Bridging yawns
INCONCEIVABLE is what you may say to a proposal of building a 14.32 km long bridge across the Strait of Gibralter. But a revolution in bridge design technology is now on the anvil which may soon allow us to span distances long believed to be unbridgeable. This new design concept, which incorporates the features of both cable- stayed and suspension bridges, would allow us to erect structures that can link distances more than 10,000 feet across land or water.
Bridge design technology has come a long way since the second World War -by the turn of this century the record will be set by the 6,500-feet long bridge over Japan's Akashi Straits. According to bridge designers and civil engineers, the existing designs and materials can probably help link as far as 10, 000 feet; thereafter, the new technology will be required.
For longer bridges, one foot of steel suspension cable would weigh as much as one foot of the rest of the bridge it would support. Additional support II must be found to carry traffic loads and withstand natural vagaries such as wind, storm, snow and earthquakes. "We do not build longer than 10,000 feet because the bridges will collapse under their own weight," says Frieder Seible, chairman of the division of structural engineering at the University of California, San Diego, US.
For distances that do not exceed 3,000 feet, designers still favour the cable-stayed bridge that came into vogue in the '50s in Germany to rebuild the war-damaged bridges on their original piers. In this design, cables run diagonally from one or more towers to support the deck.
For spans greater than 3000-feet, engineers usually take recourse to suspension bridges where ends are anchored to a rock or concrete, while vertical cables suspend the deck. The anchoring, coupled with the requirement that the fat main cable must be spun in place as the bridge is being built, makes suspension bridges more expensive than their cable-stayed counter- parts. To span distances which exceed 10,000 feet, American designers are now pondering bridges that would in corporate both 'suspension' and 'cable- stayed' features. The revolutionary concept integrates their 'belt and suspender' features where each tower supports sus- pension and stayed cables. A San Fransisco-based bridge designer, T y Linn, has proposed to span the Strait of Gibralter -between Spain and Morocco -with a 14.32 km-bridge that would include two main spans of 4.8 km each. Each of the towers would support both suspension and stayed cable. The estimated cost is us $7 billion.
Seible is presently working on a car- bon fibre-epoxy material that he says is as strong as steel a~~ one-fifth as heavy. He hopes to build a 450-feet cable stayed bridge in San Deigo with carbon tubes filled with concrete. Though the carbon fibre costs as much as ten times that of steel, Seible believes that the high cost would be offset by much shorter construction time. "Carbon is the mate- rial of the future," he says adding that a bridge with carbon fibre for key parts should last much longer than the 100 years estimated for bridges made entirely of concrete and steel.