the bridge was built in just a year and ten months, without the need for temporary scaffolding directly supported on the ground—a masterly piece of design. The structural system involved several other technological innovations, not least the design analysis methods. Civil engineers already knew how to calculate for statically indeterminate beams, but the force method needed to predict the behavior of this kind of structure, although propounded a decade earlier, had been taken seriously only a year before Eiffel designed the bridge. It has been asserted that this was the first application of the analysis of a statically indeterminate structure other than a beam, and that Eiffel discovered the method by himself.

The pioneer technique was to be used in many large arches, including two in Oporto. The first came soon after: the wrought-iron Dom Luís I Bridge for pedestrian and vehicular traffic (1886), designed by the French engineer Téophile Seyrig. It is noteworthy that it weighed almost twice as much as the 1,800-ton (1,630-tonne) Maria-Pia. The second arch in Oporto was built almost eighty years later: the 900-foot-span (270-meter) reinforced concrete Arrábida Bridge (1963) was designed by the Portuguese Edgar Cardoso. And Eiffel himself reused the design in France: in 1880 Leon Boyer of the Ponts et Chaussées (Bridges and Highways Department), who was aware of the success of the Maria-Pia Bridge, invited him to build a bridge across the La Truyère River near Garabit on the railroad between Marvejols and Neussargues. The 550-foot (165-meter) span Garabit Viaduct, completed in 1884, incorporated all the innovations of the revolutionary Portuguese structure: it comprised a 1,500-foot-long (450-meter) wrought-iron truss girder, carried to the arch on variable height piers and extended by brick approach viaducts to a total length of 1,880 feet (564 meters).

In 1996 UNESCO designated Oporto a World Heritage City. The Maria-Pia Bridge, threatened with demolition after it was replaced by a new rail crossing in 1991, is now safe and awaiting a new use appropriate to its significant place in the history of engineering.

See also

Eiffel Tower

Further reading

Loyrette, Henri. 1985. Gustave Eiffel. New York: Rizzoli.

Marrey, Bernard. 1984. Gustave Eiffel, the Engineer Who Built the Statue of Liberty, the Garabit Viaduct. Paris: Graphite.

Marib Dam

Yemen

The Republic of Yemen is located on the southwestern coast of the Arabian peninsula, the region once possessed by the ancient southern Arabian kingdoms that occupied the mouths of large wadis (valleys) between mountains and desert. The first-millennium-b.c. kingdom of Saba sprang up in the dry delta of the Wadi Dhana that divides the Balak Hills. In the eighth century b.c., at the height of their prosperity, the Sabaeans had established colonies along both sea and land trade routes to Israel, and they dominated the region. Their capital, Marib, among the wealthiest cities of ancient Arabia, stood 107 miles (172 kilometers) east of Sana’a, the capital of modern Yemen. It is generally agreed that artificial irrigation was practiced near ancient Marib as early as the middle of the third millennium b.c. About 2,000 years later a dam was built to harness the biannual floods and systematic irrigation was introduced. Some scholars believe that the Marib Dam was the “greatest technical structure of antiquity.”

Around 685 b.c., under King Karib’il Watar, Saba enlarged its borders. Territories were conquered in the southwest of the peninsula; Ausan in the south was defeated and Sabaean rule extended northwest as far as Nagran. In the second half of the sixth century b.c., two kings successively built the Marib Dam near the mouth of the Wadi Dhana, the largest water course from the Yemeni uplands. By impounding water during the two rainy seasons, the dam provided irrigation for some 25 square miles (65 square kilometers) of fields and gardens. Replenished and enriched by sedimentary deposits, this agricultural land supported a population estimated to be about 30,000.

The first dam was a simple earth structure, 1,900 feet (580 meters) long and probably only 13 feet (4 meters) high, built between rocks on the south side