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. Over 100 years, a typical bridge’s life span, experts put the chances of a damaging earthquake at about 20 percent, and of a catastrophic one at around 4 percent. Its buoyant foundation is built to withstand vertical—not horizontal—forces, and a quake’s power would be magnified by a factor of four to six times by the soft riverbed. That the Tappan Zee’s design places the majority of its mass in its foundation, not in its superstructure, would further magnify an earthquake’s effects.
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. While authorities say shipworms haven’t feasted on the bridge’s timber pilings yet, a state report noted that they were “anticipated.”
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, which estimated that nearly 60 percent of the deck needed to be replaced. The state has spent $750 million over the past decade on maintenance and estimates it would cost around $100 million a year to keep it operating in the absence of a replacement.
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 like its “stringers,” horizontal beams that hold up the deck, and joints. Replacement deck joints manage to last only about ten years. Particular concern surrounds the stringers along the causeway section, which the assessment identified as “high risk.” Major cracking has also been observed on the causeway’s outer columns, reappearing soon after repairs.
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. Despite the retrofit, the 2009 assessment found that any rehabilitation of the bridge should include impact protection around the buoyant caissons.
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. A 2006 state report found that the bridge was “vulnerable to local or major collapse from a number of different causes,” saying that the most serious risks—though still remote—were posed by overload and the failure of steel parts.
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.