Engineering: To Get to the Other Side

The golden age of bridges is now. Never before in the history of the world has man had such a wealth of means in money, materials and technology to fulfill his inborn desire to get to the other side. By using strong new steels and ingeniously strengthened concrete, he has made it possible to move himself and his goods over barriers his forebears thought uncrossable.

Not only is man building his bridges longer and stronger than ever before, he is also erecting more of them than at any other time in history. In the past six years, the U.S.’s interstate-highway program has spent $5.6 billion building almost 20,000 new bridges, will spend another $8 billion to $9 billion in the next eight years on bridge construction.

In Europe, bridge building is becoming almost as commonplace as house building. Britain has built 120 new bridges in the past five years as parts of its new highways, and figures that by the 1970s it will have built 280 more. Germany now completes 1,000 new bridges every year, at this moment has under construction nine spans more than 3,000 ft. long.

The result is not only new efficiency and new speed in getting from place to place; almost inevitably, when a great new bridge goes up, the result is also breath-taking beauty. The very nature of the barriers that man seeks to cross makes them some of the loveliest spots on the globe—gorges, bays, broad rivers, mountain valleys, the approaches to towering cities. By necessity, bridges are the purest sort of expression of the architectural concept of form following function. A steel-arch bridge over a deep canyon cannot help completing the frame of a picture of classic beauty: rushing waters below, soaring steel above, and all framing the natural art of rock shaped by wind and water.

What has allowed man to create these great structures is a new mastery over matter and mind.

> Steel has played the dominant role in modern bridges. A bridge built with today’s steels is lighter, yet nearly twice as strong as a span of equal length built just 25 years ago. Today’s bridge builders use as many as 18 different types of steel in the same bridge.

> Concrete mixers of today are producing wonders. Reinforced with steel wire and prestressed for still more strength, whole slabs of concrete now form single spans up to almost 700 ft.

in length.

> Technology has taken dramatic strides over the past two decades. Bridge designers are well-grounded in modern physics and aerodynamics before formulating their designs, then run them through computers that have already been fed data on snow and rain conditions, wind velocities, low and high temperatures, traffic loads and substrata strength.

The results, say today’s bridge builders, are awesome. Using their new tools and talents, builders think suspension bridges can be built twice as long as they are now. “I don’t think a suspension bridge of 10,000 ft. is impossible,” says Raymond Boynton of the Manhattan engineering firm of Steinman, Boynton, Gronquist & London. Bridge strength will also increase. “I tell people we put up bridges that will last 1,000 years,” says American Bridge Engineer William K. McGrath. “But I’m not sure they couldn’t last forever.”

Stone. The world’s first bridges lasted only as long as nature permitted, since they themselves were natural accidents —vines or windfallen trees blown by chance across some primeval stream. For eons, men did little but imitate nature with ropes or planks. The first real bridge engineers were the conquerors who shaped the Roman Empire more than 2,000 years ago. They built bridges in such numbers that their far-flung realm could be journeyed from the northern heaths of Britain all the way to Rome without once having to ford a stream—except, of course, the English Channel, still to be bridged. Masters of the stone arch, the Romans were the first to use cement to bind their arches, solved the ticklish engineering problems of how to rest their massive spans on underwater piers and how to protect the piers from floods and the ravages of time. Today, soaring Roman arches still stand in Italy, Spain and France as monuments to their genius.

When Rome fell, the world had to wait for Renaissance Italy to revive the art of bridge building. In the 14th century, Taddeo Gaddi spanned the River Arno in Florence with the immortal Ponte Vecchio in flat, segmented arches instead of the narrow semicircles favored by the Romans, thus making the roadway level enough for easy wagon passage. Andrea Palladio became the first to discard the arch in favor of a truss—the triangular support that is a basic method of making big bridges rigid today. By the late 16th century, Architect Antonio da Ponte was driving foundation piles with a mechanical hammer, then went on to build Venice’s haunting Bridge of Sighs.

Iron. An aft medium during the Renaissance, bridge building became a more exact engineering science in the 18th century. French Engineer Jean-Rodolphe Perronet was building a bridge across the Seine at Mantes in 1763 when he discovered that the first pier of the bridge sagged slightly toward the river until the second pier was in place. Then the first one straightened itself out.

Perronet reasoned what nobody before him dreamed: that the horizontal thrust of each arch carried along the length of the entire bridge. He reasoned that there was thus little need for the massive pier-and-arch bridge. At Neuilly, he tested his theory by building a bridge using piers 13 ft. thick to support arches 120 ft. long. The bridge not only stood, but its construction used far less stone than any bridge of similar dimensions before it. Most important, Perronet greatly increased the useful waterway underneath. Roughly a decade later, when the first cast-iron bridge was thrown across the Severn River in Britain, men started on their first real bridge-building spree since the Romans.

Steel. The spree soon ran into a storm. Engineers were building bridges of iron, but they were crossing the bridges with iron too—the iron horses of the first railroads. Their weight and vibration were too much. During the 1870s and 1880s, no fewer than 25 railroad bridges fell each year in the U.S. A train’s weight collapsed the Ashtabula Creek Bridge in Ohio in 1876, killing 80 persons. The most dramatized disaster of the times was the Firth of Tay tragedy in Scotland in 1879. During a December storm, 13 of the trusses of the two-year-old iron bridge fell into the raging waters—taking with them a trainload of some 100 passengers into the black abyss.

