Daniel Webster Hoan Bridge

Download Entire Forensic Investigation and Failure Analysis Report
(3,021 KB PDF File)

Hoan Bridge Failure Investigation and Retrofit
A Summary

 
THE BRIDGE

The 1.9 mile long Hoan Bridge, located on interstate I-794 in the City of Milwaukee, first opened to traffic in 1977. It carries 6 lanes of traffic and 36,000 vehicles daily over the Milwaukee River. It has a vertical clearance of 120 feet over the navigable waterway, and a clear span of 600 feet. Eighteen bridge spans are continuous steel three-girder spans. Beyond that, the remaining spans are of a multi-girder configuration. An aerial view of the Hoan Bridge is shown in Figure 1.


Figure 1. Aerial view of the Hoan Bridge

THE PROBLEM

On December 13, 2000, the roadway began to visibly sag. Upon investigation 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. On December 28, 2000, the most critically damaged section of the northbound roadway was removed by explosive demolition (Figure 2).


Figure 2. Damaged span being removed by explosive demolition.

A plan view of a portion of the Hoan Bridge is shown as Figure 3. The location of the fractured span is highlighted.


Figure 3. South approach Unit S2A showing location of fractured span.

A cross-section of the three-girder structure is shown in Figure 4. The framing plan and elevations for the northern end span of Unit S2A is presented in Figure 5 that also identifies the cracked girder locations at Panel Point 28.


Figure 4. Typical cross-section.


Figure 5. Framing plan and elevation of fractured span.

Figure 6 shows the fractures that developed in center girder E-28 and outside girder F-28 prior to demolition


Figure 6. Visible fractures of center girder E-28 and east girder F-28. Note diagonal lateral bracing.

The west exterior girder D did not experience a flange fracture although a 3-foot long web crack had formed in the girder web, as illustrated in Figure 7.


Figure 7. Cracked web of girder D-28.

THE FORENSIC INVESTIGATION AND CONCLUSIONS

Lichtenstein Consulting Engineers, Inc. (Lichtenstein) was retained under an emergency contract by the Wisconsin Department of Transportation to investigate the failure and design the retrofit of the Hoan Bridge. In conjunction with Lehigh University and the Federal Highway Administration, a forensic investigation was performed of the failed span that concluded the cracking was not related to fatigue and that the brittle fracture 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. The Hoan Bridge brittle fracture was the first of its kind ever observed in the United States and is likely to have a nationwide impact on bridges with similar details.

Services performed by Lichtenstein included the following tasks:

  • 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 (until long-term fatigue and fracture retrofit measures were implemented) so southbound lanes could be opened to traffic at the earliest possible date. This work was completed on February 17, 2001, and the southbound bridge was reopened to restricted traffic (4 ton weight limit).
  • Task II (Forensic Investigation) - A failure analysis was conducted on the failed unit to ascertain causes and modes of failure and recommend future action (retrofit or replacement) 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. This $7.8 million fast track permanent retrofit of all three-girder spans on the bridge and replacement of the demolished span was completed in October 2001. The project team received the FHWA Strive for Excellence Award in 2001 for its efforts.

EMERGENCY INSPECTION AND RETROFIT OF SOUTHBOUND LANES

Immediately after demolition of the failed northbound span, Lichtenstein, working closely with Dr. John Fisher of Lehigh University, developed an emergency retrofit program for fatigue and fracture prone details on the southbound bridge. The objective of this accelerated project was to ensure public safety, in preparation for a limited opening of the southbound lanes to two-way automobile traffic, through a detailed fracture- critical inspection of the southbound bridge and the retrofitting of cracks and shelf plate details in tension areas.

The retrofit utilized drilled holes at all shelf plates in the tension zone of the southbound lanes that isolated the fatigue and fracture prone details and ensured that subsequent cracks would be arrested within the bounds of the retrofit holes. Cracks that were found to extend beyond the retrofit region were isolated and the cracked section repaired with bolted splice plates. This retrofit was designed to prevent further girder fractures as well as prevent significant cracking in the girder webs. It ensured that the structural system was damage tolerant and could be safely used.

SIGNIFICANT FINDINGS FROM THE FORENSIC INVESTIGATION

Crack surfaces were examined under high magnification using a scanning electron microscope. The failure mode was positively identified as brittle, cleavage fracture. The fractures occurred suddenly and propagated through the girders at an explosive rate.

  • 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 (shelf) plate and the transverse connection/stiffener plate. Figure 8 shows a view of the joint assembly where the fracture initiated. Figure 9 shows the fracture initiation site in the web gap area.


Figure 8. Joint assembly where the lateral brace system frames into the girder web.

