2011
Group 12
John Lawlor
Conor O’Leary
Cormac Doherty
[MINNEAPOLIS BRIDGE COLLAPSE]
4A6.1 Structures
Design
The I35-W was a steel truss highway bridge spanning 581m and supporting 8 lanes of traffic. On the
1st August 2007, 18:05, the I-35W Minneapolis Bridge collapsed into the Mississippi river 64m below.
At the time it was the second busiest bridge in Minnesota carrying an average 141,000 vehicles per
day.
The I-35W had 13 reinforced piers and 14 spans. The deck consisted of 2 reinforced concrete slabs
separated by about 6 inches. The deck truss portion comprised two main Warren type trusses with
verticals. Upper and lower chords extended the length of the deck truss. They were connected by
vertical and diagonal members. The upper and lower chords designed for compression while the
vertical and diagonal members designed primarily for tension. Riveted steel gusset plates at every
node tied the members to one another and to the rest of the structure.
Failure of the Bridge
On the afternoon of the 1st of August the two outer and two inner lanes of the eight lane I-35 Bridge
were closed to accommodate the delivery of these construction materials. This delivery and
placement of the materials was completed by 14:30 that day.
The building materials together with construction machinery such as an 11,300L water tanker and
two 37 tonne cement mixers not to mention additional mixers, excavators and transportation
machinery arriving to the scene throughout the afternoon significantly added to the imposed and
dead loads the bridge was being subjected to. From 17:00hrs that afternoon rush hour traffic began
to build and within the hour traffic crossing the bridge was significant with witnesses of the collapse
stating traffic was bumper to bumper.
At 18:05 a motion detecting camera monitoring the bridge was activated. Seconds later the main
span of the deck truss of the bridge gave way to the excessive loading conditions and over 142m of
the main span plunged into the 4.5m waters of the Mississippi below. 111 vehicles were passing the
collapsing section of the bridge.
The motion activated camera positioned southwest of the centre span captured 23 images of the
collapse over a 10 second timeframe. This footage was used to identify the first areas of collapse in
the bridge. The first image captures the southern end of the structure giving way. Simultaneously in
the northern section of the bridge a bend in two of the lower chord members of the bridge was
visible while structural threads and risers were beginning to separate from the structure. Within the
following 3 seconds the centre span of the bridge separated from the structure and plunged into the
river below.
While the majority of the damage ensued by the collapse occurred in deck span of the bridge, the
approach spans were also subjected to significant damage where the cantilever end of each
approach had been supported by the central span, however these failures did not yield specific
attention as the cause of failure as the manner of collapse was deemed concurrent to that of a
cantilever with a support removal. This refocused attention towards the central span of the bridge as
the source of failure.
Investigation
The final position of the entire collapsed southern end of the deck span had displaced north towards
the river during the collapse, indicating that the lower central chord members of the deck truss were
among the first structural members to give way in the collapse. Further inspection of one of these
lower chord members showed that damage was severe enough to cause partial fracturing at midlength. The same member was identified to have broken away from the connecting node through
shearing of the connecting rivets of the member and the adjoining gusset plates.
External loading conditions on these chord members due to pier displacement prior to the collapse
was also ruled out as the initial positioning of the piers seemed both stationary and vertical
according to video footage.
Further inspection of the chord members revealed that upper truss chords had separated as a result
of additional gusset plate fracturing, by the end of the investigation it was found that a further 16
gusset plates contained fracture damage.
It was determined on January 15th 2008 by the National Transport Safety Board (NTSB) that the most
likely cause of failure was inadequate design of the steel gusset plates, as the suspect gusset plates
after collapse were found to be just 0.5inches thick while the legal minimum requirement was
double this value.
However other sources were found to contribute to the collapse of the bridge. The road
maintenance in 1977 renovating and increasing deck thickness together with the 1998 construction
of a median barrier, traffic railings and an anti-icing system increased the dead load of the bridge by
approximately 24%. Additionally the building materials deposited on the bridge throughout the day
were deposited almost directly above the weakest point of the bridge.
At the time of collapse the total estimated weight the bridge was being subject to was almost 672
tonnes; over 55% of this total load was exerted on the central span of the bridge. This loading
contributed to the eventual failure of the gusset plates and thus the bridge. A review of the original
design created by Sverdrup & Parcel found that this major flaw was evident in 16 of the 224 gusset
plates used to connect the steel beams in the Minneapolis I-35 Bridge.
Figure 1: Bending (left) in central gusset plates (2003) and eventual fracturing (right) on August 1st 2007
The NTSB also concluded that a "contributing" cause of the collapse was "inadequate design review
by federal and state transportation officials." This would have happened during the design and
approval phase, which was in the mid 1960s.
Recommendations and Lessons Learned
The final report largely exonerates state agencies from blame in the collapse - and in fact notes that
officials were inspecting bridges more frequently than required by federal guidelines - other portions
of the report are somewhat more equivocal about the state’s role. The report found that another
"contributing" factor was "the generally accepted practice among federal and state transportation
officials of giving inadequate attention to gusset plates during inspections for conditions of
distortion, such as bowing, and of excluding gusset plates in load rating analyses."
Evidence from the wreckage site pointed strongly to failure initiating at the U10 (upper) gusset plate
connections. However, there was also concern that corrosion subsequently identified on the L11
(lower) gusset plate connections may have also contributed to the collapse mechanism. The
following specific conclusions can be drawn:
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The gusset plates at joints U10 and L11 were clearly under sized for both the design forces
and for the forces present in the I-35W bridge at the time of collapse. Stress analysis of the
gusset plates predicts that some connections would have been at yield strength when the
bridge was initially constructed in 1967.
From the time the bridge was first opened to traffic up to the late 1990’s, the weight of the
bridge (steel and concrete) increased by 24%. The additional dead load greatly expanded
the region of yielding and the presence of construction/traffic loads at the time of failure
increased the area of yielded material even further.
The NTSB found that in many cases in the absence of formal and specific guidance, decisions about
the placement of construction materials may be made on an ad hoc basis and may not take into
account all the considerations necessary to ensure that temporary loads do not damage the
structure or possibly even exceed the load-carrying capacity of the structure at its most highly
stressed location. The NTSB added that following the collapse, the state agency revised its standards
for the storage of construction materials on bridges.
Additional dead and live loads over the years including increased vehicle traffic and road
maintenance and conditioning had been overlooked together with non-adherence to advice from
recommendations made by an engineering firm regarding retrofit of the existing bridge. This directly
resulted in its collapse. The replacement design clearly displays some lessons having being learnt
with a far more conservative concrete bridge incorporated to ensure such a disaster never occurs
again.
The Minnesota Legislature enacted a series of laws to mitigate against the failure in a number of
areas both administrative and engineering to ensure that this disaster is not repeated.
References
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Minnesota Public Radio
http://minnesota.publicradio.org/collections/special/2007/bridge_collapse/
National Transport Safety Board http://dms.ntsb.gov/pubdms/
Minnesota Legislative Reference Library
http://www.leg.state.mn.us/lrl/issues/issues.aspx?issue=bridges