the collapse of the i-35w bridge: an analysis of the potential causes and the aftermath
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Analytical EssayTRANSCRIPT
THE COLLAPSE OF THE I-35W BRIDGE:
AN ANALYSIS OF THE POTENTIAL CAUSES AND THE AFTERMATH
Douglas Kahl, author
Professor Sinead MacNamara, instructor
The summer of 2007 was nearing its end and the first day of August brought
the city of Minneapolis, Minnesota yet another fair and sunny day. It was early in the
evening on this particular Wednesday and most people found themselves in a hustle to
return home from their places of daily employment. Road construction on the Interstate
35 West (I-35W) Bridge, crossing the Mississippi River near the University of
Minnesota campus, continued as planned but had persisted to cause a great deal of
headaches for the regular commuters with the never-ending backups and delays as four
of its eight lanes were closed to traffic. Traffic over the I-35W Bridge had been moving
at an excruciating ten miles per hour or less and the line of vehicles was bumper to
bumper in each direction. Then, without warning and being ushered in with what some
described as a deafening crack of thunder,1
the I-35W Bridge collapsed into the
Mississippi River below.
Within seconds, millions of pounds of steel and concrete crashed into the
river. Approximately 100 cars and commercial vehicles,2
as well as the drivers and
passengers within, in addition to construction crews and machinery, accompanied the
1 “Transcript: Minneapolis, Minnesota Bridge Collapses During Rush Hour,” CNN.com,
http://transcripts.cnn.com/ TRANSCRIPTS/0708/02/cnr.01.html.
2 National Transportation Safety Board, Safety Recommendation Letter, (Washington, DC, 2008).
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bridge in its fall. A plume of debris shot skyward like a geyser
3
and enormous waves of
water crashed onto the nearby riverbanks. Within minutes sirens were heard as hundreds
of people awaited rescue. For some though, rescue did not come soon enough. The
collapse left 13 motorists dead and nearly 150 others injured and in nearby hospitals.
Strangely enough, the counting of the injured and dead may have been the easiest part
of the recovery efforts in the wake of this tragedy. As the sun set on Minneapolis on the
evening of August 1st, the state of Minnesota was left with a host of questions. Most
importantly, everyone wanted to know how did the collapse happen and could it happen
again? August 2nd opened with the daunting task of attempting to answer those very
questions.
The I-35W Bridge, officially known as Bridge 9340, was originally a part of a
major “bridge building boom”4
that took place in the first two decades after the passage
of the Interstate Highway Act under President Eisenhower in the 1950s. A total of 14
bridges at a total cost of $40 million were slated for construction in the Twin Cities,
Minnesota area, nine of which were receiving funding from the federal government.5
In
1964, construction on the I-35W Bridge had begun at a cost of $4 million; by time the
bridge opened in 1967, the costs had risen to $5 million. The design for the bridge
came from the Sverdrup and Parcel engineering and architecture firm out of St. Louis,
3 “Minnesota Bridge Collapse: Your Accounts,” BBC.co.uk,
http://news.bbc.co.uk/2/hi/talking_point/6927625.stm.
4 Rodgers Adams, “Bridge Building Boom on in Twin Cities,” The Minneapolis Star, 17 November 1964,
1B.
5 Adams, “Bridge Building Boom,” 1B.
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FIGURE 1: The underside of the I-35W Bridge, looking northwest.
FIGURE 2: Post-collapse image showing the nearly 80 foot shift as the bridge fell, looking south.
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Missouri. Additional projects from the firm include numerous bridges from around the
United States such as the Amelia Earhart Bridge (1939) in Atchison County, Kansas6
and
the Chesapeake Bay Bridge-Tunnel (1964) in Virginia Beach, Virginia.7
The I-35W
Bridge was comprised of a series of steel arch trusses that spanned a distance of nearly
1,000 feet, divided into three main lengths. On top of the intricate layout of steel
members laid a six and one half inch layer of concrete that made up the driving surface,
which had later been increased to eight and one half inches during an earlier
construction project. Additionally, there were ramped approaches that led to the
bridge‟s deck, constructed mainly out of concrete.
