On July 23, a two-minute time-lapse video of the Manhattan Bridge, undulating under traffic, appeared on YouTube. It got 140,000 hits in the first week, and the media, always short on engineering majors, gave it lots of play. WPIX news aired a clip, and Morning Joe played it to uneasy oohs and aahs from its co-hosts. The website Gawker posted it under the headline “The Manhattan Bridge Is Falling Down” (later clarifying that it had been a joke). In fact, suspension bridges are supposed to move, in multiple dimensions. The century-old Manhattan Bridge is in the final stages of a rehab that began in 1982, when it was actually in danger of collapsing. It’ll bounce, without incident, for years to come.
1. The Rust
By the late seventies, the cash-strapped city had skimped on painting for decades, and rust never sleeps. In 1986, it was discovered that the four cables slung over the towers (made up of 9,472 wires each) were severely corroded. Some components of the anchorages, where the cables are pinned to the riverbank, were half-gone. “We came so close to losing the bridge,” Schwartz says (an emergency closure, repairs, and the institution of a strict preventive-maintenance program followed).
2. The Stressed-Out Deck
Bridge builder Leon Moisseiff’s decision to put four subway tracks along the outer edges of the roadway proved unwise; when a pair of trains went over the bridge at once on the same side, the bridge deck twisted eight feet off the horizontal, causing strain and severe cracking. (Engineers realized that this was a big problem as early as the forties.) In the early nineties, the roadway was stiffened with cross-bracing, and new steel with corrosion-resistant properties replaced much of the old. Department of Transportation spokesman Scott Gastel notes that the bridge now deflects a mere sixteen inches (the movement seen in the YouTube video), which is much gentler on the structure.
3. The Hot and the Cold
The center of every suspension bridge moves up and down as the deck expands and contracts with temperature shifts. The roadway is designed to be slightly arched, and expansion joints, where overlapping pieces of steel can slide apart, allow its curvature to change. (The middle of the Verrazano-Narrows Bridge varies in height a full twelve feet from its coldest day to its hottest.)
4. Walking It
The sidewalks were closed in the fifties because the floors had rusted through, and also because “pedestrians weren’t considered important” in those years, says Schwartz. The repaired foot- and bike paths reopened in 2001 and 2005. Fun exercise: Walk out to the very center, where the big cables dip to their lowest point. Train your eyes on a nearby building that intersects the horizon. Wait for a subway to cross, and watch everything shift. But don’t worry! It’s supposed to.
5. The Wind
The Manhattan Bridge’s towers bend back and forth slightly, depending on the winds and weight of traffic. “A couple of inches on a tower that’s about 350 feet over mean high water—it’s really not much,” transit engineer and former DOT commissioner Sam Schwartz says. “I’ve been up at the top, and you can barely feel it.” The foundations for the towers are totally stable.