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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FIGURE 3: Aerial view, 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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FIGURE 5: Gusset plates, showing slight bending.

FIGURE 4: Gusset plates.

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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 7: Bridge Elevation

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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.