Minneapolis Bridge Collapse
I-35W
Group 17
Rachel Keegan
Cian O’Reilly
Caoimhe Proud-Murphy
Background:
The I-35W Mississippi River Bridge, officially known as Bridge 9340 located in Minneapolis,
Minnesota, America. It was an eight-lane, steel truss arch bridge that carried the Interstate 35W
across the Mississippi River. The Bridge was designed in 1961 to the American Association of State
Highway standard Specifications. The construction contract at the time was worth more the US $5.2
million. Construction began in 1964 and the Bridge was opened to traffic in 1967. The I-35W Bridge
was the 5th busiest in terms of traffic in the State of Minneapolis and carried 140,000 vehicles a day.
On the 1st of August, 2007 the bridge collapsed during the evening rush hour, 13 people were killed
and 145 injured.
Structural Design:
The original Minneapolis bridge (I35-W) was a 580m long 14 span steel truss bridge, built to the
current American Association of State Highway Officials Standards from 1964-1967, by Hurcon Inc.
and Industrial Construction Company. The two main vertical support locations of the bridge were
located at either bank of the river, and the centre span of the bridge was 119m long. The bridge was
built during the 1960s twin cities building boom, and in 2007, when the bridge fell, it was still the
most recent river crossing bridge in Minnesota, as in the 70’s construction slowed, and the city
invested in maintenance and safety inspection.
Fig1: Structure of the original Minneapolis Bridge
The deck of the bridge held eight lanes of traffic, and at peak hours (rush hour) this traffic would be
at a relative standstill on the bridge, so loads varied hugely throughout the day depending on traffic
density.
Failure Mechanism:
On Wednesday, August 1, 2007 at 6.05pm, the Minneapolis Bridge collapsed. The centre span of the
bridge fell into the river almost completely intact, as the failure occurred close to the vertical
supports. The bridge failed at the north end first, which left the whole centre span resting on the
south end’s concrete block support as a sort of cantilever, this only lasted a moment, as the north
end soon failed, sending the centre span of the bridge into the river.
It is widely believed that the failure was caused by the inadequately sized gusset plates used to
connect trusses. This inadequacy was worsened by the fact that the load bearing capacity required
of the bridge increased over time. It was estimated that the construction weight situated on the
bridge was in the region of 261,000kg. This huge weight, combined with the stand-still rush hour
traffic, 2 inches of concrete which had been added to the road surface, lack of structural redundancy
and corrosion of the undersized gusset plates, meant all it took was for one gusset plate to fail, and
the whole bridge collapsed in series like dominoes falling.
Fig.2: Highlighting of problematic gusset plate connection
Structural Problems:
Annual safety inspection of the bridge began in 1993, so unlike other historic structural failures,
there is a lot of detail about the problems that caused this particular collapse
The structure of the bridge being flawed was first noted in 1990, when it was declared ‘Structurally
deficient’ and was said to be in need of repair due to the severe corrosion of its bearings. . Ten years
later, a study by the University of Minnesota on the bridge discovered that cracking in the transverse
trusses that provided the bridge with lateral support, coupled with little or no movement from the
severely corroded bearings, lead to deformation in the main trusses connected to these bearings.
The same report noted little or no redundancy in the main truss system, which meant if one single
truss failed the bridge as a whole would almost certainly collapse.
The pier located on the southwest, note the rust
& bearing device on top of pier, which had failed
previously.
Prevention:
Minnesota’s state department of transport (MNDOT) were in charge of the annual inspections of the
bridge but had relied on “visual inspections with load tests and strain gauges”. It is worth noting that
these visual checks would have missed crucial details. Small incisions could have been made into the
steel bridge which would have allowed for strain gauges to be installed. This would have given a
precise picture of how the bridge was reacting to loads etc. 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. In conclusion a design flaw in the sizing of the gusset plates was the reason along with
the extra weight on the bridge however if the state had carried out a more thorough inspection and
maintenance these defects might have been detected with the advanced technology of today. #
Part of a gusset plate from the bridge with fracture.
Conclusion:
A design flaw in the gusset plate, which holds together several girders, was found to be the cause of
the collapse by the National Transportation Safety Board. They were undersized and inadequate to
support the intended load of the bridge. The NTSB found broken gusset plates in eight of the 122
locations were they hold together steel girders in the steel truss. The monitoring of the fatigue
cracks by visual inspection and testing of the stress assumed the gusset plates were at the specified
size and thickness. The actual bridge had a much lower load capacity than the theoretical
calculations could predict. Contributing to that design error was the fact that 2 inches (51 mm) of
concrete were added to the road surface over the years, increasing the dead load by 20%. The
bridge was due to be replaced in 2020; it clearly should have been replaced before then.
Sources:
http://minnesota.publicradio.org/collections/special/2007/bridge_collapse/
http://en.wikipedia.org/wiki/I-35W_Mississippi_River_bridge
http://www.nytimes.com/2007/08/03/us/03safety.html
http://www.injuryboard.com/national-news/faulty-design-caused-minneapolis-bridgecollapse.aspx?googleid=29080