I-35W Bridge Collapse

by Michael Miller
August 8, 2007

Updates added here.

I work in Epidemiology (it's about diseases, nothing to do with bridges or engineering) at the University of Minnesota. My office is in the West Bank Office Building which is the closest major building to the I-35W bridge. Like most people in the area, I drove either across or under the bridge nearly every day until it collapsed at about 6:05 p.m. CDT on Wednesday, August 1, 2007. I last crossed it about 3 hours before it collapsed, one of my employees crossed it 10 minutes before it collapsed and one of our professors was on the bridge when it went down but she was not physically injured. Our building was shut down for a few days after the collapse so that we wouldn't be in the way of emergency workers.

I made this web page to put together a few sources of information, mostly photographs and one digital video, about what might have caused the collapse.

The South Side Fell Faster Than the North Side

This video was taken from a security camera at the south side of the bridge (CNN claimed at first that it was taken from the north side, but that was an error) and it shows clearly that the south side of the central span of the bridge fell faster than the north side of the central span of the bridge. I won't say that the south side fell before the north side, but that might also be true and the south side certainly hit the water first.

Concrete Road Plays an Important Structural Role

Click on each of the following four images from the New York Times to learn about some features of the bridge:

Construction Work May Have Played a Role in Weakening the Bridge

After several weeks of construction work on the bridge, quite a bit of concrete had been removed, and this may have weakened the bridge. From an Associated Press report:
"I would be stunned if this didn't have something to do with the construction project," said David Schulz, director of the Infrastructure Technology Institute at Northwestern University. "I think it's a major factor."

The following photo (thanks to Jim Pankow for finding this) was taken by someone at around 9:00 am on Monday, July 23, 2007, which was 9 days and 9 hours before the bridge collapsed. The photo shows some fairly major work being done, more than simple resurfacing, but on what part of the bridge was this photo taken? (Photo removed at request of photographer.)

The "Construction Gash" Location: At the South End of the Central Span

I added two gray lines tracing the sides of the gash in the bridge so that we can see the vanishing point (thanks to Dan Dvorkin for suggesting this). This shows us that a building in the distance, which happens to be Elliott Hall on the University of Minnesota campus, is lined up with the "construction gash" (for lack of a better term) in the bridge. This allows us to locate the construction gash on photos of the intact bridge.

The following Google Map satellite photos were taken in late summer of 2002. We know this because a little to the west of the bridge we can see the Cirque du Soleil Alegría show's "Grand Chapiteau" tent in a parking lot near the riverfront (it is a park today). Alegría performances ran from August 21, 2002, to September 15, 2002. The date is only important because the locations of the street lamps were changed at some point after these satellite photos were taken and before either the photo above or the Microsoft aerial photos below were taken.

To find the location of the construction gash, I used a piece of paper to draw a line perpendicular to the bridge that passed through the middle of Elliott Hall. This line is shown in the following two images. The second image shows that the surface has some obvious markings at that exact point on the bridge. I believe they are openings for water drainage. This photo shows the drainage pipe coming down from the bridge at that exact point.


Here is the original Google Map of the bridge, also showing the perpendicular line to Elliott Hall.

The aerial view below was derived from this page at Microsoft's "Live Maps" web site. The markings on the surface are clearly visible in this more recent photo. This photo shows that the location of the construction gash was probably exactly on top of the columns holding up the south end of the central span of the bridge (indicated by yellow arrows).


The Gash Was Located at a Highly Damaged Part of the Bridge

This photo shows an aerial view of the bridge after the collapse. It appears to me that the bridge broke at the location of the gash but I am making no definite claims and I will leave the conclusions to the experts.