Angry in the Great White North

From the Wikipedia entry attached to Google Earth:

The I-35W Bridge is a deck-arch truss bridge that spans the Mississippi River in Minneapolis, Minnesota. It was built in 1967 by the Minnesota Department of Transportation. The bridge is notable for not having any piers in the water. Instead, the main support piers are located on the banks of the river, and are built of tubular-shaped concrete pillars. This allows for a wide, clear span across the river, making river navigation easier. Although not very decorative, the bridge is one of the widest bridges in the Twin Cities area and provides an important link for Interstate 35W traffic.

You can see what a major thoroughfare the bridge represented for the city of Minneapolis. The bridge, of course, did not follow the ground level and water level. This is an artifact of the Google Earth perspective rendering.

Here is what the I-35W bridge looked like from below before the collapse:

Designed by the Minnesota Department of Transportation, this bridge has a high water clearance and no mid-river piers in order to facilitate navigation of large vessels up to the two locks at St. Anthony Falls, which were opened four years earlier. The single cross-river arch has a span of 458 feet.

This photo makes clear the long span of the bridge. A truss, the triangular structure, can resist both tension and compression, but not twisting. That's why truss bridges are invariably straight.

Perhaps a failure in one of the four piers pulled the bridge truss over in one direction off the centreline, and the failure progressed along the truss length, unable to resist the progression of the failure until the next pier was reached, which in this design was at the far side of the span instead of at the mid-river point.

We'll learn soon whether that inherent property of trusses had a role to play in this disaster.