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
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
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
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
- 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
- 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
- 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.