people. In order to achieve vast interior spaces, the Romans exploited the semicircular arch, a technology inherited from their Etruscan forebears. The arch and its three-dimensional extensions, the vault (a prismatically extended arch) and the dome (a rotated arch), could span large distances without intermediate supports. With characteristic directness, Roman engineers found expedient solutions. A small rectangular room could be covered by a semicircular barrel vault carried on continuous parallel side walls. A square room could be roofed by a cross vault (two barrel vaults placed at right angles), supported by piers at the corners and allowing the space to be lit from all sides. Larger rectangular spaces could be enclosed by a procession of such vaults built side by side. A polygonal space received a hemispherical dome and an apse a half-dome, carried on drums above the base walls. By using concrete for these roof structures, the Romans enclosed volumes that would not be equaled for over 1,000 years.

The simplest barrel vaults consisted of a series of parallel brick arches cross-tied as in opus testaceum and filled between with concrete; that is, the concrete was packed into brick compartments. The whole structure was supported by wooden centering until the mortar had set. Other vaults and domes were directly formed in mass concrete. The technique had two main advantages: once the centering was designed and placed, it employed unskilled labor, and it enabled complex plan forms to be roofed without the cost of dressed stone construction. Often, the weight was reduced by using hollow clay boxes or even wine jars, especially in the groins of cross vaults; alternatively, vaults were lightened by forming recesses or coffers in their undersides. Domes were generally much thicker toward their base and therefore appeared externally as inverted saucers, while inside they were hemispherical. As they rose, lighter materials, such as pumice—a stone that floats—were used for aggregate.

In order to underline the achievement of the Roman engineers and architects, it is helpful to consider the size of some concrete structures; a couple of examples may suffice. The concrete barrel vaults that spanned the 76-foot-wide (23-meter) side aisles of the fourth-century-a.d. Basilica of Maxentius in Rome were 8 feet (2.45 meters) thick. The 142-foot (43-meter) concrete dome of the Pantheon, also in Rome, is 4 feet thick (1.3 meters) at its apex and 20 feet at its base. The massive loads of these roofs were carried to the ground through huge piers or thick walls, and their horizontal thrusts resisted by buttressing elements integrated with the architectural design.

See also

Baths of Caracalla; Pantheon; Reinforced concrete

Further reading

Adam, Jean-Pierre. 1994. Roman Building: Materials and Techniques. Bloomington: Indiana University Press.

Hamey, L. A., and J. A. Hamey. 1982. The Roman Engineers. Minneapolis: Lerner.

Sear, Frank. 1989. Roman Architecture. London: Batsford.

Royal Albert Bridge

Saltash, England

The Royal Albert Bridge at Saltash, completed in 1859, was Isambard Kingdom Brunel’s last bridge and probably his finest work. Certainly, it was one of the great engineering feats of the nineteenth century, because (it is widely agreed) of its size, its economy of design, its revolutionary superstructure, and not least because of the way in which Brunel solved difficult logistical problems. It was one of the first bridge projects on which compressed air was used to allow underwater foundation work to proceed.

Dividing Cornwall from the rest of England, the tidal reaches of the River Tamar were once a major maritime thoroughfare. The twelfth-century port of Saltash lies on the west shore of the Tamar Estuary near the English Channel coast, nearly facing Plymouth on the opposite side. A railroad into Cornwall, the county in the extreme southwest of England, was first proposed in 1844. The Cornwall Railway Company was formed in 1845, and it successfully applied for the necessary act of Parliament to provide either a steam ferry to transport trains across the 1,100-foot-wide (336-meter) 85-foot-deep (26-meter) river or to build a bridge. The project was delayed because the Admiralty was concerned about restricted access to the Devonport naval base, close to Saltash. Finally, in 1852 Brunel’s proposal for a bridge with two main