plates at all panel points.

• Attachment of girder connection plates to the tension flanges using bolted structural tee sections at interior panel points.

• Installation of transverse bracings at girder end diaphragms at each pier panel point to transfer wind loads.

• Non-destructive testing and remedial repair of defects by drilling crack-arrest holes or by grinding.

• Repair of cracked steel by splicing.

• Replacement of the demolished portion of unit S2A with a superstructure system similar to the demolished span.

• Reconstruction of demolished deck, parapets and deck joints.

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