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Old Posted Jan 28, 2013, 9:11 PM
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Falling Down
The Tappan Zee Bridge, as one expert calls it, is the “scary of scaries.”





By Andrew Rice
Jan 27, 2013

Quote:
LePatner, who is the author of a book called Too Big to Fall and who routinely offers dire warnings about the condition of America’s vital infrastructure, considers the Tappan Zee his “scary of scaries.” Yet even he was taken aback when, after the speech, a state official sidled up and implied that things were worse, even, than LePatner imagined. The man asked LePatner how often he crosses the bridge. Maybe twice a year, LePatner told him, to go antiquing in Nyack. “That’s enough,” the official replied.

For the record, the New York State Thruway Authority, which controls the Tappan Zee, insists the bridge is in no immediate danger of falling down. But the 140,000 or so people who travel daily across the seven-lane span, which was built to handle a fraction of that traffic load, can be forgiven for having their doubts. The Tappan Zee routinely sheds chunks of concrete, like so much dandruff, into the river below. Engineering assessments have found that everything from steel corrosion to earthquakes to maritime accidents could cause major, perhaps catastrophic, damage to the span. Last July, one of Governor Cuomo’s top aides referred to the Tappan Zee as the “hold-your-breath bridge.”

After years of dithering, stopgap maintenance, and $88 million worth of studies, Cuomo fast-tracked plans to replace the bridge shortly after taking office in 2011. In December, state officials selected a design for a proposed new, $3.1 billion span. Construction, scheduled to begin this year, could be completed as soon as 2017.

What follows is a breakdown of how the bridge was built—and how it’s been falling apart since.

1. A Bridge Too Far
Improbably enough, the three-mile Tappan Zee, the state’s longest bridge, spans the Hudson River at one of its widest points. Governor Thomas Dewey proposed the site during his 1950 reelection bid. The Port Authority wanted to build a toll bridge at a narrower (read: cheaper and less complicated) juncture a few miles south; Dewey’s location had the advantage of sitting just beyond the Port Authority’s jurisdiction.

2. Cheap and Easy
Completed in just five years for a relatively inexpensive $81 million, the Tappan Zee “was one of the last bridges to arise out of a vanished American way of doing things: build stuff first, ask questions later,” wrote the Manhattan Institute’s Nicole Gelinas in a 2011 City Journal essay. Gelinas says the designers built “thin and light,” using an “untested design that ‘floated’ above bedrock,” which, inconveniently, lay beneath hundreds of feet of silt, sand, and clay. Among other things, the novel technique involved using foundational elements called buoyant caissons to anchor the main span in the soft soil, yielding substantial savings but a less durable structure.

3. And Ugly, Too
The bridge’s design married a towering cantilevered truss, offering sufficient clearance for ships navigating the deep channel on the east side of the river, to a squat causeway, which crossed shallow waters to the west. One of its engineers called the result “one of the ugliest bridges in the East.”

4. Earthquakes!
At the time the Tappan Zee was built, engineers didn’t seriously consider seismic risk, but researchers have since determined the bridge sits atop an active fault line.

5. Worms!!
Marine organisms known as shipworms—actually a type of clam that can bore into submerged wood—weren’t a problem in the Hudson at the time the Tappan Zee was built, but after decades of anti-pollution efforts, they have returned.

6. Punch-Throughs!!!
More than 50 million vehicles traversed the Tappan Zee in 2010, up from 10 million in 1960, and while the bridge was designed to carry the 36-ton trucks of the fifties, it must now withstand today’s 45-ton behemoths. Thus the deteriorating concrete, which falls off the bridge in chunks, sometimes creating holes in the roadway through which the river below can be seen. Forty-five such “punch-throughs” were recorded in the eighteen months prior to a 2009 engineering assessment of the bridge...

7. Strong As Steel (Which Turns Out to Be Not That Strong)
Steel, like concrete, is vulnerable to the corrosive effects of rainwater, especially when it mixes with road salt used to melt snow. The 2009 engineering assessment found that the Tappan Zee’s rate of deterioration is “unusually high.” The bridge’s drainage system was designed to dump water onto the substructure below the highway, causing major corrosion in crucial components...

8. Crash
Traffic accidents occur on the bridge and its approaches at double the rate of the rest of the New York State Thruway. Its seven lanes are narrower than today’s recommended average, and it has no shoulders, meaning that even fender benders can paralyze traffic.

9. Bridge, Ho!
Another potential threat is a maritime accident. In 2006, the Journal News reported that the state had acted to shore up the towers that hold up the main span after a study found that they could be brought down by a collision with one of today’s larger ships.

10. Failure Is an Option
State transportation officials insist the Tappan Zee is still structurally sound. At least some experts disagree. Like other bridges of its vintage, the Tappan Zee has a “fracture critical” design. Because of a lack of engineering redundancies, it could conceivably be brought down by the failure of a key single component.

11. The Chopsticks Solution
In December, a selection committee that included Richard Meier and Jeff Koons recommended a design for the new Tappan Zee. The winning bid came from a consortium of companies that handled the complex—and notoriously overbudget and behind schedule—$6 billion replacement of San Francisco’s Bay Bridge. The design consists of two parallel roadways, with the main span supported by cables strung from eight columns that jut distinctly outward, like chopsticks. Though the cheapest of three finalists, at $3.1 billion, the bridge will still be one of the largest infrastructure projects in New York State. In response to complaints from transit advocates, state officials say the bridge will be built with the capacity to add light rail or bus lanes—features cut from the project for now because of the additional cost. The new bridge, state officials say, is designed to last 100 years before it needs major repairs.
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