Washington, DC - The National Transportation Safety Board
has determined the probable cause of the collapse of the I-
35W bridge in Minneapolis, Minnesota, was the inadequate
load capacity, due to a design error by Sverdrup & Parcel
and Associates, Inc., of the gusset plates at the U10 nodes,
which failed under a combination of (1) substantial
increases in the weight of the bridge, which resulted from
previous modifications, and (2) the traffic and concentrated
construction loads on the bridge on the day of the accident.
Contributing to the design error was the failure of Sverdrup
& Parcel's quality control procedures to ensure that the
appropriate main truss gusset plate calculations were
performed for the I-35W bridge and the inadequate design
review by federal and state transportation officials.
Also contributing was the generally accepted practice among
Federal and State transportation officials of giving
inadequate attention to gusset plates during inspections for
conditions of distortion, such as bowing, and of excluding
gusset plates in load rating analysis.
"We believe this thorough investigation should put to rest
any speculation as to the root cause of this terrible
accident and provide a roadmap for improvements to prevent
future tragedies," said NTSB Acting Chairman Mark V.
Rosenker. "We came to this conclusion only through
exhaustive efforts to eliminate each potential area that
might have caused or contributed to this accident.
"Bridge designers, builders, owners, and inspectors will
never look at gusset plates quite the same again, and as a
result, these critical connections in a bridge will receive
the attention they deserve in the design process, in future
inspections, and when bridge load rating analyses are
performed. By addressing all three areas in our
recommendations, we are hopeful that industry and government
bodies will take appropriate action and the American people
can continue to have confidence in the safety of our
nation's bridges," he added.
About 6:05 p.m. Central Daylight Time on Wednesday, August
1, 2007, the eight-lane, 1,907-foot-long I-35W highway
bridge over the Mississippi River in Minneapolis, Minnesota,
experienced a catastrophic failure in the main span of the
deck truss. As a result, 1,000 feet of the deck truss
collapsed, with about 456 feet of the main span falling 108
feet into the 15-foot-deep river. A total of 111 vehicles
were on the portion of the bridge that collapsed. Of these,
17 were recovered from the water. As a result of the bridge
collapse, 13 people died, and 145 people were injured.
During its investigation, the Safety Board learned that 24
under-designed gusset plates, which were about half the
thickness of properly sized gusset plates, escaped discovery
in the original review process and were incorporated into
the design and construction of the bridge.
On the day of the collapse, roadwork was underway on the I-
35W bridge, and four of the eight travel lanes (two outside
lanes northbound and two inside lanes southbound) were
closed to traffic. In the early afternoon, construction
equipment and construction aggregates (sand and gravel for
making concrete) were delivered and positioned in the two
closed inside southbound lanes. The equipment and
aggregates, which were being staged for a concrete pour of
the southbound lanes that was to begin about 7 p.m., were
positioned toward the south end of the center section of the
deck truss portion of the bridge near node U10 and were in
place by about 2:30 p.m.
Shortly after 6 p.m. a lateral instability at the upper end
of the L9/U10W diagonal member led to the subsequent failure
of the U10 node gusset plates on the center portion of the
deck truss. Because the deck truss portion of the I-35W
bridge was considered non-load-path-redundant, the total
collapse of the deck truss was unavoidable once the gusset
plates at the U10 nodes failed.
The NTSB examined other possible collapse scenarios - such
as corrosion damage found on the gusset plates at the L11
nodes and elsewhere, fracture of a floor truss, pre-existing
cracking in the bridge deck truss or approach spans,
temperature effects and shifting of the piers - and found
that none of these played a role in the accident.
As a result of its investigation, the NTSB made nine
recommendations to the Federal Highway Administration and
the American Association of State Highway and Transportation
Officials dealing with improving bridge design review
procedures, bridge inspection procedures, bridge inspection,
training and load rating evaluations.
A synopsis of the Board's report, including the probable
cause, conclusions, and recommendations, is available on the
NTSB's website, www.ntsb.gov, under "Board Meetings." The
Board's full report will be available on the website in
several weeks.
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