Roman arch bridges
The Romans began organized bridge building to help their military campaigns. Engineers and skilled workmen formed guilds that were dispatched throughout the empire, and these guilds spread and exchanged building ideas and principles. The Romans also discovered a natural cement, called pozzolana, which they used for piers in rivers.
Roman bridges are famous for using the circular arch form, which allowed for spans much longer than stone beams and for bridges of more permanence than wood. Where several arches were necessary for longer bridges, the building of strong piers was critical. This was a problem when the piers could not be built on rock, as in a wide river with a soft bed. To solve this dilemma, the Romans developed the cofferdam, a temporary enclosure made from wooden piles driven into the riverbed to make a sheath, which was often sealed with clay. Concrete was then poured into the water within the ring of piles. Although most surviving Roman bridges were built on rock, the Sant’Angelo Bridge in Rome stands on cofferdam foundations built in the Tiber River more than 1,800 years ago.
The Romans built many wooden bridges, but none has survived, and their reputation rests on their masonry bridges. One beautiful example is the bridge over the Tagus River at Alcántara, Spain. The arches, each spanning 29 metres (98 feet), feature huge arch stones (voussoirs) weighing up to eight tons each. Typical of the best stone bridges, the voussoirs at Alcántara were so accurately shaped that no mortar was needed in the joints. This bridge has remained standing for nearly 2,000 years.
Another surviving monument is the Pont du Gard aqueduct near Nîmes in southern France, completed in 14 ce. This structure, almost 270 metres (900 feet) long, has three tiers of semicircular arches, with the top tier rising more than 45 metres (150 feet) above the river. The bottom piers form diamond-shaped points, called cutwaters, which offer less resistance to the flow of water.
Asian cantilever and arch bridges
Wooden cantilever bridges were popular in Asia. The basic design used piles driven into the riverbed and old boats filled with stones sunk between them to make cofferdam-like foundations. When the highest of the stone-filled boats reached above the low-water level, layers of logs were crisscrossed in such a way that, as they rose in height, they jutted farther out toward the adjacent piers. At the top the Y-shaped cantilevering piers were joined by long tree trunks. By crisscrossing the logs, the builders allowed water to pass through the piers, offering less resistance to floods than with a solid design. In this respect, these designs presaged some of the advantages of the early iron bridges.
In parts of China many bridges had to stand in the spongy silt of river valleys. As these bridges were subject to an unpredictable assortment of tension and compression, the Chinese created a flexible masonry-arch bridge. Using thin, curved slabs of stone, the bridges yielded to considerable deformation before failure.
In the Great Stone Bridge (also called the Zhaozhou Bridge) in southern Hebei province, China, built by Li Chun between 589 and 618 ce, the single span of 37 metres (123 feet) has a rise of only 7 metres (23 feet) from the abutments to the crown. This rise-to-span ratio of 1:5, much lower than the 1:2 ratio found in semicircular arches, produced a large thrust against the abutments. To reduce the weight, the builders made the spandrels (walls between the supporting vault and deck) open. The Great Stone Bridge thus employed a form rarely seen in Europe prior to the mid-18th century, and it anticipated the reinforced-concrete designs of Robert Maillart in the 20th century.
The Middle Ages
After the fall of the Roman Empire, progress in European bridge building slowed considerably until the Renaissance. Fine bridges sporadically appeared, however. Medieval bridges are particularly noted for the ogival, or pointed arch. With the pointed arch the tendency to sag at the crown is less dangerous, and there is less horizontal thrust at the abutments. Medieval bridges served many purposes. Chapels and shops were commonly built on them, and many were fortified with towers and ramparts. Some featured a drawbridge, a medieval innovation. The most famous bridge of that age was Old London Bridge, begun in the late 12th century under the direction of a priest, Peter of Colechurch, and completed in 1209, four years after his death. London Bridge was designed to have 19 pointed arches, each with a 7.2-metre (24-foot) span and resting on piers 6 metres (20 feet) wide. There were obstructions encountered in building the cofferdams, however, so that the arch spans eventually varied from 4.5 to 10.2 metres (15 to 34 feet). The uneven quality of construction resulted in a frequent need for repair, but the bridge held a large jumble of houses and shops and survived more than 600 years before being replaced.
A more elegant bridge of the period was the Saint-Bénézet Bridge at Avignon, France. Begun in 1177, part of it still stands today.
Another medieval bridge of note is Monnow Bridge in Wales, which featured three separate ribs of stone under the arches. Rib construction reduced the quantity of material needed for the rest of the arch and lightened the load on the foundations.
The Renaissance and after
Stone arch bridges
During the Renaissance the Italian architect Andrea Palladio took the principle of the truss, which previously had been used for roof supports, and designed several successful wooden bridges with spans up to 30 metres (100 feet). Longer bridges, however, were still made of stone.
Another Italian designer, Bartolommeo Ammannati, adapted the medieval ogival arch by concealing the angle at the crown and by starting the curves of the arches vertically in their springings from the piers. This elliptical shape of arch, in which the rise-to-span ratio was as low as 1:7, became known as basket-handled and has been adopted widely since. Ammannati’s elegant Santa Trinità Bridge (1569) in Florence, with two elliptical arches, carried pedestrians and later automobiles until it was destroyed during World War II; it was afterward rebuilt with many of the original materials recovered from the riverbed.
Yet another Italian, Antonio da Ponte, designed the Rialto Bridge (1591) in Venice, an ornate arch made of two segments with a span of 27 metres (89 feet) and a rise of 6 metres (21 feet). Antonio overcame the problem of soft, wet soil by having 6,000 timber piles driven straight down under each of the two abutments, upon which the masonry was placed in such a way that the bed joints of the stones were perpendicular to the line of thrust of the arch. This innovation of angling stone or concrete to the line of thrust has been continued into the present.
By the middle of the 18th century, bridge building in masonry reached its zenith. Jean-Rodolphe Perronet, builder of some of the finest bridges of his day, developed very flat arches supported on slender piers. His works included the Pont de Neuilly (1774), over the Seine, the Pont Sainte-Maxence (1785), over the Oise, and the beautiful Pont de la Concorde (1791), also over the Seine. In Great Britain, William Edwards built what many people consider the most beautiful arch bridge in the British Isles—the Pontypridd Bridge (1750), over the Taff in Wales, with a lofty span of 42 metres (140 feet). In London the young Swiss engineer Charles Labelye, entrusted with the building of the first bridge at Westminster, evolved a novel and ingenious method of sinking the foundations, employing huge timber caissons that were filled with masonry after they had been floated into position for each pier. The 12 semicircular arches of portland stone, rising in a graceful camber over the river, set a high standard of engineering and architectural achievement for the next generation and stood for a hundred years.
Also in London, John Rennie, engaged by private enterprise in 1811, built the first Waterloo Bridge, whose level-topped masonry arches were described by the Italian sculptor Antonio Canova as “the noblest bridge in the world.” It was replaced by a modern bridge in 1937–45. Rennie subsequently designed the New London Bridge of multiple masonry arches. Completed in 1831, after Rennie’s death, it was subsequently widened and was finally replaced in the 1960s.
