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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.