This image shows the ground supports of the bridge before the
collapse:

http://www.wmur.com/2007/0802/13805989_240X180.jpg

taken from:

Nation's Bridges Face Immediate Inspection.
Fifth Victim Found; President Bush To Visit Minneapolis
UPDATED: 9:20 am EDT August 3, 2007
http://www.wmur.com/news/13801620/detail.html

A video of the collapse is also available on this page.

Note the ground supports are slender concrete columns. Note also the
steel arch only extends to the top of the support. It does not extend
down into the ground.
In contrast note the arches of the 10th Avenue bridge next to the
collapsed bridge extend into the ground:

10th Avenue Bridge.
http://en.wikipedia.org/wiki/10th_Avenue_Bridge

The strength of an arch extends from its curvature. Note that an arch of the
I-35W bridge not extending into the ground means the arch is shorter which
necessarily makes the arch straighter, and therefore
weaker.
The 10th Avenue Bridge also has supports in the middle of the river
while the I-35W bridge did not. Compared to the supports of the 10th
Avenue Bridge, the ground supports of the I-35W bridge can only be
described as flimsy.
This is a bridge that carries the most traffic in the state of
Minnesota. Moreover the 10th Avenue bridge only has to carry 2 lanes, while
the I-35W carried 8.
A professor at Northwestern argues the failure was likely due to the
joints connecting the bridge to the concrete supports:

Investigators in bridge collapse focus on chilling video.
By Jon Hilkevitch | Tribune transportation reporter
9:41 PM CDT, August 2, 2007
"The bridge must have been near a state of collapse for some time, and the
construction might have contributed to its failure," said Zdenek Bazant, a
professor of civil and environmental engineering at
Northwestern University. Bazant said he suspects there may have been a
hairline crack or fatigue in the steel joints near bridge supports,
leading to the buckling"
http://www.chicagotribune.com/servic...6.story?page=2

This page on the I-35W describes it as a truss bridge:

I-35W Mississippi River bridge.
http://en.wikipedia.org/wiki/I-35W_M...i_River_bridge

These are among the cheapest and flimsiest of bridges. They lack the
redundancy of many other types of bridges:

Investigators in bridge collapse focus on chilling video.
By Jon Hilkevitch | Tribune transportation reporter
9:41 PM CDT, August 2, 2007
"Other engineering experts said that the 1960s-design of steel-arched
bridges did not contain structural redundancies, meaning that if one
component fails, the whole structure is in jeopardy because the weight does
not shift to other points on the bridge.
"We know that we would not build a bridge like this today,'' said Kent
Harries, an assistant engineering professor at the University of
Pittsburgh."
http://www.chicagotribune.com/servic...6.story?page=2


Bob Clark