Sizing Up The Bridges

Excerpt and Chart from Big River Magazine, November-December 2007

Update from May-June 2008 Big River Magazine

St. Cloud, Minn. — Two old bridges across the Mississippi River in Minnesota will be torn down and replaced, because the Minnesota Department of Transportation found critical flaws. The DeSoto Bridge on Highway 23 in St. Cloud and the Highway 61 Bridge in Hastings, Minn., have moved to the top of the list for replacement.

Bridge inspectors closed the bridge in St. Cloud in late March after discovering that gusset plates in four parts of the bridge had bent about a quarter of an inch. They weren’t taking any chances. Under-sized gusset plates have been implicated in the collapse of the I-35W Bridge in Minneapolis on August 1, 2007, that killed 13 people.

Bridge traffic, which had averaged about 31,000 vehicles a day, was re-rerouted. The 890-foot bridge, which was built in 1957, had been scheduled for replacement in 2015. It will now be demolished this summer and replaced by 2009.

Forty-five miles south of the Twin Cities, the 58-year-old Hastings bridge will now be replaced by 2010, moved up from 2020. It is one of the most heavily traveled in the state, with about 32,000 vehicles crossing it each day. The latest inspection rating had dropped from 49.1 to 38.1 on a 100 point scale, due to severe corrosion since the last inspection.

The National Transportation Safety Board’s (NTSB) investigation of the Minneapolis I-35W Bridge collapse is scheduled for completion at the end of the year. Among other things, the investigation is trying to determine how the apparent gusset-plate design error occurred and why it was not discovered. The NTSB is also developing computer models and a 1/200th scale model to explore the bridge’s structure.

In the meantime, the NTSB has advised the Federal Highway Administration to require bridge owners to update load capacity calculations to include modifications and other changes to the bridges since they were built, then to recalculate the load carrying capacity of the gusset plates.

Update from March-April 2008, Big River magazine

I-35W Bridge Design Error

Minneapolis — Weak gussets plates — which tie together beams — appear to have failed, causing the I-35W bridge collapse that killed 13 people last August 1. Gusset plates at four nodes had fractured, according to a Safety Recommendation issued by the National Transportation Safety Board (NTSB) January 15.

The gusset plates at four nodes of beams were roughly half the required thickness. Apparently the flaw was in the design, because “The gusset plates that were undersized on the bridge were undersized on the drawings.”

Renovations since the bridge was opened in 1967 added significantly to its weight.
The report noted that inspections do not check the adequacy of gusset plate designs. However, it noted that the problem is probably not widespread. “In fact, this is the only bridge failure of this type of which the Safety Board is aware.”

The NTSB investigation is not yet complete, and no official cause of the collapse has been determined.

 

Sizing Up The Bridges: Excerpt and Chart from Big River Magazine, November-December 2007