It Took 18 Hours to Pour San Francisco’s Biggest-Ever Concrete Foundation

When I was in San Francisco for the ULI Fall Meeting I had the pleasure of attending a lecture on and a tour of Salesforce Tower, a joint project by Hines and Boston Properties that will be the tallest building west of Chicago upon completion. It was an incredible presentation and tour and the entire Transbay District will be revolutionary for the city when it is finished.

This weekend, they did the concrete pour for the base of the structure and if you're into real estate beyond just the ROI, the scale is simply incredible.

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.

That took 12,000 cubic yards of concrete—enough to fill nearly four Olympic swimming pools. That concrete began as (roughly) 7,900,000 pounds of cement, 9,000 cubic yards of sand and gravel, and 54,000 gallons of water at several mixing plants around the Bay Area. Over the course of 18 hours—beginning just before midnight on November 7—1,300 mixing trucks ferried the stuff to the site in downtown San Francisco. Then, two at a time, they dumped their contents into one of seven pumping trucks.

Those pumping trucks are awesome, by the way. Their booms reached from 150 to 190 feet long—enough to extend from the edge of the construction site to the center of the pit. “When they unfold they look like a praying mantis,” says Michael Tymoff, senior project manager for Boston Properties, and guy in charge of building Salesforce Tower.

All that concrete did not slop down into an earthy void. In the weeks leading up to the pour, workers constructed a subterranean lattice of rebar—12 layers high, with six inches separating each layer. ”We used 5 million pounds of number 18 rebar, the largest size available,” says Tymoff. At two and a quarter inches in diameter, grabbing a bar of No. 18 is like gripping the fat end of a baseball bat. It took eight iron workers to lift each 45-foot segment into place. That cagework will act as the foundation’s skeleton, but during the pour it also served as a catwalk for workers holding the cement hoses or the massive vibrators used to ensure the concrete had no air pockets.

The hardened slab, in all its mightiness, is but half of the tower’s earthquake protection. It will keep the building from toppling sideways, but what about sliding back and forth? In a big earthquake, the ground is actually trying to slip sideways underneath the building. “You need something to keep you from changing addresses,” says Joseph. Those somethings are called piles, in essence underground stilts connecting the building with the bedrock. In the lowlands of San Francisco’s Financial District, bedrock is 300 below street level. “We have 42 piles that go all the way down and are socketed 15 to 25 foot deep into the rock,” says Tymoff.

Now that the foundations are complete, the only way to go is up. Next come the parking garages, then the lobby, then 61 floors of prime office space. “It will be the tallest building west of Chicago when it’s complete,” says Tymoff.

Wait a second, what about the Wilshire Grand down in Los Angeles? At 1,100 feet, its topping spire is about 30 feet taller than Salesforce Tower’s crown. “Yes, but we have the highest occupiable floor,” says Tymoff. “From a skyscraper geek perspective, I think they’ve got the nod. But then again that’s because they have a fishing pole on top of their building.” Whoever wins, the race to the sky starts deep in the ground.

Source: http://www.wired.com/2015/11/it-took-18-hours-to-pour-san-franciscos-bi…

 
Best Response

Dingdong08 - I got in touch with someone at Boston Properties. They did a quick mix around the crane (5% of the pour) and then purposefully put a retarder in the rest to better control heat in order to maintain the integrity. They even put thermostats in to make sure it doesn't get too hot.

He said it takes 56 days to get to full strength, which is double the normal length of concrete, but that people are able to walk on it the next day. I mentioned the Hoover Dam thing you said and he laughed and asked if I was a civil engineer, so thank you for making me sound informed. He said that concrete curing is an exponential process, and the goal they aim for is 95% (which typically happens in 28 days but they are doing 56). The last 5% gradually cures toward the limit but doesn't really matter.

