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from the how-long-will-it-take-to-dry? dept.
WHEN FINISHED, SALESFORCE Tower will be the tallest building in San Francisco. For now, it's a big hole in the ground. And at the bottom of that hole is a new, massive concrete slab—14 feet thick, spread nearly an acre in breadth, and ready to support 1,070 feet of glass, steel ... and a lot more concrete.
Pouring it all took more than 18 hours on a cloudy San Francisco Sunday. An armada of trucks delivered nearly 49 million pounds of concrete and brontosaurine pumps vomited it into the hole while a small army of rubber-booted workers scurried about, directing the flow. It was one of the biggest, longest concrete pours in history.
And it's all to keep the building upright. "Skyscrapers are basically big sticks coming out of the ground, so obviously one concern is the whole thing toppling over," says Leonard Joseph, structural engineer at Thornton Tomasetti, a firm in Los Angeles. High wind or quaking earth can make buildings bend and wriggle, and if the wriggling takes the upper mass too far off center, the bottom of the building will begin to lift. This is called hinging, and it is very, very, bad: Buildings that hinge tend to collapse.
Holding back the hinge means attaching the structure to something big, solid, and subterranean. In places like Manhattan, developers can drill down and affix buildings directly to the island's shallow bedrock. But San Francisco's bedrock is below 300 feet of mud and clay, which is why the engineers for Salesforce Tower had to build a big, shallow, fake rock using concrete and metal.
As an aside, the longest continuous concrete pour took 49 hours.
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(Score: 3, Interesting) by Runaway1956 on Sunday November 15 2015, @05:13AM
I've participated in an 18 hour pour. It makes for a long, long, LONG day. But, we only used two pump trucks, and the concrete delivery didn't stream in steadily, either. The floating slab that supports the biggest paper machine in the world (in 1990 - they may have built a bigger one since then) is smaller than this foundation by a couple orders of magnitude. Still, that was a memorable day. It's the only time the employer bought two meals in the same day, and brought them out into the field for us.
Concrete work is the hardest work I've ever done, without competition.