One of the major distinguishing features to the I-35W Bridge is that it lacked
structural redundancy. In 2001, the Minnesota Department of Transportation (Mn/DOT)
authored a comprehensive report on the I-35W Bridge, titled, “Fatigue Evaluation of the
Deck Truss of Bridge 9340.” In this report, the issue of structural redundancy was
addressed; the report stated that “In any structural system, loads are carried along a
variety of simultaneous paths. The existence of these redundant load paths in a bridge
ensures reliable structural behavior in instances of damage to some of the structural
elements. However, if there is no redundancy, failure of one member may cause the
entire structure to collapse.”8
This is what happened to the I-35W Bridge. At least one
6 “Preservation Projects,” Historic Bridge Foundation,
http://www.historicbridgefoundation.com/ipages/preservation/ preservation.html.
7 “Facts and Figures,” Chesapeake Bay Bridge-Tunnel, http://www.cbbt.com/facts.html.
8 Minnesota Department of Transportation, Technical Report: Fatigue Evaluation of the Deck Truss of Bridge
9340, (St. Paul, MN, 2001), 19.
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structural member failed and without another path for the excessive loads to follow, the
structure was forced into collapse. Realizing that it may have been something as small
as one member failing, some began searching through the maintenance records back to
1967 to see if something may have been overlooked.
According to records, the bridge had been inspected every other year since
1967, up until 1993, and since then it had been inspected every year.9
The increase in
inspections was likely the result of the bridge receiving a “structurally deficient” rating
as early as 1990.10
When a bridge is deemed as being deficient, it does not mean that
collapse or failure is imminent. According to the U.S. Department of Transportation,
“Structurally deficient means that there are elements of the bridge that need to be
monitored and/or repaired.”11
Further, most deficient bridges remain open to vehicular
traffic while repairs take place. If a bridge ever were suspected of being in danger of
collapsing, immediate measures would be taken, such as closing the bridge
completely, to avoid unnecessary risks to the public. In the defense of those charged
with inspecting the I-35W Bridge prior to its collapse, no one would have ever
knowingly allowed the bridge to remain in service had they known that a collapse was
possible. The thoroughness of the inspections and what was being inspected may be in
question though.
9 “I-35W Bridge Fact Sheet,” Minnesota Public Radio,
http://minnesota.publicradio.org/display/web/2007/08/03/ bridge_background/?rsssource=1.
10 “‟Critical Factor‟ Cited in Deadly Bridge Collapse,” MSNBC,com, (2008),
http://www.msnbc.msn.com/id/22663216/.
11 I-35 Bridge Collapse, Minneapolis, MN, U.S. Department of Transportation, http://www.dot.gov/affairs/
factsheet080207.htm
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Some of the physical ailments of the I-35W Bridge that led to its low structural
rating included corrosion of paint at critical joints, poor welds throughout the structure,
bearings not being allowed to move as they should, and fatigue cracks.12
All of these
issues were acknowledged in the last three inspection reports from the Mn/DOT, dated
from 2005-2007. Additionally, each and every one of these potential problems was
being constantly monitored by inspection officials. Recommended repair actions to the
bridge included adding redundant plating at critical joints and conducting visual
inspections to identify, remove, and repair measurable defects.13
As the State of
Minnesota‟s maintenance of the I-35W Bridge has been questioned, an MSNBC.com
article from January, 2008 stated that investigations have so far revealed that there is
“no evidence that cracking, corrosion or other wear „played any role in the collapse of
the bridge.‟”14
The quality of the steel and concrete was also examined (post-collapse)
and they were both found to be well within the acceptable range of quality materials.15
Despite the I-35W Bridge being within the accepted safe limits, only 4% of the nation‟s
bridges are in worse condition. However, Minnesota‟s bridges are generally seen as
being in better condition than most others throughout the country.16
Traffic conditions have changed a great deal as well. Between 1990 and 2003
alone, a mere 13 years, traffic in the St. Paul/Minneapolis region had increased by
12 I-35W Bridge Fact Sheet.
13 Ibid.
14 “Critical Factor.”
15 Ibid.
16 Bill Dedman, “I-35 Bridge Was Rated Among the Nation‟s Worst,” MSNBC.com, (2008),
http://www.msnbc.msn.com/id/20102713.
