Mind the gap;The Big Picture

24th September 1999 at 01:00
(Photograph) - These precast concrete segments formed the base of the Vasco da Gama Bridge across the Tagus estuary in Portugal, which opened last year. Each section, weighing 250 tons, was cast 15 miles upstream from the site, transported by barge and lifted into place.

At 11 miles, it is Europe's longest bridge. When it was opened a lunch for 15,000 was held on a three-mile table on the bridge, during which guests ate seven tonnes of feijoada, a Portuguese meat and vegetable stew. It took 3,300 people to build it and during the construction, Anthony Freeman, an eminent British structural engineer, and five others were killed in an accident.

Bridges have always been incredible feats of engineering. The Romans constructed viaducts and aqueducts so strong that many of them are still standing. Their work included a granite bridge over another point of the Tagus at Alcantara in Spain.

Nowadays, as skyscrapers get taller, bridges are getting longer, pushing back the boundaries of what is buildable. World records include the Humber estuary bridge (longest span of any suspension bridge, 4,626 ft) and the Quebec rail bridge in Canada (longest cantilever span, 1,800 ft) Bridge building has come a long way since early human beings laid tree trunks between stepping stones or slung ropes across river gorges. Stone was the preferred material of classical antiquity but iron and steel belong to the railway age. Thomas Telford built the first significant metal bridge, of cast iron, at Ironbridge in 1779. New material meant new methods of construction. The suspension and cantilever bridge - where a a supporting pier holds up spans on either side as in the Forth rail bridge - exploit the potential of steel, while prestressed concrete, as over the Tagus, has been developed this century.

But, whatever elegance or engineering prowess it displays, a bridge was never built to adorn a pretty valley or on an emperor's whim. First and foremost, a bridge is functional. And therein lies its real beauty.

Harvey McGavin

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