Less Bronx-Whitestone Bridge Yielded More Stability During Hurricane Sandy

David W. Dunlap/The New York Times The Bronx-Whitestone Bridge, seen from Malba, Queens.

One very windy day in 1968, the Bronx-Whitestone Bridge began to oscillate crazily, leading drivers to abandon their cars in panic.

David W. Dunlap/The New York Times Siobhan Roberts at the former Citicorp Center, which the subject of her biography , Alan G. Davenport, helped engineer.

But on an even windier night in 2012, as Hurricane Sandy howled across Long Island Sound and buffeted the span, the bridge stood all but unmoving. The difference? Six thousand fewer tons of steel trusses, which were removed in 2004.

The trusses had been installed in 1946 to stiffen the bridge deck and lessen the chances that the 2,300-foot-long span would break apart in the wind, as the Tacoma Narrows Bridge (“Galloping Gertie“) did in 1940.

But it turned out the trusses were doing more harm than good. Their weight was shortening the bridge's life span by further stressing the structure. From an aesthetic point of view, they spoiled the slender lines of one of the most beautiful bridges in New York. And when those 70-mile-an-hour winds hit the bridge in November 1968, the deck oscillated all the same, as much as 10 inches.

Instead of trusses, the bridge is now equipped with aerodynamic fiberglass fairings along the deck, which streamline the airflow around the suspended span. During Hurricane Sandy, the bridge was closed to traffic as it sustained winds of 50 to 55 miles an hour, and gusts up to 80 miles an hour. It reopened at noon the next day.

“Our engineers were very pleased with the performance of the bridge,” said Aaron Donovan, a spokesman for the Metropolitan Transportation Authority, which owns and operates the Bronx-Whitestone through its bridges and tunnels division. “There were no instabilities recorded.”

David W. Dunlap/The New York Times Detailed view of the aerodynamic fiberglass fairings that deflect wind load around the suspended deck of the Bronx-Whitestone Bridge.

Much credit for solving the problems of the Bronx-Whitestone Bridge goes to Alan G. Davenport (1932-2009) and his colleagues at the Boundary Layer Wind Tunnel Laboratory at Western University in London, Ontario. A new biography by Siobhan Roberts, “Wind Wizard: Alan G. Davenport and the Art of Wind Engineering” (Princeton University Press), details the work that has gone into preparing skyscrapers and bridges for an event like Hurricane Sandy.

Ms. Roberts, 41, a freelance science writer, was in town during the storm. “I was definitely thinking of th e Bronx-Whitestone, knowing that it was all rigged up to record every quiver.” she said. “I was doubtful that it had any instability, given what it's gone through in the last 10 years.”

The same was true for 601 Lexington Avenue, the 59-story skyscraper-on-stilts formerly known as Citicorp Center. In her book, Ms. Roberts revisits the harrowing summer of 1978 when Mr. Davenport and his colleagues helped determine that the tower was in danger of imminent collapse in certain winds. An emergency welding program, undertaken as hurricane season approached, left the tower “fit to withstand a 700-year storm,” Ms. Roberts wrote. (She suggested the base of the building as the rendezvous for our interview, to underscore her confidence.)

Courtesy of Western University In the “Three Little Pigs” experiment, researchers at Western University blew this full-scale house apart with hurricane-strength pressure, to measure what structural systems failed and why.

Mr. Davenport's concern was not limited to long bridges and skyscrapers, Ms Roberts wrote. He worried about low-rise buildings, she said, in part because “the ability of a community to cope and recover turns on the survival of these Everyman structures.” In 2001, engineers at Western began the “Three Little Pigs” project. The goal was to subject a two-story, 1,900-square-foot, code-compliant brick house to hurricane-force pressures.

Since then, two houses have been torn asunder to provide a better understanding of what structural systems fail under hurricane conditions, and why. One house had a gable roof: the classic, inverted V-shape in which two sloping planes rise from two parallel walls and meet along a center line. The other had a hip roof, in which four planes rise from all four walls, converging either at a point or along a center line. Many other tests have been conducted on roof sheathing, window openings, soffits and sidings, as well as on the form of the structures.

“We don't tend to think about the shape of the roof when we buy a house, except aesthetically,” said Prof. Gregory A. Kopp of Western University. “But hip roofs are definitely better than gable roofs.” Their structural and aerodynamic superiority, he said, is borne out in testing and in field observations after big tornadoes, when hip-roof houses remain intact while roofless houses nearby turn out to have been gabled. “Roof shape makes a big difference,” Professor Kopp said.