Kajima Demolition Tech
In congested cities like Tokyo, there's barely room to swing a wrecking ball, and neighbors hate the caustic dust that implosions kick up. So the Japanese construction company Kajima developed a tidier technique, which it first used this past spring to take down a 17-story and a 20-story office tower: Knock out the ground floor, lower the building on computer-controlled hydraulic jacks, and repeat. Keeping deconstruction on the ground is safer for workers, and the orderly disassembly makes it easier to contain asbestos and other toxic materials. Kajima recycled 99 percent of the steel and concrete and 92 percent of the interior materials—55 percent is standard—and cut demolition time by a fifth. kajima.co.jp
Awesome! Except now we'll see a lot fewer of those fun implosion videos... Oh well.
So how do they remove the floor of the 2nd floor the extricate the jacks to hold up the 3rd floor. Skyhook? We're only getting part of the story here.
I would guess that the hydraulic lifts are punched through the floor above.. So when the second floor is removed they lower the building one floor and then punch a second set of hydraulics to the the "new" third floor and repeat : )
How do they remove the floors? Simple: floors are not structural load-bearing components of most conventional mid- and high-rises and the building weight doesn't really bear on them. Also, most of the outer walls are generally "curtain" (i.e., also not integral to structure, except in cases like the NY World Trade Centers "exoskeletal" design) so they could be pulled off each floor as it hits ground level. For a typical "pan-and-pour" floor, once you bust up the concrete, the steel pan is modular and could be cut out and removed through the open walls. I'd guess they strip out the floor of each level before they lower it.
Kajima's process is very cool, the reverse of the "build and jack" process used for some structures.