In Big Structures, the Title of ‘Greatest’ Doesn’t Last Long

DURHAM, N.C. -- Engineers pushed the limits of technology in the past century to accomplish things that were not even dreamed of in the 19th century.

"And so it will be in the 21st century, with the contents of any list of engineering achievements that will be compiled in the late 2090s being virtually unpredictable today," says Duke University civil engineering professor Henry Petroski in his latest book, Pushing the Limits, New Adventures in Engineering (Alfred A. Knopf).

Petroski says our ancestors in the 1890s did not have a car, had not even heard of an airplane, drank untreated water and had yet to hear a radio broadcast or see a television show. Spacecraft were science fiction. Lasers and fiber optics, nuclear power, and high-performance materials like nylon and Kevlar were undreamed of.

Predicting the technological future has always been a risky business, says Petroski, the Aleksandar S. Vesic Professor of Civil Engineering at Duke's Pratt School of Engineering. He notes, in fact, that a bank president in 1903 advised Henry Ford's lawyer not to invest in the Ford Motor Co. because "the horse is here to stay." And in 1977, the founder of the Digital Equipment Corp. said there was no reason for people to have a computer at home.

"Fortunately, not all inventors and engineers listen to predictions, even those made by experts; they just push the limits of technology farther and farther into the future," Petroski writes.

In his new book, Petroski turns his observant eye to large feats of engineering -- bridges and buildings and dams that are among the largest constructions on Earth.

He notes that some of the greatest engineering achievements have been dwarfed by later ones. Petroski says, for example, that a pedestrian who in five minutes can walk between the towers of the Brooklyn Bridge, once the longest span in the world, would need 20 minutes to walk from tower to tower of Japan's Akashi Kaikyo Bridge, today's longest.

"The title of greatest is one that seldom stands unchallenged for long, but even former champions still command a measure of respect," Petroski writes. "The Empire State Building, which reigned as the world's tallest for four decades, remains an awesome structure to which tourists flock."

Petroski devotes much of his book to bridges. America has more than a half million of them, and Petroski says "masterpieces of bridge engineering are everywhere, legacies of their designers' structural artwork that are as much a part of the American scene as are the red barns and white churches of the countryside, or the tall buildings and canyon-like streets of large cities."

Designers of bridges, like those who design skyscrapers and dams, tend to push the limits to span ever increasing distances. The Delaware River at Philadelphia, for example, was a challenge for bridge designers in the 1800s. In 1851, a suspension bridge of four spans of 1,000 feet each was proposed, which Petroski says "would have been a daring stretch of the state of the art." Just two years earlier, a 1,010-foot suspension bridge had crossed the Ohio River at Wheeling, W.Va., but its roadway was destroyed in a wind storm in 1854. The four-span bridge idea for the Delaware was rejected, as were other proposals.

Finally, in 1921, a 1,750-foot suspension bridge linking Philadelphia and Camden, N.J., was accepted. The design would result in the longest suspension bridge in the world. Petroski says the span, the bridge's width and its traffic capacity required cables of unprecedented size. Each cable, 30 inches in diameter, was made up of 20,000 individual steel wires.

The bridge, now known as the Benjamin Franklin Bridge, was completed in 1926 and was called "an engineering monument." But its reign as the longest suspended span in the world lasted only until 1929, when the Ambassador Bridge between Detroit and Windsor, Ontario, was finished. In 1931, the George Washington Bridge crossed the Hudson River between New York and New Jersey with a main span twice as long as the Ben Franklin.

An era of great bridge building in America reached its peak in the 1930s, Petroski says, when the George Washington, Golden Gate and San Francisco-Oakland Bay bridges were completed, each pushing the limits in its own way.

The era ended, Petroski writes, when some engineers a few years later allowed "their hubris to get the better of the judgment," leading to the 1940 collapse of the Tacoma Narrows Bridge near Bremerton, Wash. The Ben Franklin, however, had none of the inherent flaws of the Tacoma Narrows Bridge, and Petroski says the Ben Franklin stands as a monument "to the role of civil engineering in forging vital links across the waters between neighboring states."

A redesigned Tacoma Narrows Bridge opened in 1950. The Verrazano-Narrows Bridge across New York harbor was completed in 1964 with a world record 4,260-foot central span. The 4,626-foot Humber Bridge in England captured the record in 1981, and the Akashi Kaikyo Bridge near Kobe, Japan, which opened in 1998 with a 6,529-foot main span, is likely to be the world's longest suspension bridge for some time, Petroski writes.

There are always ideas for immense bridges around the world, including an intercontinental link across the Strait of Gibraltar.

"Whether dream bridges will be realized within the foreseeable future depends at least as much on political and economic conditions -- and will -- as on technical details," Petroski says.