failure

Topics: I-35W Mississippi River bridge, Truss bridge, Mississippi River Pages: 12 (2645 words) Published: November 15, 2013
SYSTEM FAILURE CASE STUDIES
SEPTEMBER 2009 VOLUME 3 ISSUE 06

A Half-Inch to Failure
At 6:05 pm, on Wednesday, August 1, 2007, the Interstate-35
West (I-35W) bridge over the Mississippi River in
Minneapolis collapsed. On the day of the collapse, four of
the bridge’s eight lanes were closed for planned
construction. Four weak connector plates fractured under
the combined burden of rush hour traffic, concentrated
construction equipment, and previous heavy renovations.
The bridge fell 108 feet into the Mississippi River. The
police, fire department, and U.S. Coast Guard immediately
initiated rescue operations. Of the 190 people on or near the bridge, thirteen died and 145 were injured.

BACKGROUND
he I-35W bridge supported a 1,907 foot long, 8-lane
wide roadway that served Minneapolis for forty
years. The state inspected the bridge annually using
the National Bridge Inspection Standards set by the Federal
Highway Administration (FHWA). Inspectors had been
labeling the bridge “structurally deficient” since 1991. This label indicated that the bridge required significant
maintenance and repair to remain in service, but not that it was unsafe (inspectors would have closed the bridge if they
believed it was unsafe). A structurally deficient rating is not uncommon; approximately 12% of U.S. bridges are rated
structurally deficient.

T

Steel truss bridges, like I-35W, were more frequently labeled structurally deficient; approximately 31% of the 465 steel
truss bridges in the U.S. were listed as structurally deficient at the time of the collapse. Such bridges consist of straight beams of steel formed into triangular units (Figure 1). In
large steel truss bridges, the ends of the beams are connected with riveted metal plates called gusset plates. I-35W’s
gusset plates connected three beams at each node: two
diagonal beams and one vertical beam (Figure 2).
Over the course of forty years, the state of Minnesota
conducted significant renovations on the bridge three times. In 1977, the State increased the bridge deck thickness about two inches. This renovation increased the dead load (weight
of the structure itself) by 13.4%. In 1998, the State increased the dead load another 6.1% when it installed a median
barrier. Together, these two renovations increased the
weight of the bridge 19.5% over the original design.

Figure 1: The I-35W Bridge

A third set of renovations began in June 2007, two months
before the collapse.
The Minnesota Department of
Transportation (MnDOT) hired a construction contractor,
Progressive Contractors, Inc. (PCI), to resurface the bridge. The renovations involved removing two inches of concrete
on the roadway and replacing it with fresh concrete.
Resurfacing required heavy construction equipment to mix
and pour concrete. In the weeks leading up to the collapse,
the construction contractor had poured concrete on seven
bridge deck sections.
The contractor placed heavy
construction equipment on bridge ramps for five concrete
pours; one pour stationed some equipment on the bridge
August 1, 2007: The I-35W Bridge that
spanned the Mississippi River in Minneapolis
collapsed, killing 13 and injuring 145.
Proximate Cause:
• Weak gusset plates fractured under the weight of rush
hour traffic, previous bridge renovations and
concentrated construction materials.
Underlying Issues:
• Inadequate gusset plate design and insufficient review
process.
• Added weight from renovations, traffic, and
construction materials.
• Lack of attention to gusset plates in inspections and
load ratings.
• Communication issues between the construction
contractor and the State.

deck and some on the
ramps,
and
another
positioned the equipment
and materials on the
bridge deck itself.
About a week before the
collapse, the contractor
was preparing for one of
the concrete pours, and
the foreman asked the
state bridge construction
inspector if equipment
and materials...
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