Minneapolis I-35W Bridge Collapse

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Minneapolis I
-
35W Bridge Collapse


At 6:05 p.m. on Wednesday, August 1, 2007, during evening rush hour, the main spans of the I
-
35W bridge collapsed, falling into the Mississippi River and onto its banks. Thirteen people died
and at least one hundred more
were injured.

The eight
-
lane bridge was in the midst of repairs
when the bridge buckled, sending people, cars and hundreds of tons of concrete plunging 100 feet
into the Mississippi.

This bridge was on the western part of Interstate 35 which travels throug
h
the Minneapolis
-
Saint Paul metropolitan area.


Possible Discussion Questions




What are possible root causes for this event? (please brainstorm with your group. This is
your primary assignment.)



What concerns you the most about this catastrophe?



How migh
t you investigate the bridge failure?



What initial recommendations do you have for both correcting the problem, and
preventing a similar tragedy from occurring?



Any other thoughts or comments.


Bridge Facts



Opened in November 1967



Dimensions:


o

Length:


190
7 ft (structure length; 458 ft (length of max. span)

o

Width:


8 traffic lanes, 108 ft

o

Height:


Roadway deck was approximately 115

feet above the water level



Type structure:


Steel Arch Deck Truss




Location:


I
-
35 West at Mississippi River



B
ridge was built w
ith a single 458 foot long steel arch to avoid putting any piers in the
water to impede river navigation.



B
ridge featured an anti
-
ice system. PVC pipes carried a deicer fluid to outlets drilled into
the deck of the bridge. When the temperature for ice was
right, the deicer fluid would be
pumped onto the bridge deck.



Daily Traffic:


141,000 vehicles per day as of 2004 count.

Inspection History and Results



The I
-
35 bridge was

inspected annually by Minnesota Department of Transportation
(MnDOT). The most recen
t inspection was
May 2007

to address minor co
ncerns re:

weld
details.

No imminent dangers were observed.




Prior to that inspection, the brid
ge was inspected in June 2006.

Th
e bridge was rated 4
over all.

The 4 rating means that the state can continue to op
erate the bridge without load
restrictions (0= shut down, 9 = perfect).

General Observations



One quarter of U.S. bridges need repair or replacement (13.4% are considered
functionally obsolete, and 12.4% are considered structurally deficient). The I
-
35W bri
dge
was first identified as structurally deficient in 1990.



The American Society for Civil Engineers estimates that there is a $1.6 trillion deficit in
needed infrastructure spending in the U.S. (highways, airports, freight and passenger rail)
through 2010

just for repairs and maintenance.

Site Facts



The I
-
35W Mississippi River bridg
e was
downstream from Saint Anthony Falls.



In 1987 the Saint Anthony Lower Hydro
-
Electric Plant
(Dam itself built in 1956)
experienced an undermining failure and in 1988 the
fac
ility was razed. I
t was near the
north bank foundation pier of the I
-
35W bridge.



T
he bridge's south abutment was a site of industrial chemical pollu
tion. For 90 years prior
to bridge

construction,

a
coal
-
to
-
gas p
rocessing plant operated on
the south bank o
f the
river at the later site of the bridge.

This plant was demolished

in the 1960s.

Half the site
was

sold to Conoco
,
which stored/
processed petroleum products on the site for 20 years.



L
ong
-
term industrial use resulted in a toxic
-
waste dump below the bri
dge.15,000

tons of
contaminated soil were removed from below the bridge, ground up with wood chips and
burned as a fuel additive for a power plant between 1993 and 1998 at a cost of US $30
million. No causal relationship has been proven between the above s
ite facts and
subsequent bridge failure.

Design and C
onstruction



This bridge was de
signed to 1961 h
ighway
specifications current at the time.



The bridge was over 1900 feet long and consisted of fourteen spans: five south approach
spans, three main spans, a
nd six north approach spans. The three main spans were of deck
truss construction. All but two of the approach spans were steel multi
-
girder construction.
The remaining two approach spans were concrete slab construction.



The bridge had no piers built into
the river bed. Instead, the center span of the bridge
consisted of a single 458
-
foot steel arched truss over the 390
-
foot wide navigation
channel.



The two support piers for the main trusses, each with two load
-
bearing concrete pylons at
either side of the
center main span, were located on opposite banks of the river
.



Construction from 1964
-
1967 in

an era of large
-
scale projects related to building the
Twin Cities freeway system.

When the bridge fell, it was still the most recent river
crossing built on a ne
w site in Minneapolis.

After the building boom ebbed during the
1970s infrastructure management shifted toward inspection and maintenance.

Black I
ce



In 1996, the bridge was identified as the single most treacherous cold
-
weather spot in the
Twin Cities free
way syst
em

because of
black ice
.

The bridge's proximity to Saint
An
thony Falls contributed
to the icing problem. The bridge

was noted for frequent
spinouts and collisions.



By January 1999, Mn/DOT began testing magnesium chloride solutions to see whether
ei
ther would reduce the black ice.



In October 1999, the state embedded temperature
-
activated nozzles in the bridge deck to
spray the bridge with potassium acetate solution to keep the area free of winter black ice.

The system came into operation in 2000.

The

potassium acetate may have contributed to
the collapse of the 35W bridge.

Maintenance and I
nspection



Since 1993, the bridge was inspected annually by Mn/DOT.
Due to construction work, no
inspection report was completed in 2007
.




S
everal
prior
reports cite
d problems with bridge structure.
In 1990, the federal
government gave the I
-
35W bridge a rating of "structurally deficient," citing significant
corrosion in its bearings.



