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:
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 arches 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. Originally built with 6 lanes, in 1988 the brides
shoulders were removed to accommodate 2 more lanes.
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 close it 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.
An investigation into the collapse of the bridge revealed that the prime cause of failure was the
gusset plates. These are used to connect the steel girders in the trusses together. They were
undersized and inadequate to deal with increased loads the bridge had to support over time.
Other possible factors include the construction work which was underway on the bridge at the time,
as four of the lanes were closed for resurfacing and heavy construction equipment was operating on
the deck, and the accumulation of pigeon droppings on some members and joints, which had been
mentioned in previous bridge inspection reports. It had been suggested that pigeon droppings
accelerate the oxidising process of steel, and would therefore cause increased weakening of the
material over an extended period of time.
http://www.theglobeandmail.com/v5/content/pdf/_done_0802bridge_800.jpg
CONCLUSION
The failure of this bridge highlighted a number of shortcomings in the US system of both design and
subsequent inspection procedures. The bridge had been designed for a 50 year lifespan, a practice
which was brought into question by the events of August, 2007. The extent of the corrosion on
portions of the bridge including, significantly, the expansion bearings also points towards the use of
under-specified steel or inadequate protection.
Furthermore, most of these issues had been spotted and reported by Department of Transport
inspectors and yet their significance seems to have been underestimated or ignored. Many of the
issues brought up in the periodic inspections were left unresolved. The bridge was in the bottom
4% in federal inspection ratings nationally, but somehow slipped through the net with catastrophic
results.
The bridge collapse caused the loss of 13 lives, wounded over 100, and additionaly cost the city of
Minneappolis approximately $400k daily in lost revenue due to the severing of the important I-35
roadway.
SINCE THE DISASTER…
A new bridge was opened on the same site on September 2008, three months ahead of schedule.
The planning, design and construction stages were expedited because of the importance of this
crossing and the connected roads.
The new, 371 metre bridge was called St. Anthony Falls (possibly a poorly chosen name given the
circumstances of its construction), and represented the great leap forward in bridge technology in
the interim period between the construction of the original I-35W in 1967, and the bridge built in its
place.
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TIFF (Uncompressed) decompressor
are needed to see this picture.
The new bridge had was designed with a 100 year lifespan, twice that of the original, and currently
accomodates ten lanes of traffic with shoulders on both sides that could in the future be replaced
with more traffic lanes. Provisions were also made for the incorporation of a light-rail line. All this
was done to facilitate the growing traffic numbers and expansion of the city.
This bridge primarily consists of post-tensioned precast reinforced concrete, chosen for its durability
and ease of maintenance. The concrete construction allows for multiple layers of redundancy (the
lack of which on the previous bridge caused concern in the inspection reports), and the integration
of sensors to allow for in-depth monitoring of the bridge during the course of its lifespan.