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42%.
17
This increase in traffic was not an isolated incident either. Countless cities
across the United States have seen rapid increases in traffic levels. The I-35W Bridge by
itself carried over 144,000 vehicles per day, with nearly 5,000 of them being
commercial vehicles.18
Traffic increased by so much on the I-35W Bridge that the
original four lane bridge was eventually reconfigured to accommodate eight lanes.
Some cite not only the increase in daily traffic as a rising problem, but also the rising
weights of individual vehicles. From 1995 to 2005, the weight on America‟s highways
has increased by half and since 1970, it has increased by seven and one half times.19
Commercial trucks are seen as the main culprit to the increasing vehicle weights. At one
point in America‟s history, large stationary warehouses were used to store the goods of
the nation. Today, those goods are stored in semi trucks that are in continuous transport.
In addition to trucks, even the everyday automobile is getting heavier. In 2003, the
average weight of a new car was 4,021 pounds, a weight that broke the two-ton barrier
for the first time since the 1970s.20
The I-35W Bridge did not have a weight limit but
higher-weight vehicles did require permits; bridges with weight limits are typically
placed on the “functionally obsolete” list.21
On August 1, 2007, no known structural defects or deteriorating conditions
17 Stephen Flynn, “Minn. Bridge Collapse Reveals Brittle America: Expert Op-Ed,” Popular Mechanics,
(2007).
18 I-35W Bridge Fact Sheet.
19 “More, Heavier Vehicles Take Toll on U.S. Roads,” Associated Press, (2007),
http://www.msnbc.msn.com/id/20218349/.
20 Danny Hakim, “Average U.S. Car Is Tipping Scales at 4,000 Pounds,” The New York Times, (2004),
http://www.nytimes.com/2004/05/05/business/05weight.html
21 “More, Heavier Vehicles.”
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warranted the closing of the I-35W Bridge. Repairs, under the guidelines of the
“structurally deficient” rating, were to continue as planned until the bridge was to be
replaced sometime after 2020.22
Numerous reasons for the collapse had been cited
early on, some of them far-fetched and some of them much more acceptable. Some of
the proposed reasons for collapse revolve around the idea that exposed materials on the
bridge began to corrode and rust. In several past inspection photographs, large areas of
rust were quite visible. Additionally, some believed the rust and corrosion process to be
accelerated by the existence of bird droppings, primarily from pigeons.23
According to
the Newsweek article, “Foul Play?,” from August 25, 2007, “The build-up of pigeon
excrement on the I-35W bridge was substantial enough to be noted in several Minnesota
Department of Transportation inspections over the year.”24
Engineer William Schutt,
president of Matcor, a corrosion-protection firm in Pennsylvania, stated that “Pigeon
dung can be a serious issue – it‟s acidic and will easily eat away almost any metal.”25
The reason for the acidity is due to the fact that pigeons do not urinate; they crystallize
the ammonia in their bodies into uric acid. When uric acid dries, the salt left behind is
what contributes to the acceleration of the corrosion process. After thoroughly
examining the effects of the pigeon waste on the I-35W Bridge, it is largely believed that
acidic droppings would not have altered the structure enough to cause it to collapse.
22 Michael D. Lemonick, “Why Did the Bridge Fall?” Time, (2007), http://www.time.com/time/nation/article/
0,8599,1649423,00.html?cnn=yes
23 Eve Conant, “Foul Play?,” Newsweek, (2007), http://www.newsweek.com/id/78344.
24 Conant, “Foul Play?.”
25 Conant, “Foul Play?.”
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Individual bolts or other small pieces here and there may have been damaged (even
with the lacking redundant design), contributing to the collapse, but pigeon droppings
are not seen as the cause.26
Another potential cause for collapse that had been examined dealt with the
existence of a de-icing system. The I-35W Bridge was fairly close to the Lower St.
