NTSB examines St. Cloud's DeSoto Bridgeby Tim Post, Minnesota Public Radio
Investigators from the National Transportation Safety Board were in St. Cloud today to examine the steel truss bridge that state officials closed last week after they found distortions in the bridge's gusset plates.
NTSB engineers are looking at similarities between that bridge and the I35-W bridge that collapsed in Minneapolis last summer.
Meanwhile, MnDOT officials hope to determine within a few weeks whether the bridge in St. Cloud can be repaired or should be replaced.
St. Cloud, Minn. — National Transportation Safety Board engineers spent the day suspended under the Desoto bridge in St. Cloud, in a bucket, attached to a crane on a truck.
The NTSB is investigating the St. Cloud bridge to see if its gusset plate problems are a precursor to collapse. Gussets are the steel plates that hold the beams of the bridge together, and NTSB officials have said they may have been a factor in the I35-W bridge collapse.
Both bridges were built in the middle of the last century by the same company and in the same style.
The NTSB crew wasn't talking to the media, but state transportation engineer Bob Bush said federal investigators are using the DeSoto Bridge as a sort of living model of the I35-W bridge pre-collapse.
"They're looking at creating a body of knowledge of work out there, as far as seeing what this bridge is doing and see if it would apply to anything of lessons learned at the I35-W bridge and all other bridges of this same type and nature throughout the nation," Bush said.
Bush said MnDOT engineers have a different job. They're focusing on whether the DeSoto Bridge should be replaced or repaired.
The span, which carried 31,000 cars a day across the Mississippi River, was scheduled to be replaced in a few years anyway. Now, MnDOT is considering moving that construction up. The price tag for a new bridge is an estimated $35 million.
Acting MnDOT Commissioner Bob McFarlin said no matter what happens, the crossing won't be reopened until bridge repairs are made, or the bridge is replaced.
McFarlin said on Minnesota Public Radio's "Midday" program that the decision to close the bridge was not out of fear that it was about to collapse.
"What we saw with the St. Cloud bridge was a very slight, almost imperceptible, bend in gusset plates in four locations which indicated there was some type of load factor causing them to have that slight bend in a symmetrical fashion," McFarlin said.
But another bridge expert thinks that description doesn't go far enough, and referring to it as a "bend" doesn't show how serious the problem is.
Hassan Astaneh, professor of structural engineering at the University of California-Berkley, is an expert on steel truss bridges.
Astaneh is doing his own investigation of the St. Cloud bridge. This week he's spent hours under the span, taking pictures and describing his findings into a tape recorder.
Astaneh said what's happening to the St. Cloud bridge is gusset plate buckling, something more serious than bending.
"All the evidence I see here is buckling. If it proves to be buckling, it means over stressing of this truss bridge," Astaneh said.
That Astaneh said could come from the extra concrete that was layered on the bridge over the years, something MnDOT officials have also suggested.
Astaneh, who also investigated the I35-W bridge collapse in Minneapolis, hopes his work in St. Cloud adds to knowledge about why certain bridges collapse when one piece of their structure fails.
"All I'm trying to find out is what we can learn. Whether or not this buckling could end up progressively collapsing this bridge," Astaneh said.
Astaneh's goal is to help engineers and lawmakers understand how to best shore up the nation's aging bridges and also protect them from terrorist attacks.
In the meantime, MnDOT will continue its investigation into what's wrong with the DeSoto Bridge in St. Cloud.
MnDOT could announce whether the bridge will be replaced or repaired as soon as April 10th.
- All Things Considered, 03/26/2008, 5:47 p.m.