This image shows the ground supports of the bridge before the collapse:
Nation's Bridges Face Immediate Inspection. Fifth Victim Found; President Bush To Visit Minneapolis UPDATED: 9:20 am EDT August 3, 2007
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.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"I-35W Mississippi River bridge.
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."Bob Clark