Anthony Falls Lock and Dam. Mist from the dam would constantly freeze on the deck of
the bridge on colder days, forming an invisible layer of what is known as black ice. The
black ice problem had caused numerous accidents over the years but in 1999, the de-
icing system was installed. The I-35W Bridge was the first major bridge in the United
States to be outfitted with such a system.27
The chemical used in the de-icing process
on the I-35W Bridge is known as CF7 (liquid potassium acetate) and it is widely
accepted as being non-corrosive to bridges and roads. In fact, it was chosen specifically
for its environmental responsibility in being able to easily biodegrade.28
However, in a
separate investigation, the same chemical, CF7, had been blamed for corroding an
airport runway in Portland, Oregon.29
The company that manufactures CF7, Cryotech, based in San Diego, California,
eventually issued a technical bulletin of their test results, stating that “a slow reaction
26 Ibid.
27 Esme Murphy, “De-Icing Chemical May Have Corroded 35W Bridge,” WCCO, (2007),
http://wcco.com/topstories/ de.ice.de.2.369740.html
28 Pam Louwagie and Dan Browning, “Bridge‟s De-Icing System Comes Under Scrutiny,” The Star Tribue,
(2007), http://www.startribune.com/local/11593846.html
29 Murphy, “De-Icing Chemical.”
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can occur when potassium acetate [CF7] and zinc come into prolonged contact.”
30
Since zinc is used in the galvanization process, it was again suspected that some agent,
in this case, CF7, was accelerating the corrosion process on the exposed metals of the
I-35W Bridge. After another post-collapse analysis, it has been found that the
galvanization on the bridge components was in fact thick enough and its exposure to
CF7 did not put the bridge in danger of causing failure in any of the structural
members.31
Transportation officials maintained that the chemical, CF7, is non-corrosive
and would continue to be used on the neighboring I-35E Bridge.
Weight has been considered to be another potential cause for the collapse of the I-35W
Bridge. In addition to the actual weight of the bridge itself, it has been estimated that
there was an additional 1.26 million pounds (630 tons) on the bridge at the time of
collapse.32
Adults were estimated at 200 pounds each and children were 50-100
pounds. Figuring the weight of each vehicle, including the construction equipment of
the crews working that day, was rather straightforward. Included with the construction
crews and their equipment were their supplies, primarily, sand. An estimated 198,000
pounds (99 tons) worth of sand situated into four mounds sat across the closed lanes of
traffic. This weight, in retrospect, should not have been enough to bring down the I-35W
Bridge.33
Safety inspections had been conducted on a regular basis since the bridge
30 Louwagie, “De-Icing System.”
31 Ibid.
32 Matthew L. Wald, “Mounds of Sand Stressed Minnesota Bridge, Report Says,” New York Times, (2008),
http://www.nytimes.com/2008/03/18/us/18bridge.html.
33 Wald, “Mounds of Sand.”
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opened and stress calculations were performed on the design years before construction
on the bridge even started. On paper, the I-35W Bridge was structurally sound. If the
individual members did not fail, what did? The answer lay in the structural elements
holding the truss members together: the gusset plates.
The gusset plates for the I-35W Bridge were flat sheets of metal, of varying
sizes and thicknesses, used to join two or more truss members together, creating a
joint. During the recovery of the wreckage, a number of gusset plates had been
identified as having been fractured or deformed in some manner. As these plates were
further examined, it was blatantly obvious that they were much to thin to support the
loads that had been placed upon them.34
The plates needed to be almost twice as thick.
Even before the collapse, inspection photographs revealed a slight bend in one of the
plates, however, most state that the photograph is not enough to perform a proper
evaluation or reach a solid conclusion. Typically, the gusset plates are seen as the
strongest portion of a truss and for this reason, they were never bothered with during the
safety inspections. When accounting for all of the new construction projects over the
years and the increase in traffic, the gusset plates were assumed to be infallible and
able to support the increasing loads. Even the mounds of sand played an important roll
in the collapse; they were unfortunately sitting on some of the weakest locations on the
bridge. Weight though, combined with the flawed gusset plates are the likely causes for
34 National Transportation Safety Board, NTSB Urges Bridge Owners to Perform Load Capacity Calculations
Before Modifications; I-35W Investigation Continues, (Washington, DC, 2008).
