How San Francisco's Massive New Suspension Bridge Will Withstand Megaquakes

By Veronique Greenwood | November 16, 2011 3:28 pm

One cable holds the bridge up.

San Francisco has its share of massive earthquakes, but the Bay Bridge, one of the city’s main transit arteries, is not as quake-safe as you’d hope. That’s why, alongside it, the state is building a massive new replacement structure—the largest self-supporting suspension bridge ever built. Jim Giles at New Scientist went to visit the bridge and provides a primer on its engineering:

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. 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…

As the [road] 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.

Read more at New Scientist.

Image courtesy of Bay Bridge Information Office.

  • Jockaira

    Thanks for the pic and the explanation of the principle of this bridge. As a long-time resident of the San Francisco Bay Area, I’ve seen many articles on his subject.

    Yours is the best. With very few words and a single picture, you’ve told essentially the whole story of this bridge and its design. Very Nice!

  • NOYB

    A sure Single Point Of Failure (SPOF).

    Remember the ‘Unsinkable ship’ (Titanic)?

    What a fiasco.

  • Edmund Metheny

    Just a minor correction – the City of San Francisco is not building the bridge, it is a State project.

  • Marc Vader

    For $6 Billion they could probably (and should) build a whole new public transportation system.

  • Cathy

    It’s not a single point, it’s four points that happen to be close to each other. Even if one of the towers breaks lose, the other three will hold it in place.

  • Veronique Greenwood

    @Edmund, whoops, you’re right! Fixed.

  • Brian Too

    Not sure I agree with the negativity on this one.

    What do we always say we hate about government? Inaction, gridlock, inefficiency. Yet here is a real response to an identified problem. The government isn’t waiting until after the Big One, they have acted in advance of that.

    OK, so it’s expensive. When has building heavily reinforced, earthquake resistant structures been cheap? Would you bet your life on a cheap earthquake resistant bridge? Sometimes you get what you pay for.


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