American Scientist

To the Editors:

Regarding Henry Petroski’s Engineering column “Tacoma Narrows Bridges” (March–April): Aerodynamic loading of suspension bridges was experienced as far back as 1854, when Charles Ellet’s bridge across the Ohio River at Wheeling was destroyed in a violent windstorm. Washington Roebling added diagonal stays to the vertical suspenders and cables in 1870 to contain wind-induced movements during subsequent rebuilding of the bridge in 1870. The system of diagonal stays on the Brooklyn Bridge had—in part—a similar purpose.

The Bronx-Whitestone bridge in New York City originally had a much-admired light and delicate appearance, similar to that of the Tacoma Narrows bridge. It also lacked a stiffening truss, having only plate girders along the deck’s sides. It experienced wind-induced oscillations soon after it was finished. It now has an added stiffening truss and stays. France also experienced this problem during the 19th century. By not considering aerodynamic forces acting on suspension bridges, engineers during the 1930s simply forgot—or ignored—what already was known. George Santayana was correct: Those who don’t learn from the mistakes of history are destined to repeat them.

Myron Levitsky
New York, NY

Dr. Petroski responds:

Dr. Levitsky is correct. In fact, the destructive force of wind loading on suspension bridges was evident even earlier in the 19th century, when the light decks of structures such as the Menai Strait Suspension Bridge and the Brighton Chain Pier (structurally also a suspension bridge) were extensively damaged. These and other wind incidents were reported in the literature of the time. It was by surveying that literature that John Roebling concluded that wind forces must be addressed when designing a successful suspension bridge deck. He decided that a stable bridge deck needed sufficient mass and stiffness and its suspenders must be supplemented by stay cables to weather any storm. Roebling published his findings and applied them to the Niagara Gorge Suspension Bridge, the first of its type to carry railroad trains. Roebling also applied his principles to designs for the suspension bridge over the Ohio River at Cincinnati and the Brooklyn Bridge, which he did not live to see built.

The lessons of the 19th-century wind failures were forgotten in the 20th century. Suspension bridges began to be designed and built without stay cables and, later, without stiffening trusses. This devolutionary trend culminated in the destruction of the Tacoma Narrows Bridge. A report by investigating engineers concluded that wind was the enemy of suspension bridges—something Roebling learned a century earlier. Later engineers evidently did not recognize that it was relevant to their modern structures.

The Bronx-Whitestone Bridge was among several built in the late 1930s, when aesthetic goals drove design, that had wind troubles. A stiffening truss was added to that bridge—obstructing a great view of the Manhattan skyline—but that truss is no longer in place. A few years ago, fairings designed to ameliorate the wind effects on the plate girders, along with other motion-checking and –damping devices, were installed to steady the deck.