How Do Cranes Get on Top of Skyscrapers? How Do They Get Down?

Answers to your questions about the news.
May 22 2012 7:13 PM

Raising Crane

How do cranes get to the top of skyscrapers?

One World Trade Center.
The rising steel frame of One World Trade Center has just become the tallest building in New York City.

Photograph by Don Emmert/Getty Images.

Perched above New York City’s skyline, two tower cranes are piecing together the new One World Trade Center, which just became the city’s tallest skyscraper. How do those cranes get up there?

Forrest Wickman Forrest Wickman

Forrest Wickman is a Slate staff writer. 

By using their own hoisting power to make themselves taller. While that’s the most common method, there are actually three ways: 1) the external climbing method, in which the crane—the arm plus its tower—expands upward along the outside of the building, 2) the internal climbing method, in which the crane builds a few floors at a time from the inside and then “jumps” to a higher spot, and 3) the “skycrane” method of airlifting in a crane on a helicopter. (Engineers at 1 WTC are using both of the first two methods.)

In the external climbing method, the base of the crane is fixed in a concrete slab in the ground, and the crane tower is erected beside the building using smaller, mobile cranes. Once the building is about 180 feet tall (some 15 stories), the crane is fastened to the building with steel collars, and new segments are inserted into the crane’s backbone. To add segments to itself, the crane has a special climbing section—a big metal sheath that scales the outside of the crane tower. This sheath raises the arm above the last vertebra and temporarily supports it, then takes in a new tower segment from the arm. The sheath holds that segment in place while workers bolt it in. They repeat this process every 180 feet or so: The crane constructs the building, then it’s fastened to the building, then it receives new vertebrae to grow taller.

The second way is the internal climbing method. The crane stands inside the center of the building, in a kind of makeshift courtyard, where it constructs the skyscraper around itself about a hundred feet at a time. A hydraulic cylinder at the crane’s base elevates it through the hollow middle of the building to a higher floor. Then workers slide steel beams underneath the crane to give it a sturdy new footing, and the crane begins building again.

The third method is to have a heavy-lift helicopter (or “skycrane”) fly the crane to the top of the construction site. This must be done piece by piece—just a single segment of a crane’s tower can weigh between 3,000 and 20,000 pounds. However, because of the cost, and because flying a load-bearing helicopter over a populated area is logistically very difficult, this method is the most rare, used only a few times a year nationwide.

Bonus Explainer: How do cranes get back down? Often by constructing the very gallows that will destroy them. To disassemble themselves, tower cranes construct derricks on the rooftop of the finished project. (Derricks are tower cranes’ simpler great-granddaddy.) These derricks then help dismantle the tower cranes, and—in the case of internal climbers—lower their parts one by one to the ground using extremely long cables. Once the parts reach the ground, they’re taken back to the rental service on flatbed trucks, then the workers take apart the derricks. Most of the derricks’ parts can just take the construction elevator. (External climbers, however, can jack themselves all the way back down.)

Got a question about today’s news? Ask the Explainer.

Explainer thanks Peter Juhren of the National Commission for the Certification of Crane Operators and Morrow Equipment Co.

Video Explainer: Why Don't Women Sing in a Falsetto?

This video was produced from an original Explainer by Will  Oremus. Want more questions answered? You can now watch video Explainers at Slate's News Channel on YouTube.

TODAY IN SLATE

Politics

Talking White

Black people’s disdain for “proper English” and academic achievement is a myth.

Hong Kong’s Protesters Are Ridiculously Polite. That’s What Scares Beijing So Much.

The One Fact About Ebola That Should Calm You: It Spreads Slowly

Operation Backbone

How White Boy Rick, a legendary Detroit cocaine dealer, helped the FBI uncover brazen police corruption.

A Jaw-Dropping Political Ad Aimed at Young Women, Apparently

The XX Factor
Oct. 1 2014 4:05 PM Today in GOP Outreach to Women: You Broads Like Wedding Dresses, Right?
Music

How Even an Old Hipster Can Age Gracefully

On their new albums, Leonard Cohen, Robert Plant, and Loudon Wainwright III show three ways.

How Tattoo Parlors Became the Barber Shops of Hipster Neighborhoods

This Gargantuan Wind Farm in Wyoming Would Be the Hoover Dam of the 21st Century

Moneybox
Oct. 1 2014 8:34 AM This Gargantuan Wind Farm in Wyoming Would Be the Hoover Dam of the 21st Century To undertake a massively ambitious energy project, you don’t need the government anymore.
  News & Politics
Politics
Oct. 1 2014 7:26 PM Talking White Black people’s disdain for “proper English” and academic achievement is a myth.
  Business
Moneybox
Oct. 2 2014 8:07 AM The Dark Side of Techtopia
  Life
Quora
Oct. 2 2014 8:27 AM How Do Teachers Kill the Joy of Reading for Students?
  Double X
The XX Factor
Oct. 1 2014 5:11 PM Celebrity Feminist Identification Has Reached Peak Meaninglessness
  Slate Plus
Behind the Scenes
Oct. 1 2014 3:24 PM Revelry (and Business) at Mohonk Photos and highlights from Slate’s annual retreat.
  Arts
Brow Beat
Oct. 1 2014 9:39 PM Tom Cruise Dies Over and Over Again in This Edge of Tomorrow Supercut
  Technology
Future Tense
Oct. 1 2014 6:59 PM EU’s Next Digital Commissioner Thinks Keeping Nude Celeb Photos in the Cloud Is “Stupid”
  Health & Science
Bad Astronomy
Oct. 2 2014 7:30 AM What Put the Man in the Moon in the Moon?
  Sports
Sports Nut
Oct. 1 2014 5:19 PM Bunt-a-Palooza! How bad was the Kansas City Royals’ bunt-all-the-time strategy in the American League wild-card game?