I-35W Bridge Collapse Myths

The busy Interstate 35W bridge over the Mississippi River in downtown Minneapolis, Minnesota, collapsed at the peak of rush hour on Wednesday, August 1, 2007. This disaster put the news media on a 72 hour continuous coverage feeding frenzy. Since no one could prepare for such an event, there were few sources of information available to reporters at the point in time where people were the most eager to hear details. As a result, some incorrect information made it to air, along with mistakes, and some out and out inventions. Lets take a look at some of these myths, along with the correct answers to these questions.

1	The I-35W bridge spanned the river between Minneapolis and Saint Paul.

Not true. The bridge was fully inside of Minneapolis. It is 2 miles from any part of Saint Paul, and the bridge runs north and south, whereas Saint Paul is east of the bridge. There are Mississippi River bridges that cross between Minneapolis and Saint Paul, such as the Lake Street Bridge and the Intercity Bridge (on Ford Parkway), but the I-35W Bridge did not.

2	It was the longest bridge in Minnesota.

Not even close. The Bong Bridge between Duluth and Superior is 11,800 feet, 5 times as long.

How about longest bridge fully inside of Minnesota? No, the Lafayette Bridge is 3,369 feet, nearly twice as long as the 1,907 foot long I-35W bridge. There are 7 bridges in the metro area across the Minnesota River that are even longer. This includes the two spans of the Bloomington Ferry Bridge (5,856 feet SB, 5,824 feet NB), Cedar Avenue Bridge spans (5,185 feet SB, 5,164 feet NB), I-494 Bridge spans (4,509 feet WB, 4,393 feet EB), and the Mendota Bridge (4,118 feet, which is 4/5 of a mile long).

How about the longest clear span? Again, no, the Hennepin Ave Bridge has a 625 foot clear span, while the I-35W bridge had a clear span of 458 feet. That does not, however, diminish the importance of this river crossing.

3	The I-35W bridge was the busiest bridge in Minneapolis or Minnesota.

No, it was the 3rd busiest river bridge in the metro area, and the state. The I-94 Dartmouth carries 157,000 cars per day, and the I-694 bridge carries 150,000 cars per day, while the I-35W was measured at 140,000 cars per day. That is a lot of traffic, but not the most. Note that these are 2006 traffic volumes.

4	The I-35W bridge was named the Washington Avenue Bridge.

Not true. The Washington Avenue Bridge is downriver 6/10s of a mile. That is the famous bridge with the roof top walkway painted in the maroon and gold U of M colors.

The I-35W bridge was once proposed to be called the Saint Anthony Bridge, but that name was never officially adopted. As a result, the bridge had no official name. The area on the north side of the bridge was once the City of Saint Anthony. The City of Saint Anthony merged with the City of Minneapolis in 1872. This is not the current City of Saint Anthony. The current City of Saint Anthony is the former Village of Saint Anthony, which incorporated to be a city in 1945.

5	The I-35W bridge connected the east and west bank campuses of the University of Minnesota.

Not exactly. There are university buildings on the south side of the I-35W crossing, and a student housing area on the north side, but the I-35W bridge was on the far west fringe of the U of M campus. The Washington Ave bridge would be the bridge that ties the two campuses together.

6	The bridge was ready to fall down because it was "structurally deficient".

Not true. The bridge was inspected every year since 1993, and every other year before that. This was not known as a problem bridge or a bridge in trouble. It did have some maintenance items over the years. A key factor with this bridge is that it was an older non-redundant design, which the engineers call "fracture critical". The term "structurally deficient" is an engineering term that can be applied for a variety of reasons. A number of inspections found the bridge to be serviceable, and projected several decades of future service, though none of them would build a bridge like that today knowing what we know since 1967.

7	The I-35W bridge is in the worst condition of any metro area bridge.

A bridge is rated on a scale of 0 to 100. The I-35W bridge was rated at 50. A rating of 50 or below is eligible for federal funds for replacement. There are many other such bridges in the state. At a rating of 50, the I-35W bridge was actually rated the highest in that category. In fact, there is another heavily used metro area Mississippi River crossing that scored worse than the I-35W bridge, and that is the Lafayette Bridge in Saint Paul. In 1999, an inspection of the Rock Island Toll Bridge over the Mississippi River in Newport found the structure to be in such bad condition that it was closed on the spot, and it has not been reopened since that time.

8	The bridge fell 64 feet.

The US Army Corps of Engineers states that the bridge is 64 feet from the level pool height (normal water level) to the low steel on the bridge. Since the bridge had this huge truss under it, there is quite a distance from low steel to the deck. While I have not determined the actual deck height, various estimates put it anywhere from 98 feet to 118 feet above the normal water level. As a result, people on the center of the channel span fell 100 feet before they hit the water. It is a miracle that any of those people survived.

Update—the editor of the Wikipedia page on the I-35W bridge has turned up some definitive information. The low steel to river bottom is 78 feet. High steel to river bottom is 132 feet. The river is 15 feet deep at normal pool depth. That means that low steel is 78 - 15, or 63 feet above normal water level. The high steel (bottom of the deck) is 132 - 15, which is 117 feet. Add in a foot or so for the deck, and we have 118 feet from the deck to the water.

Update—Road Scholar Adam Froehlig located an old bridge plan in the archives at MN-DOT that included elevation measurements. The plan lists the middle point of the main span at 841.1 feet. Given that the flat pool elevation of the water is 724.64 feet, that results in a deck height of 116.46 feet. I consider that to be the final answer.

9	The bridge was having a new deck put on.

Not true. The construction project was to fix the curbs, update the lighting, and fix some potholes in the deck. Yes, they were using jackhammers and pouring concrete. Two lanes were closed in each direction. But the deck itself was not being replaced, and was not estimated to need replacement for at least a decade.

10	Some sources say that the main span was 458 feet while others say 390 feet.

The main span between the support piers on each side of the river was 458 feet. The US Army Corps of Engineers states that the navigation channel is 390 feet wide. This means that the river is 458 feet wide (since the piers are right at the river edge), but river boats can only use 390 feet of that 458 foot wide passage. The reason is that the US Army Corps of Engineers has to dredge the river to make it deep enough for boats, and it doesn't dredge the full width of the river. This is typical of the upper section of the Mississippi River. It can be a mile or more wide in places, but the navigation channel is normally a well defined relatively narrow path down the river.