A bridge expert intimately familiar with the I-35W bridge said the structure's gusset plates are prone to fatigue caused by twisting or bending of a truss member.
"What gusset plates tend to do on beam bridges is they create out-of-plane bending -- things work back and forth," the expert told the Star Tribune. "The beam is not designed to see that kind of stress and that tends to increase fatigue-prone details."
The expert said that the NTSB's interest in studying the material makeup of the gusset plates could mean that investigators are checking the quality grade and strength of the steel.
"A gusset plate is fabricated out of what would be considered a lower quality, a lower tensile strength steel than what it's being attached to," the expert said. "... Some places of the bridge matter less than others -- a secondary piece of bracing wouldn't have to be of the same strength as, say, a main member."
What's crucial, the expert said, is how the gusset plates were attached. When the I-35W bridge was built in the mid-1960s, welds were commonly used to fasten truss members. Today, gusset plates are usually bolted to truss members, the expert said. The expert said that bridge inspectors considered the welding of the plates to be a design flaw in the bridge.
"A majority of the repairs we see made are due to poor design detail," the expert said.
In the latest Minnesota Department of Transportation inspection report of the I-35W bridge, deficiencies were found in at least seven gusset plates. The flaws included heavy flaking, pack rust, pitting, section loss, a weld overlap and one case of loose bolts, according to the June 2006 report. Some gusset plates on the bridge are as wide as 5 feet.
At an afternoon news conference Wednesday, Bob McFarlin, MnDOT's assistant to the transportation commissioner, would not respond to whether the investigation into the bridge collapse is now focused on the plates or whether the latest NTSB report leads in that direction.
"I don't want to rush to judgment," McFarlin said.
Other areas of focus
The NTSB emphasized that the gusset plates are only one area of inquiry. It is working with the Federal Highway Administration to conduct a computer-aided structural analysis of the bridge and that part of the investigation is expected to take "several months," the agency said Wednesday.
"We are continuing to make progress on this investigation, and each area of inquiry gets us closer to ultimately determining the cause of this tragedy," NTSB Chairman Mark Rosenker said.
The NTSB's 19-member team interviewed vehicle occupants, construction employees and crew members on a dinner cruise ship that was in the Mississippi River lock near the bridge at the time of the collapse, the agency said.
On Monday, the NTSB team used a helicopter to shoot high-resolution pictures of the fallen bridge superstructure at its north end. Several fractures were observed, but nothing that looked like a failure that could have caused the collapse, the NTSB said.
A MnDOT fact sheet released Wednesday said that while the bridge roadway slab was 9 inches thick, PCI in most instances removed only the top 2 inches and replaced it with new concrete.
"Other concrete removal work was done using 45-pound jackhammers. Nothing larger was used to remove the concrete," the agency stated. MnDOT officials said the company had stopped using the jackhammers earlier in the day or the day before.
Terry Williams, a NTSB spokesman, said he could recall no instance in which the NTSB had concluded that an existing bridge had collapsed because of repair work.
The speculation whether the repair work was a contributing factor meanwhile continued.