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the collapse of the I-35W Bridge. It has been calculated that the stress at the two
weakest gusset plates was 83% higher than what it could handle. Had there been
redundancy incorporated into the design of this bridge, perhaps the failure and
subsequent collapse could have been avoided.
Unfortunately, no one knows how the flawed gusset plates made their way into
the design of the I-35W Bridge, since there is no record of the calculations. However,
once those flawed plates were in place, it would have been next to impossible that they
would ever have been discovered during even the most thorough of examinations.35
It is
believed that the bridge was built according to the structural details and no major
deviations in materials or thicknesses were made. Bridges across the country, similar to
the I-35W Bridge or not, were examined with a fine-tooth comb in the wake of the
collapse. Currently, no one has reason to suspect the under-sized gusset plate problem
extends beyond Minneapolis, Minnesota.36
Turn to page 18 to see calculations and
diagrams showcasing how the gusset plates failed.
There was a moment during the twentieth-century when the United States and
its transportation infrastructure was the envy of the world.37
The country that created the
continental railway system in the 1800s continued with its innovative nature when it
created the interstate highway system in the 1950s and 1960s, linking major cities
across the nation. At the same time, the United States was in the process of creating a
35 “Critical Factor.”
36 Ibid.
37 Flynn, “Brittle America.”
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massive air-travel system as the development of jet aircraft continued to advance.
Indeed, the world was to follow the great example of the United States. But how far and
how long does anyone follow a mentor? As of 2008, the failing transportation
infrastructure of the United States is becoming more and more obvious, and the
collapse of the I-35W Bridge is just another symptom to the overall problem. Many of
the great public works created during the early to mid twentieth-century have surpassed
their designed life span and need to be replaced.38
Not only in Minnesota but in all areas of the country, the support systems that
sustain day to day life are failing. In 2005, Hurricane Katrina wiped out poorly
maintained levees that protected the city of New Orleans from flooding. As a result, the
city did in fact flood and entire neighborhoods were destroyed, leaving the city
struggling to survive, even today. Two weeks prior to the I-35W Bridge collapse, a
steam pipe in New York City had exploded, showering a busy Manhattan intersection
with mud and debris. The pipe had originally been installed in 1924 but had just
recently been repaired. In December of 2007, the final investigation into the explosion
revealed that a faulty repair caused the steam pipe to clog.39
Throughout much of New
England, water dams that are sometimes hundreds of years old are constantly coming
close to failing and their repairs and maintenance would end up costing millions of
38 Ibid.
39 2007 New York City steam explosion, Wikipedia.org,
http://en.wikipedia.org/wiki/2007_New_York_City_steam _explosion.
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dollars.
40
Whether it is a levee, a steam pipe, a dam, or a bridge, these are things that
society has come to depend on and their maintenance and upkeep is vital to the smooth
operations of daily life.
While most agree that maintenance is necessary, no one is willing to spend the
money to do so. The lack of repairs is a result of government agencies (at all levels),
being unwilling to spend the necessary funds.41
Nationwide, it is estimated that the
yearly upkeep and repair of just the existing roads and bridges would cost roughly $75
billion. Currently, states and local governments are making due with a meager $60
billion.42
No politician is ever going to get elected by saying they are going to raise
taxes to fix a failing infrastructure; it is just not the “sexy”43
thing to do. This is a point
that even President George W. Bush has grasped on to. He, along with other politicians,
has completely rejected the idea of raising taxes to aid the nation‟s deteriorating
transportation system.44
When there are no funds for maintenance or inspection crews,
the standards for safety begin to suffer.
Even in the wake of tragedies, no one is willing to address these problems and
propose ways to fix them. It is very doubtful that any one of the victims of the I-35W
Bridge collapse ever thought that they would have found themselves in that particular
situation. They were on a bridge that happened to collapse; who among us has not been
40 “Many New England Dams Need Repair,” US Water News Online, (2006),
http://www.uswaternews.com/archives /arcpolicy/6manynewx1.html
41 Dedman, “Nation‟s Worst.”
42 Ibid.
43 Conrad deFiebre, “Broken Bridges, Broken System,” Minnesota 2020, (2008), www.mn2020.org.
44 deFiebre, “Broken Bridges.”
