Sunday, August 10, 2008
Located at the mouth of the Douro River, Oporto is the capital of northern Portugal and the second-largest city in the country, rising steeply from the deep river valley. In 1875 the railway between Lisbon and Oporto was almost complete, and the final problem facing its builders was crossing the Douro. An international competition attracted only four entries, three from France and one from England. Gustave Alexandre Eiffel’s winning proposal for the “transparent” Maria-Pia Bridge was not only the least expensive—two-thirds that of the next tender and only one-third of the highest price—but it also involved revolutionary structural design.
Although Eiffel is best remembered for the Eiffel Tower in Paris, much of his professional life was given to building bridges. Upon his graduation from the École Centrale des Arts et Manufactures in 1855, he was employed by a firm in southwestern France that produced steam engines and railroad equipment. In 1858 it won a contract to erect a railway bridge over the Garonne River near Bordeaux; Eiffel oversaw the construction, which was completed in 1865. The following year he set up business as a “constructor,” designing and fabricating metal structural work, especially in wrought iron. After 1872 foreign contracts came his way, and three years later he designed the Maria-Pia railway bridge in Oporto.
Eiffel supported the railroad deck 190 feet (57 meters) above the river with a graceful, filigreed wrought-iron arch spanning 525 feet (160 meters); the approaches to the center span were borne on lacy framed pylons of varying heights to accommodate the sloping banks. Construction started in 1877, and 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.