Commercial Real Estate Developer
 

Good stuff. Thanks. I wonder if they have to wait until they start putting the steel up to use the concrete as the anchor. I've never been involved in building a tall building and especially not in a seismic zone but I'd imagine you wouldn't want 1000' of steel going up with a swampy foundation if an earthquake could knock it over. Wind sheer shouldn't be too bad until you put in windows but a quake could cause some issues. But I'm just bs'ing and I'm nowhere near a structural engineer.

I also thought it was pretty interesting that they drill 300 ft down to bedrock as another anchor. If you like that sort of thing, check out how the church on Copley Plaza in Boston in built. The Back Bay is all a swampy landfill (hence why there are few tall buildings there compared to the Financial District, which sits on bedrock scraped away in the last ice age), so when they built Trinity Church in the late 1800's it was so heavy with thick masonry walls they floated it on thousands of wooden pilings that literally float in the muck and water table below it. And as long as the wood isn't exposed to oxygen it doesn't rot.

 

cool article, the salesforce tower is insane. Does anyone know the lease comp? I don't know anything about the SF market but those guys must have a crazy low land basis or the highest rent in the city. Can't imagine their construction costs alone being under $300 psf with union labor, 300 ft cast in place footings, a 14 ft mat slab, 3 level garage and ridiculous quantities of curtainwall in the worst glass market in 15 years

 
Ricky Rosay:

I don't know anything about the SF market but those guys must have a crazy low land basis or the highest rent in the city.

I know that the land for 181 Fremont (another MASSIVE building under construction right down the street) was purchased for $170 Million, but that included all entitlements (a 5 year process in SF. 5 YEARS) and the building design as well

Commercial Real Estate Developer
 

what is that on a PSF basis tho? would be interesting to see what kind of back of the envelope yield on cost they are solving to on the lease comp in terms of land basis + all in construction cost.

one thing I find really interesting about these development deals with long entitlement periods is that the traditional IRR returns are often not that impressive because carrying the land for so long is really punitive. I would imagine this deal may have a levered return under 15% even with a mammoth prelease, but the deal probably generates huge profit on the cost yield alone due to its enormous size.

 
Dick Whitman:

You know there are some geologists expecting a monster earthquake and are questioning every minute of this project.

I'm definitely not a scientist, but the building was marketed to us as the safest building in San Francisco. They are apparently going exponentially deeper into the bedrock than similar buildings.

Commercial Real Estate Developer
 

CRE, just want to thank you for always posting quality stuff. I'm just recently making the transition to CRE from residential (just landed an acquisition analyst role with a development shop) and it's things like this that makes me happy I made the move.

Where do you typically get your CRE news from? I'm always looking for more sources on major trends, large transactions, and sub market activity as well. Thanks in advance!

Also, if you could give a little insight to your experience at the ULI meeting I would appreciate it.

 
undefined:
Where do you typically get your CRE news from?

Man, a little bit from everywhere. This article, for example, is from Wired.com which is a tech website.

A lot of the best stuff I'll find on LinkedIn though. Either a contact of mine will post it or a company I follow (as is what happened in this case) will.

Commercial Real Estate Developer
 

Also, if you haven't looked up the park that will connect Salesforce Tower and 181 Fremont, you really should:

http://transbaycenter.org/project/transit-center/transit-center-level/c… http://transbaycenter.org/uploads/2009/07/TTC-SECTION-1.jpg http://media.except.nl/media/cache/uploaded_images/asset_image/San_Fran… http://assets.inhabitat.com/wp-content/blogs.dir/1/files/2010/04/Transb… http://assets.inhabitat.com/wp-content/blogs.dir/1/files/2010/04/Transb… http://assets.inhabitat.com/files/transbay3.jpg

It will be a 5.5 acre park that's something like 5 stories above ground, a retail center, and a transit terminal that both of the buildings will open up to.

Commercial Real Estate Developer
 

I actually did concrete/heavy construction, this made me want to throw up with all the possibilities for failure and time/effort to orchestrate this efficiently.

"It is better to have a friendship based on business, than a business based on friendship." - Rockefeller. "Live fast, die hard. Leave a good looking body." - Navy SEAL
 

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