Hoan Bridge Failure



Attachment A

Summary Conclusions of the Forensic Investigation

This is a brief summary of the conclusions reached by the forensic investigation team. The primary investigation was jointly performed by the FHWA, Lehigh University, and Lichtenstein Engineers, with assistance of the Wisconsin DOT. The final report of the failure analysis is in the final stages of preparation. This attachment provides a brief summary of the conclusions of the investigation.

Background

On December 12, fractures were discovered in all three girders of one of the southern approach spans to the Hoan Bridge carrying I-794 over the Milwaukee River. Figure 1 shows an underside view of the fractures. The interior and east exterior girders were fractured full depth, and the web had several 3 ft. fractures in the west exterior girder.

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Figure 1 Underside of the South Approach Span Showing the Fractured Girders

The failure analysis effort began with a brief close-up visual inspection of the fractures. After the span was removed by explosive demolition, sections of the cracked girders were excavated and sent to the Turner-Fairbank Highway Research Center (TFHRC) and Lehigh University for evaluation. Material properties were thoroughly evaluated and fractographic and metalographic studies were performed. The investigation also included detailed structural analysis, local stress analysis, fracture mechanics analysis and some live load testing of other sections of the structure.

Findings From the Investigation

• The 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 at the joint where the lower lateral bracing system framed into the web. The initiation site was located in the gap between the shelf plate and the transverse connection plate. Figure 2 shows a view of the joint assembly where the fracture initiated.

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Figure 2 Joint assembly where the lateral brace system frames into the giorder web.

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Figure 3 Fracture initiation site in the web gap Area.

• There was no evidence of fatigue cracking prior to fracture initiation. This indicates that 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.

• The web material properties met modern standards for A36 steel. Toughness met the 2001 AASHTO requirements for zone 2, fracture critical use.

• The flange material properties met modern properties for A588 steel. Toughness met the 2001 AASHTO requirements for zone 2, non-fracture critical use.

• Witnesses reported seeing two trucks on the bridge at the time of failure. Subsequent weigh-in-motion testing indicates that the truck weight was probably between 80,000 and 100,000 lbs. The live load stress due to this loading was low. The stress due to the sum of all loads (DL+LL+WL+Thermal) was within acceptable design limits.

• The narrow gap between the gusset plate and the transverse connection plate created an area of triaxial constraint in the web gap at the fracture initiation site. Constraint prevented yielding and the local stress state in the web gap was well beyond the yield strength of the material. The result is a lowered resistance to brittle fracture initiation.

• The dynamic toughness of the interior girder flange was insufficient to arrest a high rate fracture. The two exterior girder flanges had higher dynamic toughness and crack arrest occurred in the west exterior girder.

• The first fracture initiated in the interior girder in the web gap. The fracture continued to propagate through the flanges and completely severed the girder. This set off a chain reaction that causes fractures to initiate in the web gaps of the two exterior girders. The fracture continued through the flange in the east exterior girder, but arrested in the flange of the west exterior girder.

• 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 were retrofitted with arresting holes. The forensic investigation has determined that these prior cracks were fractures similar to the ones resulting in failure. However, all prior web cracks arrested at the flange and didn’t trigger the chain reaction failure.

Significance of Factors Contributing to Failure

The joint connecting the lower lateral bracing to the web is clearly the initiation site of failure in the Hoan. There are many know 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 has 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 add up and cause the chain reaction failure, but some are more significant than others in the process.

• Joint Details

The primary cause of fracture initiation was determined to be the geometry and fabrication tolerance of the joint wher the lateral bracing frames into the web. The joint was detailed with a narrow web gap that caused locally high constraint and reduced fracture resistance. As ideally detailed, the joint has only 1/8 in. separating the welds on the two plates. The fabrication tolerance resulted in reduced gaps and intersecting welds in many locations throughout the structure. Stress analysis showed that the intersecting welds increased the constraint level and made the constrain problem worse.

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

• Retrofit Hole

An asymmetric hole was drilled in one side of the shelf plate at the joint where failure initiated. This hole caused a further increase in stress concentration at the point where the failure initiated. Because fractures were also found to have initiated in joints without the hole, the presence of the hole is relatively insignificant.

• Effect of Truck loading

Weigh in motion results indicate that overloaded trucks were common on the bridge. However, the live load stress ranges in the gap area are relatively small compared to the dead load. This is consistent with the fact that no fatigue cracking was observed at the failure sites. Analysis shows the stress at the failure location was within design limits even when the overloads were on the bridge. Therefore, overloaded trucks had only a minor role in the failure.

• 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 causes a reduction in fracture initiation resistance that is relatively insensitive to temperature. Therefore, temperature had a minor effect on the fracture initiation at the time of failure.

The dynamic fracture resistance of the flange plates, however, 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 increases significantly. 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 should be noted, however, that the toughness specification used for bridge steels is based on preventing fracture initiation, not crack arrest.

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