Tudor Van Hampton
NTSB Chairman Mark Rosenker uses a gesture to describe the bridge's north end, which slid backwards off its pier. This side of the bridge became NTSB's focus after the southern end was cleared as a causal zone.
Minnesota Dept. of Transportation officials have issued an urgent request for proposals for design-build teams interested in replacing Minneapolis' I–35W bridge over the Mississippi River. The 40–year–old bridge collapsed on Aug. 1, killing at least five people and injuring dozens more.
The agency already has selected New York City–based Parsons Brinckerhoff to examine its inspection protocols and help expedite emergency inspections for the state's ongoing bridge sweep.
Having received authorization Aug. 4 from the National Transportation Safety Board to begin clearing deck debris, MNDOT says it is putting the replacement on a fast track. A design-build contract could be awarded as soon as early September, sources say.
Selecting a team on such short notice "is unlike any process that we have ever followed before," says Bob McFarlin, MNDOT's assistant commissioner. The "A+B" contract will take into account qualifications, unlike typical low–bid projects the state usually procures, he says.
Though MNDOT has not released a cost figure for the new bridge, sources at the agency say it could be as high as $250 million. President George W. Bush and members of Congress have vowed to foot the bill with federally–appropriated funds. On Aug. 4, Congress authorized $250 million for bridge repair and replacement. The measure awaits President Bush's signature. MNDOT officials hope to open the new bridge late next year.
In an another attempt to douse the speculative brushfire sizzling over the state's inspection program of the I–35W bridge, McFarlin also reiterated that MNDOT's decision to inspect, rather than simply add stiffener plates to 52 fracture–critical truss members, was not based on cutting costs.
"That was an engineering decision, not a money decision," McFarlin says. Engineers wanted to inspect and repair as–needed, "first and foremost, because of concerns about drilling all of those holes."
The agency estimates that adding stiffeners would have cost about $1.5 million, "not a significant budget item," McFarlin adds.
As on–scene documentation of the collapse continues, federal investigators have shifted their focus from the southern end of the bridge to the truss members on the northern end.
Courtesy U.S. Coast Guard
The collapse left structural truss remnants of bridge's northern connection pointing skyward.
After examination of the southern end, which slipped laterally about 81 ft before it hit the ground, "we found no defects," says Mark V. Rosenker, NTSB chairman. He adds that the behavior appears to be "a secondary action."
Federal Bureau of Investigation officials have completed a laser scan of the debris field and NTSB plans to fly a helicopter equipped with a remote camera to get a closer look at the northern end of the deck truss. One span sheared, then slipped off a pier and fell backwards, sending twisted metal beams and rebar tilting up toward the sky.
"If we begin to see anything there, we will take a piece of it off," says Rosenker. "If not, our focus will shift to the center section." The Federal Highway Administration will begin modeling failure scenarios as soon as Aug. 6, Rosenker adds. NTSB also is interviewing employees of Progressive Contractors Inc., the St. Michael, Minn.–based firm that was performing a $9–million deck overlay at the time of accident.
Sources are not commenting on the role of construction in the collapse, but engineers say that the overlay project was routine and would not be expected to damage any truss members.
The scope of Progressive's work was to scarify 2 in. of existing low–slump concrete and perform repairs to the bridge's 7–in. structural concrete deck. That included mostly shallow and medium–depth Type 1 and Type 3 patches, says Barry Nelson, MNDOT project engineer.
Such repairs required the contractor to dig out spalled or delaminated concrete with jackhammers. Workers typically then clean out the affected area, without cutting into the deck's two rebar mats, and then place new concrete into the voids.
A few areas of the bridge also needed full–depth repairs, requiring extra scaffolds and falsework hanging over and wrapping around the deck. The equipment allows the contractor to catch construction debris from falling through the deck truss and into the river, as well as containing the fresh concrete as it cures. It also provides access for workers.
Engineers note that the full–depth work was minimal. "This was just a normal operation," Nelson says. MNDOT engineers say they are not certain whether or not scaffolding was set up at the time of collapse.
They do know, however, that the overlay project was more than 50% complete at the time of the collapse. Working in two lanes on each side of the bridge while the other lanes remained open to traffic, Progressive had already finished the first set of four and was working on the last set when the bridge came down.
The bridge had no capacity restrictions on it at the time of the collapse, according to MNDOT.
NTSB says it has ruled out seismic action as a factor. The collapse is "baffling," McFarlin says.
MNDOT announced Aug. 5 that it that it had awarded a debris removal contract. Carl Bolander & Sons. Co., St. Paul, began mobilizing four cranes onsite on Aug. 6. Removing vehicles from the site is the first order of business, an MNDOT official says. Then the contract will clear a 56–ft–wide pathway on the south side of the river to clear a channel for the resumption of barge traffic. MNDOT estimates the contract's value to reach $15 million.