Predictions of Risk

There are reports tonight of a bridge collapse in Minneapolis, Minnesota. As I write this, the number of dead and injured is unknown.

The reason I add it to this to a blog on IS&T failure and success is that recently I spoke with Dr. Henry Petroski, professor of civil engineering at Duke University on success and failure of design, as articulated in his recent book, Success Through Failure. Dr. Petroski has written extensively on the history of bridge failure, and one of his predictions using historical evidence is that about every 30 or years or so, there is a major bridge collapse that surprises everyone. We are/were overdue for one.

It is too early to tell yet why this bridge collapsed, which is about 40 years old from news reports. But we shouldn't be surprised if it turns out that it was because of a design flaw hidden in plain sight.


From the 02 August edition of the New York Times:

A 2001 evaluation of the bridge, prepared for the state transportation department by the University of Minnesota Civil Engineering Department, reported that there were preliminary signs of fatigue on the steel truss section under the roadway but no cracking.

It said there was no need for the transportation department to replace the bridge because of fatigue cracking.

Governor Pawlenty said the bridge had an unusual design and was inspected in 2005 and 2006. No structural deficiencies were detected, he said.


From an LA Times report:

When it opened 40 years ago, the bridge was hailed for its novel design: an unbroken, 458-foot-long arch across the river. Engineers did not support the bridge mid-span with piers or pylons that would impede barge traffic on the Mississippi.


There is a story titled "Generation of Bridges was Built With Less Steel," in Sunday's Washington Post describing the engineering assumptions used during the time the Minneapolis bridge was built:

The 40-year-old bridge that collapsed in Minneapolis last week was built during an era when designers were confident they knew enough about bridge strength and weight loads that they could build bridges lighter and cheaper.

But a number of bridge collapses have taught engineers painful lessons about the frailty of bridges and the punishment they take from heavy trucks, strong tides and even the errant barge that slams into bridge supports, according to engineers, bridge builders and academics.

There are lessons in the article that IS&T designers should take to heart about engineering hubris.


Risk Factor

IEEE Spectrum's risk analysis blog, featuring daily news, updates and analysis on computing and IT projects, software and systems failures, successes and innovations, security threats, and more.

Robert Charette
Spotsylvania, Va.
Willie D. Jones
New York City