AP Crews are removing about 100 vehicles.

It may take more than a year of waiting before the world knows what caused Minneapolis’ Interstate-35W continuous-steel deck truss to collapse on Aug. 1, killing at least nine people and injuring dozens of others. Several theories are bouncing around: Fatigue, corrosion, poor inspections, irregular loading and faulty connections all are possible factors in the high-profile tragedy. As divers recover more bodies and cranes clean up about 100 vehicles, the investigation is deepening.

The National Transportation Safety Board, which is leading a federal probe, noted just a week after the collapse that investigators “have observed a design issue with gusset plates at particular locations,” calling into question the original design of the 40-year-old bridge. The agency also is looking at stresses on the plates and the types of materials used to build and fasten them.

In addition, NTSB is verifying the weight of equipment and materials staged on the bridge just prior to the collapse, but it has not yet drawn any connection to this summer’s paving job and its stress on the gussets, located in dozens of spots where steel members framed into the floor and main trusses.

Workers for St. Michael, Minn.-based Progressive Contractors Inc. had shut down four of eight lanes and were  about to start concrete paving work when the bridge fell more than 60 ft into the Mississippi River. Some people who crossed the bridge before the collapse say it felt “squishy.” Highway experts not involved in state or federal probes  have written off such anecdotes as uninformed reactions, as natural deflection typically causes some bridges to flex.

Related Links: NTSB's Gusset Alarm Perplexes Engineers Full Coverage: Catch up on all of ENR's recent coverage of the I-35 bridge collapse, and find more bridge news, in our Bridge Collapse Update Center

Shortly after NTSB’s announcement, however, U.S. Transportation Secretary Mary E. Peters advised state bridge engineers to double-check construction loads. “Given the questions being raised by the NTSB, it is vital that states remain mindful of the extra weight construction projects place on bridges,” said Peters.

The Federal Highway Administration followed up with a technical advisory directing state officials to make sure paving projects are complying with the latest editions of the American Association of State Highway and Transportation Officials’ design books, “Standard Specifications for Highway Bridges” or “Load and Resistance Factor Design Bridge Design Specifications.”

As if the failure of the well-documented, 1,907-ft-long bridge is not confounding enough, the government warning calls into question summer bridge-paving loads and their effects on potentially underdesigned gusset plates. Some criticize the warning as confusing, and even premature, at best.

“I thought the comment and directive was ill-advised,” says Joseph Yura, professor emeritus of civil engineering at the University of Texas at Austin. Because state and federal collapse probes are just beginning, the government’s warning raises more questions than answers, Yura says. Engineers need to look at all the evidence before coming to any conclusions. Otherwise, “you scare a lot of people and cause a lot of unnecessary concern,” he says.

Other engineers agree, adding that the routine nature of deck work and the proven design of gusset plates are unlikely triggers. Without more detail, the warnings have an alarmist tone, they say.

Some gusset plates apparently broke during the accident. But if the gussets were the cause, “I’d be a little bit surprised,” says Nick Altebrando, bridge practice leader for New York-based STV Group Inc. “Poor fatigue details inside the boxes were what people were worrying about, since they’re not visible.”

Investigators likely are looking at the thickness of the gusset plates and how they were fastened, adds Gene Corley, senior vice president of Skokie, Ill.-based CTLGroup. As for deck construction, he says the workers “weren’t even supposed to be near the main members.”

The bridge was partially closed for deck replacement. “I cannot conceive of the construction repair loads for working on the deck coming anywhere near the load of those cars,” says Richard M. Gutkowski, an engineering professor at Colorado State University, Ft. Collins.

“We don’t know the story yet,” warns Ralph Anderson, state bridge engineer for the Illinois Dept. of Transportation. Bridge engineers “would like to make sure we know what happened in Minnesota before [federal officials] send out warnings piecemeal.”

AP Federal investigators say gusset plates may have been underdesigned.

When asked to provide further clarification, Nancy Singer, a spokeswoman for Washington, D.C.-based FHWA, called the warning about construction loads “an extra caution.” As for gusset plates, she says investigators are focusing on load calculations used in the I-35W bridge’s design. However, FHWA did not feel it was necessary to raise questions about gussets on other bridges. “Overall, gusset plates have a sound history,” Singer says.

Inspection reports offer little information on the bridge’s gusset plates, except to note that they were spots ripe for corrosion, which, engineers say, is not unusual. Gussets are often a handy place for birds to roost and rainwater to collect, aggravating corrosion. Routine painting can cover up deficiencies and make gusset inspections more difficult, but bridge truss members of the 1960s tended to be overdesigned anyway, especially at gusseted connections.

MDOT Crews are removing about 100 vehicles.

That makes the gusset-plate question “a mystery to the owners,” says Anderson. “It might be one of those things you got to look at a little bit more. Until we get the [NTSB] report, we don’t really know what the concern is.”

Jacobs Engineering Group Inc., Pasadena, Calif., which in 1999 bought Sverdrup Corp., did not return phone calls. The engineering unit designed the bridge, before the structure opened in 1967, as Sverdrup & Parcel. Jacobs says in a statement that it is “prepared to provide any assistance” to investigators.

The contractor slated to wrap up partial deck replacement on Sept. 30 claims it did nothing wrong. “We’ve done thousands of these overlays,” says Mike McGray, president of PCI. “We just proceeded as we had in the past, always under the inspection of the owner.”

NTSB Chairman Mark V. Rosenker says PCI is “an experienced bridge and deck repair contractor.” But the FHWA’s warning is the first official indication that the staging of materials, equipment and construction workers on the bridge may have contributed to its failure. PCI was more than 50% finished with its overlay and partial full-depth deck replacement on the eight-lane bridge when the structure fell. The state had planned to finish critical truss inspections and identify structural repairs and retrofits this fall.