James B. Eads led the way back out of the abyss. A self-taught engineer who built ironclads for the Union Navy, Eads’s experience with iron taught him the defects of the metal. When he began after the war to push his scheme for bridging the Mississippi at St. Louis, he conceived the notion of a great triple arch of steel. In those days, steel was an untried structural metal that cost three times what it does today. But Eads knew it also had twice the strength of wrought iron and could be worked in a way that iron never could. It took Eads more than seven years and $7,000,000, but what he built was a magnificent, 1,524-ft. bridge that was also one of the world’s first important steel constructions of any kind. Scientific American was so impressed that it proposed Eads for President.

Cable. While Eads was working with rigid steel, other innovators were developing the concept of the suspension bridge—a primitive invention never much fancied by later bridge builders because of its nasty tendency to dump travelers or blow down. But with the invention of steel cables, the principle of bearing the load from above took on new fascination. As it turned out, suspension bridges were found to be the sole reasonable way of bridging long spans, since only suspension bridges can economically support dead weight beyond 1,600 ft.

Early experiments were shaky; in 1850 a regiment of French soldiers fell to their death from a suspension bridge at Angers. But a year later, German-born John Roebling began assembling a suspension bridge—over, of all places, the Niagara gorge and to carry, of all things, a railroad.

Wind. It took Roebling four years to build the 821-ft. Niagara bridge, but beginning in March of 1855, trains began regular crossings over a span held up by wire cables for the first time in history. Twelve years later he began planning his greatest work, the Brooklyn Bridge. Surveying the East River for the location of the main piers, he had his foot crushed. The injury gave him tetanus, and he died three weeks later. The man who took over the job was another Roebling—his son, Washington, who saw the bridge to completion in 1883. At a cost of $15 million and 20 lives, the Brooklyn Bridge set a record length of 1,595 ft. and set builders striving for even greater spans. In 1931, Builder Othmar Ammann spun the George Washington Bridge 3,500 ft. across the Hudson River; in 1937, Cincinnati Engineer Joseph Strauss carried the Golden Gate 4,200 ft. across the entrance to San Francisco Bay.

The long inverted arch of the suspension bridges was not only economic, it possessed inspiring beauty. But that very beauty blinded some builders, who wanted to create an even slimmer bridge by cutting down on the depth of the stiffening girders. Such a bridge was the Tacoma Narrows Bridge, built out over Puget Sound in 1940. Motorists crossing the bridge often noticed that the car in front appeared to sink into the roadway or even vanish for an instant. Nobody was alarmed at first, and engineers and drivers alike enjoyed explaining the advantages of “Galloping Gertie’s” flexible suspension design. Then, four months after the bridge was opened, Gertie galloped herself to pieces in a high wind. Gertie’s extremely narrow, slender and flexible design was strong enough to withstand foreseeable forces. But the wind that killed the bridge came at more than 40 m.p.h. across and under the bridge, and started the span on a vertical oscillation, which so fed itself that the deck was whipped clear of its supporting cables. The bridge, ruled the experts, was “aerodynamically unstable.”

Concrete. Gertie’s final gallop convinced bridge builders that they did not know everything about bridge building. Back to school they went to learn more about aerodynamics, stresses and strains. The new technology produces far more than just better suspension bridges. One of the most ingenious uses of prestressed concrete is in the $21 million floating bridge across the Hood Canal in Washington’s Puget Sound. Carried on 23 concrete pontoons, the bridge has retractable center sections that slide into the main body of the bridge, allowing waterborne traffic to pass through instead of under. The greatest use of prestressed concrete is in the 51-mile bridge over Venezuela’s Lake Maracaibo—the longest prestressed concrete bridge in the world.

By necessity, since nearly all of their big bridges were destroyed in World War II, some of the busiest users of the new technology are the Germans. They are also some of the most inventive. Nearly all the steel bridges built in Germany today use a German-developed steel plate called orthotropic. On a conventional bridge, the concrete roadway is supported on steel stringers. Not on an orthotropic bridge, which has instead of a concrete slab a half-as-heavy steel deck serving both as roadway and stress-carrying component of the bridge spans.

Bridge building is almost as frenzied in other parts of the world. Britain’s new bridges include the majestic Firth of Forth suspension span (3,300 ft., longest in Europe), soon to be completed. Already under construction in Portugal is the even longer (3,323 ft.) Tagus River span, scheduled for completion in 1967.

Biggest. But nowhere on earth is there such a surge of bridge building as in the U.S., which already has 500,000 bridges. So far the most spectacular new span is the masterwork of George Washington Builder Othmar Ammann (now 85)—the Verrazano-Narrows Bridge across the main entrance to New York Harbor. Nearly everything about the bridge is the biggest: it cost $325 million, it outspans Golden Gate by 60 ft., it hangs from 145,000 miles of cable wire. Its twelve traffic lanes will carry 48 million cars a year between Brooklyn and Staten Island.

What next? Bridge builders are now talking about suspensions almost two miles long in a single span, and such talk is likely to lead to startling results. Prospects, perhaps sooner than later: bridges vaulting Italy’s Messina strait, Turkey’s Bosporus and New York’s Long Island Sound.