Newswise — A tugboat captain guides a 150-foot, 800-ton barge up a bay and points it at a bridge with one goal in mind: to ram it.
It sounds like the beginnings of a tragedy. But that's just what the massive experiments that began last week in Apalachicola Bay in Florida's Panhandle are designed to prevent. Headed by University of Florida engineers and sponsored by the Florida Department of Transportation, the experiments " which will involve at least a dozen planned barge vs. bridge rammings this month " are aimed at reducing the cost of bridge construction while making them safer.
"There is very, very little information available that deals with the actual impact load when a barge strikes a bridge," said Henry Bollmann, FDOT's senior bridge designer. "This will fill in the blanks, and it will affect national and international bridge-building codes."
The barge tests are not designed to bring down the recently closed "old" St. George Island Causeway Bridge spanning the bay from the small town of East Point to St. George's Island.
Its replacement, the 4.1-mile-long Bryant Grady Patton Bridge, opened last month, and most of its 1960s-era predecessor is slated for demolition following the tests. Boat traffic will be restricted in the area during the tests and demolition.
The planned tests are expected to jiggle the innards of more than 150 carefully placed sensors on the barge and the bridge. The result: What UF and FDOT engineers describe as the first-ever microsecond-by-microsecond glimpse of the forces that unfold in a real-world calamity.
Gary Consolazio, a UF assistant professor of civil engineering and the lead researcher on the project, explained that when bridge engineers design structures currently, they rely on nationally adopted standards. Those standards are based on tests using scale models.
While scale model tests are thought to provide good data, they are not as accurate at determining the forces on the bridge resulting from the crash as the real thing, he said. However, until now, no one has taken the opportunity to conduct tests on real barges and bridges.
"Quite simply put, you can't run a barge into a bridge intentionally if the bridge is in service," Consolazio said. "There are just massive safety issues involved."
Today's standards typically require engineers to design bridges that will remain standing while sustaining several million pounds of "static" load, or load that doesn't change over time, Consolazio said. Those specifications vastly increase the amount of concrete, steel and other materials used in the supportive piers, or underpinnings, and significantly raise the cost of building the structures, he said.
The planned tests may reveal that this huge load declines quite a bit after impact rather than remaining constant, which may mean the standards could be loosened in some cases, significantly reducing the cost, Consolazio said. Alternatively, the tests could reveal that the standards are correct or may need to be strengthened in some cases, he said.
"If it turns out the loads are smaller than what we are currently designing for that could have a major economic impact, because the foundations of a bridge are such a massive component of the cost of construction," he said. "On the other hand, if it turns out that the loads are larger than we are currently designing for, then obviously from a public safety standpoint we want to know that."
The possibility of improving bridges' safety is a top goal of the tests " and one of the reasons the FDOT is spending an estimated $1 million on the project, Bollmann said. Boh Brothers Construction, the contractor that just completed the replacement bridge, also is assisting.
Although rare, fatal accidents involving downed bridges are not that uncommon. In Florida, the most infamous dates back to 1980, when a freighter knocked out part of the Sunshine Skyway bridge, plunging 35 people to their deaths. Two years ago, a barge struck the Interstate 40 bridge spanning the Arkansas River in Oklahoma, killing 14. In 2001, another barge vs. bridge incident in Texas killed eight.
With its extensive coastline and lengthy Intracoastal Waterway, Florida is a hot spot for barges toting fertilizer, coal, petroleum products and other cargo, Bollmann said. Several hundred of the state's roughly 10,000 bridges span bays and waterways deep enough for barge traffic, he said. Although there hasn't been a fatal barge vs. bridge accident here in recent memory, the FDOT is concerned about the possibility " particularly in light of the Sept. 11, 2001, terrorist attacks.
"People are concerned that terrorists could use a vessel to knock a bridge down," he said, adding that the experiment was already in the planning stages before the Sept. 11 events.
As part of the tests, researchers plan to shove the barge into two different piers, a large one near the shipping channel and a smaller one farther away. The big pier will get the maximum punishment, with a tug slamming a barge into the pier at maximum speeds of about 6 mph, typical for barge traffic. Load sensors, accelerometers and other sensors will record the force the barge imparts to the pier, how much the pier moves following the impact and other data, Consolazio said.
Initial analysis of the experimental data will take about six months. Researchers will combine the data with computer models to develop revised bridge design specifications, Consolazio said.
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