NTSB hearing: Design errors, weight factor in bridge collapse

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The National Transportation Safety Board hearing into the collapse of the 35W bridge concludes Friday in Washington.

NTSB investigators have found the weight of the bridge and construction materials and equipment placed on it forced a poorly-designed and under-sized gusset to fail. That failure sent the entire bridge into the Mississippi River on August 1, 2007.

We've known it for a while, some of the 35W bridge's gusset plates were too small for the weight of the bridge. Where they should have been an inch thick, they were half an inch. One of them, gusset plate U10, has long been the focus of the NTSB investigation. But, investigators said that wasn't the only undersized plate. Three other sets of plates throughout the bridge were also undersized.

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"So we know then for a fact that this terrible tragedy began 40 years ago with an inadequate design of a gusset plate or in this case a number of gusset plates and in this case we have learned that the beginning of the sequence began at U10," said Mark Rosenker, acting chairman of the NTSB.

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In 1962, when the bridge was designed, the size of the gusset plates likely weren't calculated. Investigators couldn't find any documents from the original design showing planners had calculated how thick the gussets needed to be.

This became critical for the future of the bridge as MnDOT began adding to the bridge's weight. In 1977 MnDOT added two inches of pavement to the bridge deck and in 1998 it added a de-icing system and barriers. More traffic, more weight. One investigator said the bridge was stressed almost from the day it opened and to that, Acting Chairman Mark Rosenker had one question about August 1, 2007.

"What was different about that day at the hours between three and six on the day that it collapsed than any other day preceding it?"

What was different about that day at the hours between three and six on the day that it collapsed than any other day preceding it?"

On that day, August 1, 2007, the bridge reached a critical point, according to investigator Cark Schultheisz.

Schultheisz conducted the computer modeling of the bridge collapse. He said year by year, the bridge's gusset plates showed greater signs of stress.

When the construction firm PCI started re-decking the bridge in the summer of 2007, weight wasn't on anyone's mind. The NTSB's Robert Accetta studies bridge construction, he said the company's foreman asked MnDOT if he could move his construction materials onto the bridge.

"To save the time and labor that would be required, the MnDOT construction inspector did not seem concerned about the staging of the materials [on the bridge] which the foreman took as permission," Accetta said. MnDOT didn't have any guidelines for how much static weight could sit on the bridge, let alone how much weight one spot on the bridge could sustain. NTSB Board Member Steven Chealander thought that was unusual.

"Somebody missed the whole idea that we're going to put 287 tons of weight on the bridge and maybe somebody should look at that," Chealander said.

But in fact, the NTSB found that most states, including Minnesota, didn't have rules on how to place construction materials on bridges.

By 3:30 p.m. on the day of the collapse, the final load of materials was moved out onto the bridge. Nearly 300 tons of equipment and rock and concrete were placed on the bridge deck. Those materials weighed as much as a 747 aircraft. They were placed just above the U10 gusset plate.

Bridge deck
A collapsed portion of the I-35W bridge deck.
National Transportation Safety Board

Two and a half hours later, in the middle of evening rush, the bridge gave way.

Acting NTSB Chair Rosenker repeatedly stressed the connection between the weight on the bridge and the under sized gusset plate.

"On August the first, at the time that day that this bridge collapsed, that was different from any other day in the bridge's 40 year history," Rosenker said, "and that would have been the concentrated load over the U10 node on the west side of the bridge."

On Friday, NTSB investigators will finish presenting details of their investigation. The session will concluded with recommendations they say will prevent a similar disaster.

When the hearing resumes Friday, investigators will make recommendations.