Big challenges demand innovative solutions. Using new designs and techniques, builders worldwide are pushing the limits of the possible.
New Bay Bridge (see image)
For 15 seconds on 17 October 1989, a magnitude 7,1 earthquake shook northern California. The quake shifted the east span of the San Francisco-Oakland Bay Bridge by 18 cm on its Oakland side, causing a 15 m, 227-ton section of the bridge’s upper deck to collapse, killing one motorist.
The structural failure was a disturbing and highly visible sign of the vulnerability of one of San Francisco’s major transportation links. The collapsed section was repaired and re-opened a month later, but engineers knew that simply returning the bridge to its pre-earthquake state wouldn’t be enough. They needed to come up with a solution that could withstand some of the worst that California’s fault zones are capable of dishing out.
In fact, the new bridge’s designers faced two challenges: earthquakes and the demanding Bay Area public. In engineering terms, the simplest solution would have been to build a more robust version of the existing structure, a series of low trusses supported by piers that extends 3,5 km between Yerba Buena Island and Oakland.
But the public wanted a high, visually striking signature span to grace the bay. “The Bay Area is known for its spectacular bridges,” says Bart Ney, a spokesman for the California Department of Transportation (Caltrans). “It’s part of our DNA, so naturally the aesthetics are a key part of the project.”
Caltrans ultimately decided to create a two-stage bridge, marrying a 2 km Skyway to the first ever single-tower Self-Anchored Suspension (SAS) bridge. This revolutionary new structure hangs 567 m of roadway from a single central tower, with the shorter western side rising from Yerba Buena Island, and the longer eastern side extending to meet with the Skyway.
Most of us think of a bridge as a stationary, immobile object. But a bridge in northern California has to be designed like a machine that moves. The US Geological Survey estimates there is a 62 per cent chance that a magnitude 6,7 or larger quake will hit the area by 2032. The Bay Bridge is flanked on the west by the San Andreas Fault and on the east by the Hayward Fault – putting it right in the strike zone.
Since the new bridge’s design specifications require that it last for 150 years, the engineers had to build in state-of-the-art seismic defences (see “How to quakeproof a bridge” on p 4). The SAS tower, for instance, incorporates deformable structural elements to absorb quake forces, much as a car’s crumple zone takes the brunt of a head-on collision. Thanks to this innovation, the structure should be able to accommodate seismically induced movement of up to 90 cm.
The conflicting demands of aesthetics and physics required engineers to come up with some clever solutions. Officially, the Bay Bridge is designated as a “lifeline” structure, meaning it needs to be able to serve emergency vehicles immediately after even the most powerful predicted earthquake.
“The community representatives were very excited about a single tower,” says Brian Maroney, who oversaw the project’s engineering design for Caltrans. “But if you think about the size of such a structure, I didn’t see how we could realistically repair it in the event of a quake.”
To dissipate quake forces, Maroney’s team designed the main SAS tower as four columns that function as one support system, but can move independently. And all of the bridge’s shock-absorbing elements can be replaced within a day of quake damage to get the bridge up and running. But perhaps the most difficult engineering challenge involved the suspension cable system. Normally, suspension bridges require multiple towers, from which the roadway is hung like a hammock. The new Bay Bridge works more like a sling, suspending its twin roadways on a single tower. One long cable loops under the eastern end of the SAS span, then crosses over itself at the top of the tower and is fastened at the western end.
The project’s design, approval and construction has been delayed by lengthy political bickering, which helped boost the estimated cost from R9 billion to over R41 billion. But when it opens to traffic in six years, the Bay Area will have a bridge worthy of the region’s engineering legacy – an icon of both beauty and strength.
How to quake-proof a bridge
1 Shear link beams (see image): Steel beams connecting the four vertical elements of the central tower are designed to shear under excess load, absorbing damage.
2 Hinge pipe beams: Twenty 18 m-long tubes connect sections of the roadway. The soft steel centre of the tube functions like a replaceable fuse, crumpling during quakes.
3 Piles: The Skyway portion of the bridge sits atop bay mud, which can liquefy during a quake. Engineers drove 160 angled (or “battered”) piles 91 m down into the goo for a more solid footing.