San Francisco's huge new quake-proof bridge

The new bridge across San Francisco bay, the largest of its kind ever built, must be able to sway to withstand a big earthquake – and one is expected soon

AS I drive out onto San Francisco bay's new bridge, an empty five-lane freeway curves towards me. In the distance I catch sight of the structure's architectural signature: a single suspension tower, 160 metres high. When finished, cables will arc down from the tower's perch and hold the roadway aloft. At almost 400 metres long it will be the largest self-anchored suspension bridge ever built - a beautiful structure worthy of comparison to the nearby Golden Gate Bridge.

Ahead of us a crane as large as a skyscraper lowers a 1000-tonne slab of road and prepares to fit it into the final slot. The segment will complete the bridge's freeway, which by late 2013 will carry 280,000 cars across the bay every day.

This moment is a key one in an extraordinary $6-billion engineering challenge. But, like others before it, the occasion has an air of caution. To one side, traffic streams towards Oakland across the existing Bay Bridge. And a major earthquake of magnitude 6.7 or greater, expected to hit the area within the next 30 years, would likely topple sections of the old bridge. It is a race against the forces of nature, and everyone involved in the project hopes that the new bridge will be completed before the "big one" hits. "The only milestone that matters is when we can move traffic from that bridge to this one," says Steve Heminger, executive director of the Metropolitan Transportation Commission, the region's transport planning body. "Recent seismic activity reminds us of that," he adds, referring to a low-magnitude quake that shook houses on both sides of the bay just a week before.

The final segment is one of 28 that will be supported by the single suspension tower and an unusual cabling system. In a regular suspension bridge, the cables that support the roadway are hung between two or more towers, like a hammock between trees, and anchored at each end by a connection to land. The new bridge is more like a sling. A single cable loops under the roadway, over the tower and beneath the roadway on the other side of the tower (see diagram). The enormous forces placed on the cable by the road cancel out, leaving a structure that is balanced but not directly supported by a land anchor.

Despite the urgency, a project of this size can't be rushed. On our way out from Oakland we drove over the 2-kilometre "skyway" section of the bridge, 452 concrete segments that sit atop towers which have foundations drilled into the bay's bedrock. Even this relatively conventional design has given the engineers headaches. The skyway and the suspension part of the bridge may move apart by more than a metre during a quake. But the bridge is a designated "lifeline" structure, meaning that it must remain open to emergency vehicles after a quake.

A machine that simulates a large truck is used to conduct stress tests on the rubber joints between the bridge segements. "The rubber will inevitably fail but observing the process will help us design a schedule for monitoring and replacing the joints," says engineer Mike Whiteside.

As the segment fell into place it revealed the full length of tower that stands behind it, an elegant structure made up of four concrete pillars. These drop into enormous steel foundations, parts of which were built in Texas and shipped to California via the Panama canal. The pillars are connected by "shear beams" - relatively weak steel components that are designed to break if the towers move. The two roadways, one each for east and westbound traffic, hang from the cables but are not attached directly to the tower. This arrangement means that the four pillars and two roadways will sway when a quake hits, but remain intact even through the strongest shaking that geologists expect the region to experience over the next 1500 years.

That swaying will be picked up by motion detection sensors, which send information about the bridge's movements via radio. The data will be relayed to a control centre in Sacramento, the Californian state capital that lies 2 hours' drive inland from the bay. Officials there will study the acceleration that different parts of the bridge experience and plan repairs accordingly.

The existing Bay Bridge would not fare so well. As we turned around and headed back, the photographer drew my attention to a small section of the bridge that collapsed on 17 October 1989, killing a motorist, when the San Andreas fault triggered a 6.9-magnitude quake. The new bridge will take two years to complete. Hopefully the next big quake won't come sooner.