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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: 0) by Anonymous Coward on Sunday November 15 2015, @04:57AM
Cars with independent rear suspension have good road grip and tend not to turn-over.
Cars with solid rear suspension have lesser road grip and do not corner well.
Buildings seem to follow solid suspension principles and keeping the entire thing solid and relying on the foundation concrete slab to not break into pieces, ever.
I would suggest developing the building's foundation to work like a good car's suspension and not topple over.
*I am not an engineer, but I can suggest things.
(Score: 1) by Francis on Sunday November 15 2015, @01:35PM
The purpose of the foundation is mostly to be extremely heavy and to distribute the weight of the building evenly on the ground. It gives you something to anchor on to so as to keep the center of mass as low as possible where it's not going to be moving very much. The higher you go in the building the more flex you'll get and the more likely the building would be to topple in an earthquake or strong winds.