"Structurally deficient" is a classification term which does not in itself indicate
a lack of
safety. Approximately 75,000 other U.S. bridges had this classification in 2007.



According to a
2001 stu
dy
, cracking had been previously discovered in the cross girders
at the end of the approach spans. The main trusses connected to these cross g
irders and
resistance to motion at the connection point bearings was leading to unanticipated out
-
of
-
plane distortion of the cross girders and subsequent stress cracking.



The situation was addressed prior to the study by drilling the cracks to prevent furt
her
propagation and adding supports struts to the cross girder to prevent further distortion.



The report also noted a concern about lack of redundancy in the main truss system, which
meant the bridge had a greater risk of collapse in the event of any singl
e structural failure.



Although the report concluded that the bridge should not have any problems with fatigue
cracking in the foreseeable future, the bridge instrumentation by strain gages and
continuous structural health monitoring had been suggested.



In
2005, the bridge was again rated as "structurally deficient" and in possible need of
replacement, according to the U.S. Department of Transportation’s National Bridge
Inventory database.




Problems were noted in two subsequent inspection reports.

The inspec
tion carried out
June 15, 2006 found problems of cracking and fatigue. On August 1, 2006, Minnesota
Governor Pawlenty said his interpretation of the reports indicated that the bridge would
last until 2020 or beyond.



Federal National Bridge Inventory databa
se of inspection records show that the I
-
35W
bridge ranked near the bottom of federal inspection ratings nationwide. The scale used
was a "sufficiency rating" which ranges from the highest score, 100, to the lowest score,
zero. A score below 80 indicates t
hat some rehabilitation may be needed, while a score of
50 or less shows that replacement may be in order. This measure includes safety elements
(such as structural integrity) and factors in elements such as the bridge's size with respect
to its current tr
affic. The I
-
35W bridge was given a rating of 50 in the year 2005.



In December 2006, a steel reinforcement project was planned for the bridge. The project
was cancelled in January 2007 in favor of periodic safety inspections, after it was
revealed that dri
lling for the retrofitting would weaken the bridge. In internal Mn/DOT
documents, bridge officials talked about the possibility of the bridge collapsing and
worried that it might have to be condemned.



The construction taking place for the few weeks prio
r t
o the collapse was
cosmetic,
focusing on replacing lighting, concrete and guard rails and some work on joints.



B
ef
ore August, Mn/DOT
conducted repairs on the surface of the bridge an
d
announced
that it would reduce traffic flow on the bridge to one lane in

each direction during the late
evenings of July 31 and August 1 to accommodate construction. At the time of the
collapse, four of the eight lanes were closed for resurfacing, and there were 575,000
pounds (261,000 kg) of construction supplies and equipmen
t on the bridge.

Theories



A local writer
, John Weeks
,

notes

that the south end bearings were frozen, not allowing
the entire bridge to expand and contract as t
emperatures change. He theorizes

that the
bridge would have to flex and bend instead. The tempera
ture on the day of collapse was

91


F.



Another

theory concerns the ongoing repair work on the deck. The normal eight lanes
were reduced to two lanes each direction. Northbound lanes had a large pile of gravel
placed right above the U
-
9 and U
-
10 connector j
oints. While the bridge should have been
able to handle the extra weight, it altered the normal distribution of weight on the
structure and added a constant additional load to the already heat stressed connector
joints.



Another theory concerns the gusset p
lates that held the beams together at the connector
joints. These plates vary from one
-
half inch to one inch in thickness. There is speculation
that the one
-
half inch plates may have been too thin to support the weight of the bridge, a
possible design flaw
. Recent inspections also found that rust was attacking these gusset
plates. The L
-
11 gusset plate had lost one
-
half of its thickness in places due to rust. To
aggravate this situation, anti
-
ice solution and salt may have collected in this area speeding
up

the corrosion, and bird droppings may have accumulated in this area making
inspections more difficult.



The Minneapolis Star Tribune reported that connectors L
-
9, U
-
10, and L
-
11 all showed
evidence of pre
-
collapse damage. The failure of any one of these gu
sset plates could start
a chain reaction of failures that could lead to the collapse of the bridge.

Sources




Wikipedia, I
-
35W Mississippi River Bridge



Fatigue Evaluation of the Deck Truss of Bridge 9340, Minnesota Department of Transportation



Questions Ri
se About Previous Knowledge of Bridge,
http://www.wcco.com



I
-
35W Bridge Collapse Fact Sheet, U.S. Department of Transportation



Background About the I
-
35W River Bridge,
http://www.wcco
.com




Statement by Calvin L. Scovell III, Office of Inspector General, U.S. Department of
Transportation, Federal Highway Administration’s Oversight of Structurally Deficient Bridges,
House Committee on Transportation and Infrastructure, Addressing the Na
tion’s Critical Bridge
Needs, September 5, 2007,
http://transportation.house.gov/Media/File/Full%20Committee/20070905/OIG%20Final%20State
ment.p
df
)



Federal Highway Administration, National Bridge Inventory, 2006



Testimony by William G. Cox, American Road and Transportation Builders Association, House
Committee on Transportation and Infrastructure, Addressing the Nation’s Critical Bridge Needs,
Se
ptember 5, 2007,
http://transportation.house.gov/Media/File/Full%20Committee/20070905/ARTBA%20William%2
0Cox%20Testimony.pdf
.



America
n Society for Civil Engineers, “America’s Infrastructure Report Card


2005”



I
-
35W Bridge Collapse Investigation Focus Area

October 2007, John A. Weeks III,
http://www.visi.com/~jweeks/i35w/i
35wdetail.ht ml