Bridge
From Encyclopædia
A bridge is a structure designed to carry pedestrians or vehicles across such obstacles as ravines, rivers, or other roads or
railroads. The earliest bridges may well have consisted of tree trunks or flat stones thrown across a stream--like the "
clam" bridges of southeastern
Cornwall or the "clapper" bridges of
Dartmoor, which were the first beam bridges. Alternatively, the earliest types may have been primitive suspension spans formed of twisted bamboo or creepers hung across a stream, with their ends tied to tree trunks on either side; such bridges can still be seen in
Africa and
Asia. Little doubt exists that these were built long before the first masonry ARCH, even though both types were preceded by natural rock arches such as those at Ardeche in France or in
Lexington, Va. In all three types of bridge--beam, arch, and suspension--the foundations must carry the full weight of the bridge and the traffic on it. The vital differences, however, are that arched bridges, by virtue of their shape, are in compression and thrust outward on their end supports or bearings, whereas the cables of a suspension bridge are in tension and exert a continual pull on their end anchorages.EARLY BRIDGES OF NOTEThe finest early bridges were the semicircular masonry arches of the Romans, built during the 500 years of their supremacy. Some of these--such as the Alcantara bridge over the Tagus in Spain, with its tall, majestic spans of 30 m (98 ft)--are still standing after nearly 2,000 years. The most significant contributions of the Romans were the use of a natural cement called pozzolana that enabled them to make concrete below water in bridge foundations, and the use of timber cofferdams--made by driving piles around the site of a pier in midstream and then draining out the water inside--so that the ground could be excavated as necessary and the pier built on dry surface. The Ponte Sant' Angelo in Rome, which is still standing, was built on cofferdam foundations in the Tiber more than 1,800 years ago; most Roman bridges still standing, however, have piers built on solid rock.By the
middle ages the first ogival (pointed) arches were built in
Europe; these arches resulted from Persian and Muslim influence. Bridges were often fortified to be used in defense of a
city, such as the Pont Valentre at Cahors, France, which had arrow slits, machicolations, and protected stairways to the towers. To pay for the maintenance of many medieval bridges, tolls were levied not only on travelers crossing over but also on vessels passing under the bridge.Toward the end of the 12th century, work began on two outstanding bridges, Old LONDON BRIDGE and the Pont St. Benezet at
Avignon on the Rhone in the south of France. Begun in 1177 and completed ten years later, the Pont St. Benezet had 20 lofty elliptical arches, each spanning 30 m (98 ft). Unfortunately, little of the bridge remains today. The construction of the first stone bridge over the Thames in London, begun in 1176, presented even greater difficulties, for it was the first large bridge with masonry foundations to be built in a swiftly flowing river with a tidal range of 5 m (16 ft). The design consisted of 19 pointed arches on wide, protected piers and a drawbridge that served for defense and to let
ships pass at high water. For more than 600 years the bridge supported a famous street of shops and houses flanking the narrow roadway. It even survived the Great Fire of London in 1666, which ravaged the northern end of the bridge. The street was not demolished until 1831, by which date a new bridge had been completed by Sir John RENNIE.THE
Renaissance ERAThe next great bridge-building era occurred during the
Renaissance. An example of the unbounded confidence of the
Renaissance was Leonardo da Vinci's offer to build a masonry arch bridge with a span of 240 m (787 ft) over the Golden Horn at Constantinople. In
Florence, the Ponte Vecchio (c.1350), which still stands over the Arno, was eclipsed by the Santa Trinita bridge (1569) with its three "basket-handled" arches that had a rise-to-span ratio of only 1 to 7 instead of the usual 1 to 4. In 1588 Antonio da Ponte's design was accepted for the Rialto Bridge in
Venice--a low circular arch that supports a 23-m-wide (75-ft) roadway lined with shops on a span of 27 m (89 ft) across the Grand Canal. To support the bridge in the soft alluvial
soil of
