Investigation into fatal bridge collapse continues

posted on:
November 8, 2008

author:
Jeff Adelson

medium I 10 twin spans accident nov09 2008 Investigation into fatal bridge collapse continuesMore than a week after tons of concrete and 10 men went crashing down into Lake Pontchartrain’s chilly waters, leaving one construction worker dead, officials have no clear idea how a common procedure in a common construction design quickly turned into tragedy at the site of the new twin spans bridge.
Officials are sure they know what happened: A 70-ton girder rolled off the cap that tops the pillars supporting the bridge.

But as the investigation has ramped up, they still have no clear explanation as to what caused the Oct. 30 accident and expect that a full investigation could take months.

They are confident, however, that the mishap does not point to potential design flaws with the $800 million Interstate 10 bridge, the largest public works project in Louisiana’s history.

Investigations by the state Department of Transportation and Development, Boh Bros. Construction Co., the lead contractor on the bridge, and the federal Occupational Safety and Health Administration are all under way, with a wide range of factors under consideration, from potential faults in the construction materials to jolts from other work nearby.

Whatever the cause, transportation department spokesman Mark Lambert said it is unlikely that the accident is indicative of any serious flaws in the design of the more than 5-mile bridge and should not cause concern about the safety of the span once it is completed about 2012.

“It doesn’t seem reasonable at all to think there is a systemic problem if we’ve been doing the same thing over and over again and nothing happened,” Lambert said. “There’s some anomaly here.”

Dangerous process

Construction is, by its nature, the most precarious part of a bridge’s lifespan, as materials are stressed under conditions that differ dramatically from those in the completed project, Lambert said.

“You don’t design these things for construction use. You design them to operate as a bridge,” he said.

“We design all of the parts — the girders, the caps, the roadway segments — all of that stuff is done, and it’s designed for this to operate as a bridge. The most stress you ever have on one of these girders is when you have actually attached the concrete forms. That’s more stress than they ever have once the concrete is set.”

Hundreds of girders have already been installed on the bridge without incident, so investigators are faced with determining how circumstances were different on Oct. 30 than in the construction that preceded that day.

Investigators, who are working with eyewitness accounts and analysis of the site, expect to soon recover the girder itself from the bottom of Lake Pontchartrain, Lambert said.

The accident killed foreman Eric Blackmon, of Slidell, and sent nine others tumbling into the lake. Though attached to the falling materials by harnesses, the other workers were able to free themselves and escape without serious injury.

Construction on the bridge resumed Monday, but Boh Bros. will not work at the site of the accident until the investigation is complete.

Ann Barks, a spokeswoman for Boh Bros., declined to speculate about the cause of the accident, owing to the preliminary nature of the investigation and the complicated process of weeding through information and analysis to determine the cause. To aid the state transportation department and the company in their investigations, Boh Bros. has hired an outside contractor that has analyzed disasters including the attacks on Sept. 11, 2001, and in Oklahoma City in 1995, Barks said.

In addition to its structural analyses after the high-profile terrorist attacks in New York, Washington, D.C., and Oklahoma, CTL Group has also worked on accidents in Florida where girders rolled off bridges during construction.

Common technique

While the $800 million bridge is the most expensive public works project in Louisiana’s history and incorporates new techniques, including the use of denser concrete designed to double its lifespan to 100 years, engineering experts say the construction and design of the bridge is far from unique. The process of erecting pillars, capping them and laying girders and a road surface on top is standard procedure for modern bridge building.

The main differences between the new twin spans and the old, which were torn apart when storm surge from Hurricane Katrina lifted entire segments of roadway out of place, are all aimed at increasing the stability of the bridge. The new roadway will be about 35 feet above the water, high enough to avoid the strongest storm surges, and beams will be cemented to the caps to ensure they cannot be moved.

In rebuilding the twin spans, designers looked to a similar project undertaken for similar reasons. The Interstate 10 Escambia Bay Bridge near Pensacola, Fla., was destroyed during Hurricane Ivan and rebuilt using a system similar to the one now being employed on the twin spans.

“The type of bridge that was going up, it’s a fairly regular kind of operation,” said Brian Estock, the senior supervising engineer for PB America, which oversaw the Escambia bridge project.

It is rare, but not unheard of, for a beam to fall off the caps, an accident that is typically the result of an impact, Estock said. In one case in Pinellas County, Fla., a dump truck knocked a girder loose when it attempted to drive under the bridge and struck the girder with its raised bed.

No such accidents occurred during the Escambia County project, he said.

The most dangerous part of bridge construction tends to be when the girders are placed but not yet connected to the other beams in the span by concrete attachments known as diaphragms, which stabilize the entire section, Estock said. Boh Bros. officials have said diaphragms were already in place at the time of the accident.

Because the girders are so heavy, the weight of workers and equipment is not typically enough to overload them or cause them to roll over, Estock said. And strict monitoring and testing standards at concrete plants mean that faults with the materials are exceedingly rare, he said.

All these potential causes are on the table in the investigation of the twin spans accident.

Investigators are also looking into whether nearby construction activity may have played a role in the accident, including the possibility that nearby materials may have struck the girder or caused a shift in weight, Lambert said.

Officials are expecting a long investigation, but work could restart in “weeks rather than months” if a clear explanation of what happened emerges and policies are put in place to prevent a similar accident, Lambert said. That could occur even before investigators determine the exact cause of the accident if officials are assured that all possibilities are accounted for.

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