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on a bridge? Politicians and the average citizen alike must realize that infrastructure
failures can happen to anyone, anywhere, at any time. The only way to counter these
sleeping giants is to provide funding for the proper maintenance and upkeep. The
politicians though have assured the public that they are well aware of the failing
infrastructure and the problems it presents, but they call for people to not over-react to
the situation.45
The national response to the collapse of the I-35W Bridge in Minneapolis,
Minnesota was quite universal. Government agencies and those in ownership of most
bridges called for and performed immediate inspections. In the week following collapse,
one could drive from the nation‟s Midwest to the eastern seaboard, through several
states, and find crews working on nearly every bridge and overpass. No one wanted to
find themselves in the same situation as those in Minnesota. From this tragedy though,
there appear to be few lessons learned. The major problems with the nation‟s
infrastructure still remain and no new funds appear to be allocated to fix them. What
everyone needs to realize is that while the initial repair and maintenance costs may be
higher than most want to pay, they will be dwarfed by the long-term costs of a structural
failure to a city or region. The cost to make the suggested improvement to the I-35W
Bridge (mentioned earlier in this text) prior to its collapse would have been in the range
45 “Senate OKs $1 Billion to Repair Bridges,” MSNBC.com, (2007),
http://www.msnbc.msn.com/id/20719867/.
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FIGURE 8: Diagram of joint U10, with one of the failing gusset plates
CALCULATIONS
What we know… -Point load of 540 kips -The vertical member contains 28 rivets, less than the other members and thus bearing more of the potential stress -There are two gusset plates per joint -The gusset plate is ½ inch thick
Assumptions… -The rivet diameter is 1”
To get the forces acting on each plate… 540 kips / 2 plates = 270 kips / plate
To get the forces acting on each rivet… 270 kips / 28 bolts = 9.6429 kips / bolt
To find the cross sectional area… bolt diameter x plate thickness = 1 inch x ½ inch = ½ inch2
To find the stress… forces acting on each rivet / cross sectional area = 9.6429 kips / ½ inch2 = 19.2857 ksi
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If the plate was ¼ inch thick
To find the cross sectional area…
bolt diameter x plate thickness = 1 inch x ¼ inch = ¼ inch2
To find the stress…
forces acting on each rivet / cross sectional area = 9.6429 kips / ¼ inch2
=
38.5716 ksi
If the plate was 1 inch thick
To find the cross sectional area…
bolt diameter x plate thickness = 1 inch x 1 inch = 1 inch2
To fin the stress…
forces acting on each rivet / cross sectional area = 9.6429 kips / 1 inch2
=
9.6429 ksi
As we can tell from the calculations, the number of rivets and the thickness of the
gusset plates are directly responsible for what stresses are felt. From the calculations,
as the plates became thinner, the stresses increased and as the plates became thicker,
the stresses decreased. Likewise, if there were fewer rivets, the same load would need
to be distributed evenly to each of those, eventually creating greater forces per rivet.
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Images
FIGURE 1: I-35W Bridge: Looking Northwest, Ca. 2004, Digital Image, Washington, DC,
National Transportation Safety Board.
FIGURE 2: I-35W Bridge: Post-collapse, Looking South, 2007, Digital Image,
Washington, DC, National Transportation Safety Board.
FIGURE 3: Aerial Image of I-35W Bridge, Post-collapse, Looking South, 2007, Digital
Image, Minneapolis, Minnesota, Department of Transportation.
FIGURE 4: I-35W Gusset Plates, 2003, Digital Image, Washington, DC, National
Transportation Safety Board.
FIGURE 5: I-35W Gusset Plates, Showing Bending, 2003, Digital Image, Washington,
DC, National Transportation Safety Board.
FIGURE 6: New I-35W Bridge Rendering, 2007, Digital Image, Minneapolis, Minnesota,
Department of Transportation.
FIGURE 7: Douglas Kahl (Barron 1982 - ), student, I-35W Bridge Elevation, 2008,
Digital CAD Image, Syracuse, New York, Personal Collection.
FIGURE 8: Joint U10 Gusset Plate Diagram, 2007, Digital Image, Washington, DC,
Federal Highway Administration.