Venice, 6,000 wood piles were driven to a depth of 3.35 m (11 ft) beneath each abutment.Other great examples of this age were the Pont Notre Dame (c.1500) and the Pont Neuf (1604), both masonry arch bridges over the Seine in Paris. These preceded the masterpieces of Jean Perronet (1708-94)--the Pont Neuilly over the Seine, the Pont de Sainte-Maxence over the Oise, and his last work, the Pont de la Concorde (1791).BRIDGES OF THE 18TH AND 19TH CENTURIESDuring the 18th and 19th centuries a wide variety of timber
truss bridges were built in North
America, including the Colossus bridge of 104-m (340-ft) span over the Schuylkill River. In the late 18th century another innovation began with the building of the
world's first all-
iron bridge, the semicircular arch of 30-m (100-ft) span at Coalbrookdale over the River Severn in England.In the 19th century, as more and more
railroads were built, cast
iron was superseded by wrought
iron, because the latter was malleable, ductile, much stronger, and could be riveted instead of bolted. In 1826, Thomas TELFORD completed the 177-m (580-ft) span Menai suspension bridge, which was supported by cables of wrought-
iron links; it carried two lanes of roadway traffic over the straits. In addition, Robert STEPHENSON's Britannia Railway Bridge, also across the Menai Straits, was opened in 1850. It had four continuous spans, two of 70 m (230 ft) and two of 140 m (460 ft), consisting of wrought-
iron tubes of rectangular
cross section, through which the trains ran. This was the prototype of the modern steel box girders. The main difference today is that the traffic is carried on a roadway above the tubes or boxes instead of through them. Only a few years later, in 1859, I. K. BRUNEL completed his last and greatest project, the Royal Albert bridge at Saltash, England, with its two spans of 142 m (465 ft), each carrying one broad-gauge railway over the Tamar estuary.Before the end of the 19th century the
Mass production of mild steel plates and sections, with an ultimate strength of about 4.1 kilonewtons (kN) per sq m (30 tons per sq in) in both tension and compression, led to their use in building bridges. The first large bridge in the
world to be built of steel was the St. Louis bridge (1874) over the
Mississippi River. Designed by James B. EADS, it had three arch spans of more that 152 m (500 ft) each. This was followed by the construction (1869-83) of the BROOKLYN BRIDGE, a suspension bridg? of 486-m (1,595-ft) span traversing the East River and linking Brooklyn with
Manhattan. Designed by John A. ROEBLING, the bridge is nearly half again as long as any bridge previously built; it has six lanes for vehicular traffic and a footpath. The four main cables each comprise parallel
wires of galvanized cast steel 4.82 mm (0.19 in) thick, with an ultimate strength of 9.85 kN per sq m (71.5 tons per sq in). The cables were spun in place by a method that has subsequently been used in every large suspension bridge built in the
United States. To prevent failure brought on by oscillations built up in the deck by wind or traffic, steel-stiffening trusses were incorporated in the deck over the entire length of the bridge.The next major advance was the Forth Railway bridge (1882-90), with two
cantilever spans of 521 m (1,710 ft) each, which carries a double railway track over the Forth Estuary at South Queensferry,
Scotland. Unlike the Britannia and Royal Albert bridges, where the spans were floated out and hoisted up, all the steelwork of the Forth bridge was cantilevered out at the site from the main piers and riveted in place.NEW CONSTRUCTION TECHNIQUESThis period also saw the building of the first modern types of movable bridges, such as the bascule bridge, lift bridge, and swinging drawbridge. The largest bascule bridge to date--in which the opening spans swing upward as in the Tower Bridge in London--is the railway bridge built in 1941 at Sault Ste. Marie,
Michigan. The bridge has an opening of 102 m (335 ft). In lift bridges, the opening span remains horizontal and is counterweighted at the ends and lifted vertically. The lifting span of the Arthur Kill bridge (1959) at
Elizabeth, N.J., has a length of 170 m (558 ft). Swinging draw-bridges have swing spans that remain horizontal and are usually swung round on a
Central pivot or pin; thus they cause more obstruction to river traffic than do other movable bridges. The longest swing bridge yet built (1965) is that at al Firdan over the
Suez Canal in
Egypt; it has two arms that swing round on
turntables at each end, giving an opening of 168 m (552 ft). Floating bridges have been built since the 1st millennium BC, but their life is short and they require much maintenance. The well-known BAILEY BRIDGES were standardized steel-latticed trusses, designed to make quick replacements for bridges destroyed in wartime.
