We have known it for a while -- some of the Interstate 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.
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But investigators say U10 was not the only undersized plate. Three other sets of plates throughout the bridge were also undersized.
"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," Mark Rosenker, NTSB acting chairman, said to summarize what investigators laid out in the first day of the hearing.
In 1962 when the bridge was designed, the size of the gusset plates was likely not calculated. Investigators could not find any documents from the original design to show planners had calculated how thick the gussets needed to be.
The gusset plates became critical when 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 Aug 1.
"What was different about that day, at the hours between 3 p.m. and 6 p.m. on the day that it collapsed than any other day preceding it?" Rosenker said.
According to investigator Carl Schultheisz, the bridge had reached a critical point.
"Any variation in the overall load could have been responsible for the overall collapse," said Schultheisz.
"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."
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 redecking 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, 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, did not 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, just above the U10 gusset plate. The materials weighed as much as a 747 airplane.
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 undersized gusset plate.
"On August 1, at the time that day that this bridge collapsed, what was different from any other day in the bridge's 40-year history? And that would have been the concentrated load over the U10 node on the west side of the bridge," Rosenker said.
Today, NTSB investigators will finish presenting details of their investigation. The session will conclude with recommendations they say will prevent a similar disaster.