WASHINGTON -- Federal investigators probing the collapse of the Interstate 35W bridge in Minneapolis said Thursday that the structure was heavily loaded with construction equipment -- equivalent to the weight of a 747 airplane -- hours before a set of improperly designed joints failed catastrophically.

The added weight, combined with errors in the original design of the so-called gussett plates, appeared to produce the breaking point in the Aug. 1, 2007, disaster that killed 13 people and injured 145.

The bridge's age, the investigators told the National Transportation Safety Board (NTSB), had nothing to do with the collapse. The board is expected to issue a final accident report today, at the end of a two-day hearing.

In the investigators' first public testimony, buttressed by shards of jagged metal and slow-motion video, a picture emerged of an under-designed bridge that could not withstand the increasing loads added over its 40-year life, culminating with a rush-hour collapse with 287 tons of construction materials concentrated on the center span.

"It was historically the largest load that bridge had ever held," said Bruce Magladry, the NTSB's director of the Office of Highway Safety. Board members compared the added weight to that of a 747 airplane.

But Magladry and a host of government witnesses told the five-member board that despite the unusual load, the underlying cause of the collapse was the gusset plates that were supposed to hold together the bridge's steel beams.

"Had the gusset plates been properly sized, this bridge would still be there," Magladry said.

In St. Paul, Minn., a group of collapse survivors gathered at a National Guard armory to watch the NTSB presentation on the Internet.

Falling through a crack

Some NTSB members expressed surprise that contractors and state officials had not considered whether the bridge could sustain the added weight of the construction equipment that was being stockpiled for a repaving project.

 "You would think that someone would have realized that the aggregate of weight would be an issue," said NTSB member Steven Chealander.

Testimony on Thursday showed that officials at the Minnesota Department of Transportation (MnDOT) had no national standards to follow and no policy for handling increased loads from construction materials and equipment.

"We have a situation where there's an absence of guidance," said Vern Ellingstad, NTSB's director of research and engineering. "What we have here is a crack that we've sort of fallen through."

The use of quick-drying concrete required it to be mixed on site, which led workers to stage their equipment on the bridge, the bulk of it directly over the "U-10" gusset plate nodes that first failed.

Robert Accetta, an NTSB construction expert, said that a foreman for Progressive Contractors Inc., the St. Michael-based construction company, conferred with a MnDOT inspector about staging the equipment in the middle of the bridge. The inspector didn't object, Accetta said.

MnDOT officials told NTSB investigators that the company should have made a formal written request, which probably would have been denied. But the federal investigators also cited a state official who said MnDOT had no specific policy on stockpiling the construction materials on the bridge, and that it was "50-50" whether state officials would have denied the company's request.

NTSB member Debbie Hersman voiced some frustration with the varying accounts given by MnDOT, saying "it doesn't seem like they're reading off the same sheet of music."

MnDOT Commissioner Tom Sorel, who attended Thursday's hearing, said such construction loads would not be permitted today. "We've moved forward from that," he said. "We've changed our specifications to make sure these things don't happen again."

Corrosion not to blame

Gov. Tim Pawlenty weighed in on the findings, saying in a statement that the original design flaw was unrelated to subsequent inspections or maintenance of the bridge.

Pawlenty, a Republican, has been sparring with DFLers in the Legislature and in Washington who have focused on perceived aging and maintenance defects in the bridge, including a finding that some of the failed gusset plates had bowed. 

But NTSB officials said that even if construction inspectors had reexamined the bridge's load capacity, it's unlikely they would have looked at the gusset plates, which -- until this investigation -- were generally assumed to be stronger than the beams that they tie together. "Gusset plates get no respect," said board member Kathryn (Kitty) Higgins. "They're taken for granted."

Investigators, validating previous statements of NTSB Chairman Mark Rosenker, said the extra construction loads placed on the bridge would not have caused a fatal catastrophe if it had been properly designed.

"The collapse was the result of a serious design error," said Mark Bagnard, the NTSB's lead investigator on the 35W bridge collapse.

But even as the NTSB appears to be laying the blame on under-size gusset plates, some corrosion and pre-existing cracks were found in other parts of the structure.

Although the corrosion did not contribute to the collapse, "it is a source of concern," said Jim Wildey, an NTSB engineer who presented a structural analysis of the bridge's last moments.

Wildey said that the critical U-10 gusset plate nodes on the south span of the bridge that failed first showed no signs of corrosion. Rather, they showed signs of sudden fracture associated with excessive loads.

"Clearly all the metal was intact, and it was subjected to a very high load," Wildey said.

The heavy loads, Wildey and other NTSB investigators found, resulted from two previous bridge modifications in the 1970s and 1980s, as well as construction materials.

Concerns not dismissed

Despite the findings, which confirm initial conclusions that the NTSB reported in January, board members were careful not to dismiss public concerns about aging infrastructure and maintenance.

Rosenker said concern about the condition of the nation's highway infrastructure remains understandable, although the board's mandate is to understand exactly why the 35W bridge fell.

"We could do no less to assure the millions of Americans who cross our bridges every day that we have done all we can to see that such a tragedy doesn't ever, ever happen again," he said.

Kevin Diaz • 202-408-2753