  • There was no evidence of fatigue cracking prior to fracture initiation. This indicates there was no observable damage prior to the sudden fracture. Even the most rigorous fracture critical inspection would not have provided warning of the impending fracture.
  • Web material properties met modern standards for A36 steel. Toughness met the 2001 AASHTO requirements for zone 2, fracture critical use.
  • Flange material properties met modern properties for A588 steel. Toughness met the 2001 AASHTO requirements for zone 2, non-fracture critical use.


Figure 9. Fracture initiation site in the web gap area.

  • Subsequent weigh-in-motion testing on a detour route indicated that average truck weight was approximately 80 kips with a (+/-) 20 kip variation. 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 dynamic fracture toughness 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.
  • Inspection reports indicate that web cracks were found in other locations of the bridge as early as 1995. The cracks were thought to be fatigue cracks and retrofit actions were taken based on this assumption. The forensic investigation has since determined these prior cracks were fractures similar to the ones resulting in failure. However, all prior web cracks arrested at the flange and did not trigger the chain reaction failure.

SIGNIFICANCE OF FACTORS INVOLVED IN THE FAILURE

The joint connecting the lower lateral bracing to the web was clearly determined to be the initiation site of the failure in the Hoan Bridge. There are many known cases of fatigue cracking from this type of detail, but this is the first known case of brittle fracture. The Hoan Bridge case is unique in that there was no evidence of fatigue prior to failure. The forensic investigation studied all of the factors present at the time of failure and a relative assessment can be made regarding their significance in the failure process. It took a combination of factors to cause the chain reaction failure, but some were more significant than others in the process.<.p>

  • Joint Details - The primary cause of fracture initiation was determined to be the geometry and fabrication tolerance of the joint where the lateral bracing frames into the web. The joint was detailed with a narrow web gap that caused local high constraint, increased stiffness, and reduced the apparent fracture resistance. As ideally detailed, the joint has only 1/8 inch separating the welds on the two plates. The fabrication tolerance resulted in reduced gaps as well as intersecting welds in many locations throughout the structure. Stress analysis showed that the intersecting welds increased the rigidity of the joint and made the constraint problem worse. This non-ductile behavior in the joint caused by a triaxial constraint and state of stress has never before been documented as being a potential problem in bridge detailing. This is the first time this problem has been reported.

Additionally, the “K” pattern in the lower lateral brace system introduced an axial force in the girder to satisfy equilibrium in the joint area. A stress analysis showed that this increased the live load stress range at the outside ends of the shelf plate, but that there was little effect in the gap area.

  • Effect of Temperature - Without high constraint, the web plate toughness is sufficient to prevent fracture initiation down to the lowest anticipated service temperature of -30ºF. The constraint in the joint assembly caused a reduction in fracture initiation resistance that was relatively insensitive to temperature. Therefore, low temperature probably had a minor effect on the fracture initiation, but it significantly reduced the ability of the structure to arrest dynamic cracks. The failure sequence where multiple fractures caused the structure to unzip would have become less likely at higher temperatures.

This is supported by the fact that the dynamic fracture resistance of the flange plates was shown to decrease rapidly as a function of temperature. Therefore, temperature had a significant effect on the ability of the flanges to arrest cracks. At higher temperatures, the probability of crack arrest significantly increases. The low temperature at the time of failure was the significant factor that allowed the web fracture to progress to a chain reaction failure of the structure. It is noted, however, that the toughness specification used for bridge steels is based on preventing fracture initiation.

LONG-TERM RETROFIT DESIGN

The investigation of the failure of the Hoan Bridge concluded that the high degree of constraint present at the girder shelf plate assembly resulted in a triaxial condition at these joints. Constraint was a primary factor in the sudden fracture of the girders. The investigation recommended the shelf plate assemblies and lateral bracings should be removed to restore ductility and prevent a recurrence of such fractures. Figure 10 depicts a view of the underside of the bridge after the lateral bracing has been removed.


Figure 10. Underside of bridge after lateral bracing has been removed.

Rehabilitation efforts consisted of the retrofit of all three-girder units in the approach spans and replacement of the demolished portion of steel superstructure span in unit S2A. 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.

ASSESSMENT OF OTHER STRUCTURES

A review of the bridge inventory conducted by FHWA and State DOTs to identify structures that have design features similar to the Hoan Bridge indicated that at least 41 States and the District of Columbia have bridges with similar design characteristics as the Hoan Bridge. These structures should be assessed in further detail, on a case-by-case basis, to determine if the similar details might be subject to the same vulnerability as experienced in Wisconsin.

[Overview] [Bridges] [Highways] [Rail Transit] [Historic] [Codes & Standards] [Construction] [Jobs]
[Home] [Sitemap] [E-Mail Directory] [Locations]
Comments on our site?  Please send them to
webmaster@LCE.us
Lichtenstein Consulting Engineers
45 Eisenhower Drive
Paramus, New Jersey  07652
(201) 368-0400