Concept Artwork: projects.dot.state.mn.us/35wbridge/
A Bridge to the Future
Construction of a new St. Anthony
Falls Bridge began on October 29,
2007. The preliminary bridge designs feature structural redundancy
enhancements, as well as safety
and aesthetic features far superior
to the previous structure. High
performance concrete is employed
for durability of the bridge deck
and piers. The new bridge will be fitted with a sensor system to monitor structural integrity of all parts of the bridge over its lifetime. A final design was quickly selected in
the months following the disaster by a voting process that included local leaders and
community members. If completed on schedule, the new bridge will open to traffic on
December 24, 2008.
Additional Reading
For the more information regarding the events of August 1, 2007, the NTSB’s findings,
or the St. Anthony Falls Bridge, visit any of the resources listed below.
John A. Weeks III: I-35W Bridge Collapse
www.johnweeks.com/i35w/i35wdetail.html
NTSB Press Release SB-08-02 January 15, 2008
www.ntsb.gov/Pressrel/2008/080115.html
Minnesota Public Radio: Minneapolis bridge collapse
http://minnesota.publicradio.org/collections/special/2007/bridge_collapse/
13 seconds in August: The 35W bridge collapse
www.startribune.com/local/12166286.html
I35W St. Anthony Falls Bridge - Minnesota Dept. of Transportation
I-35W Mississippi River Bridge
Deconstructing the Fall
John Wethington
February 18, 2008
English 314, Section 4
Cover Photo Credits: www.dot.state.mn.us/i35wbridge/photos/
Photo Credit: maps.live.com
lack of redundancy in the truss system. The
significance of the redundancy issue would
only become apparent after the bridge’s collapse, as it implied that any single structural
failure in the bridge would make it much
more susceptible to total failure.
Gravel pile used for ongoing deck repairs
at the time of collapse
Collapse and Aftermath
Disaster Strikes Minneapolis
Photo Credit: mbax.deviantart.com/art/35W-BridgeCollapse-61214609
On August 1, 2007, one of the greatest bridge disasters
in the United States unfolded when the I-35W Mississippi River Bridge in Minneapolis, MN collapsed, leaving 13 dead and 144 injured. The cause of the collapse
was not immediately apparent, with several possible
failure scenarios hotly debated, as well as a host of
conspiracy theories and myths surfacing in the absence
of a clear cause.
Location and History
The I-35W crossing site is located near Saint Anthony
Falls, a natural waterfall along the Mississippi River.
As part of a massive infrastructure building project in
the Minneapolis area, construction on the I-35W Mississippi River Bridge began in 1964, with the bridge
opening in 1967. The land in the Saint Anthony Falls
area was previously used by industrial facilities, including a coal-to-gas processing plant and a petroleum processing plant. As a result, the riverbed below the bridge was riddled with toxic waste byproducts from over 100 years of industrial pollution.
In the early 1990s, 15,000 tons of material were removed from below the bridge. Interim reports published by the National Transportation Safety Board (NTSB) have not
indicated that any prior use of the
site contributed to the collapse.
At 6:05 p.m. on August 1, 2007, the central
span of the I-35W bridge collapsed suddenly, setting off a chain reaction that immediately spread to the adjoining spans. It
took about 13 seconds (the exact time is disputed due to missing frames in a security
camera video capturing the fall) for the entire bridge to fall, with the steel structure
and deck collapsing into the river and onto
the banks below.
U-10 Connector
L-11 Connector
L-9 Connector
Locked Bearing
Photo credit: www.johnweeks.com/i35w/i35wdetail.html
Various critical connecting and support elements contributed to the bridge’s instability
An estimated 100 vehicles were stranded on the collapsed segments of bridge or
washed into the river. Recovery efforts lasted three weeks, with a final toll of 13
dead and 144 injured.
Why Did The I-35W Bridge Fall?
The NTSB issued several press releases in August of 2007, but the most significant
findings were published in a January 2008 interim report. The findings of this report outlined the primary cause of failure to be a flaw in the design of the bridge.
Designing engineers had specified the thickness of critical gusset plates too thin for
the load-bearing requirements of the bridge. The primary function of gusset plates
is to tie the load-bearing steel girders of a bridge together. Over time, as weight was
added to the bridge in the form of hardware improvements and deck
renovations, the probability of failure increased dramatically, with the
total load on these under-designed plates becoming greater and
greater. The bridge was deemed “fracture critical,” indicating that
failure of any major member would have caused a collapse due to the
lack of redundancy in the support system. Recommendations set
forth by the NTSB pending its final report release included close inspection of the designs of similar bridges to ensure structural soundness and safety.
Independent analysis of previous inspection reports of the bridge,
combined with pre-collapse photographs, suggest that several other
Design and Maintenance
Graphic Credit: NTSB Interim
Report, February 2008
factors contributed to the failure. The Minneapolis Star Tribune reThe over 1,900-foot long bridge was
ported that the L-9, U-10, and L-11 connectors all showed evidence of
designed with no piers in the river,
pre-collapse damage. Other reports suggest that bearings in the piers of the bridge,
instead relying on a single 458-foot steel arched truss for its center
critical in allowing movement during temperature changes, were locked in place.
span. A common problem plaguing the bridge — black ice and exAdditionally, the construction taking place on the bridge immediately before it coltremely dangerous winter conditions — had been remedied with an
lapsed redistributed loading in such a way that may have put more direct stress on
automatic potassium acetate solution dispensing system, in use since
the extremely important and inevitably weakened U-10 connectors.
2000. Experts are still debating whether the chemical deicers may
Photo credit: James Lammens,
Associated Press
Photo credit: www.johnweeks.com/i35w
have led to weakening of key members in the bridge.
As far back as 1990, federal inspectors rated the bridge "structurally
deficient" due to significant amount of corrosion in its bearings. A
2001 University of Minnesota study discussed cracking in girders at
various locations along the bridge and also indicated concern over the
Photo Credit: en.wikipedia.org/wiki/
Image:I-35W-bridge-Minneapolis-20070801.jpg
Graphic Credit: www.msnbc.msn.com