I-35W Mississippi River bridge Collapse
GROUP 17
JONATHAN DUNPHY
JONATHAN ROONEY
PETER GORMAN
CILLIAN SUITER
INTRODUCTION
Constructed in 1967, the I-35W Mississippi River bridge (officially known simply as Bridge 9340)
was an eight-lane steel truss arch bridge that carried Interstate 35W across the Mississippi River in
Minneapolis, Minnesota, United States. The bridge was designed in by Sverdrup & Parcel to 1961
American Association of State Highway Officials standard specification. Construction began in 1964
and the bridge was opened to traffic in 1967. The bridge was Minnesota's fifth–busiest, carrying
140,000 vehicles daily. The bridge catastrophically failed during the evening rush hour on August 1,
2007, collapsing to the river and riverbanks beneath. Thirteen people were killed and 145 were injured.
DESIGN:
The I-35W Mississippi River Bridge was a steel truss arch bridge that carried eight lanes of interstate
traffic across the Mississippi River in Minneapolis, Minnesota.
In total the bridge had 14 spans extending to 580m in length. The three main spans were steel truss
constructions, with the remaining eleven smaller spans consisting of steel multi-girder construction
and concrete slab construction. The main centre span consisted of a single 140m steel arched truss
over the 119m wide river to avoid putting any piers in the water which would impede river navigation.
The two support piers for the main trusses were located at either side of the main centre span. Each
pier consisted of two load bearing concrete pylons as can be seen in the photograph. At the top of the
main trusses were the deck trusses, 3.6 m in depth and integral with the main trusses. The transverse
deck beams which are part of the deck truss were placed on top of the main trusses. These deck beams
supported the concrete pavement and road surface. The roadway deck was approximately 35m above
the water surface.
FACTORS CONTRIBUTING TO FAILURE:
On Wednesday, August 1, 2007, the I-35W Bridge suffered structural failure and collapsed into the
Mississippi River. The collapse happened at the height of rush hour with an estimated 100 cars on the
structure. The death toll counts 13 dead and 144 injured.
This bridge is considered to be a non-redundant structure. That is, if any one member fails then the
entire bridge can collapse. A key factor is that there are only four pylons holding up the main truss
arch. Any damage to any one pylon would result in a collapse of the bridge.
Previous inspections had noted that several steel girders had suffered fatigue cracks, numerous fatigue
cracks were also found in spans #3 – 5 and #9 – 10. The bride was declared by the US Department of
Transportation to be structurally deficient. However it was not considered to be deficient enough to
warrant being closed down.
The above photograph is a close-up of the southwest pier. There is a large build up of rust. On top of
the concrete pylon is a bridge bearing device. This device allows the bridge to slide back and forth as
the bridge expands and contracts during heating and cooling cycles. The bearings had failed on this
bridge years prior to the collapse resulting in additional loads being placed on the bridge as it was
prevented from moving freely.
Failure Methods Of The Minneapolis Bridge
The Minneapolis Bridge was over 40 years old and was built at a time when vehicular traffic
and weights were much less than they are today. It was also at a time when bridge steels and
redundancy where not at today's standards. An Independent consulting firm found one of the
possible design flaws which were the steel gusset plates. The steel gusset plates were used to
connect girders together in the truss structure. These design specified steel plates were
undersized and inadequate to support the intended load of the bridge. During the wreckage
recovery, investigators discovered that gusset plates at eight different joint locations in the
main centre span were fractured. The bridge failure mechanism was not due to oscillation like
the famous Tacoma Bridge but was a much more sudden failure which suggests that it was
due to a single-point failure that triggered the structure’s destruction, this suggests that the
failure of the Minneapolis bridge was due to a serious error in the sizing of some of the gusset
plates in the main truss and also the monitoring and maintenance of the bridge.
Prevention
More thorough inspection and better quality maintenance of the bridge could have prevented
the collapse. An official report carried out by the Minnesota Department of Transportation
Metro District found that cracks were located in negative moment regions where the
diaphragm web stiffener was not welded to the top flange and at one location the web had
cracked through entirely. Most existing cracks were drilled out, and any fractured beams
were reinforced with bolted plates. To reduce the stress levels, the diaphragms were also
lowered. Prior to the collapse a wide-scale steel reinforcement project was planned for the
bridge but was later cancelled in favour of periodic safety inspections, as the drilling might
have weakened the bridge structure. Perhaps if the steel retro-fitting had been carried out the
collapse could have been prevented. Studies found by the university of Minneapolis noted a
lack of redundancy in the main truss system, which therefore meant that there was a greater
risk of collapse if a single structural failure occurred. The bridge was also cited as structurally
deficient in 1990. A closer more rigorous inspection should have been carried out as visual
inspections miss a lot of cracks in the structure. Bridge instrumentation by strain gages and
continuous structural health monitoring should have been carried out to prevent the collapse.
Conclusions
The Minneapolis bridge collapse was a tragic disaster that left 13 people dead and 144 injured was a
result of poor maintenance and monitoring of the bridge’s structural integrity, the bridge should have
been blocked from the public and analysed in more detail when it was declared as structurally
deficient. It also is a good demonstration of why non-redundant structures can be unsafe if they are
not monitored carefully as a single failure in the bridge will cause the entire structure to collapse. It
also highlights the importance of the role of the engineer in the design of structures. If the gusset
plates had been designed to the correct width dimensions to withstand the loading then the collapse
could have been prevented.
References
http://www.theglobeandmail.com/v5/content/pdf/_done_0802bridge_800.jpg
http://en.wikipedia.org/wiki/I-35W_Mississippi_River_Bridge
http://www.channel4000.com/download/2007/0803/13814343.pdf
http://www.break.com/index/security-tape-of-mississippi-collapse.html