How skyscrapers are built
A tall building goes up in a fixed order, set by what carries what. The ground is tested first, the core rises before the floors, the glass follows the frame from below.
The sequence, empty lot to tower
Three to six years pass between the first drilling rig and the last tenant. The Empire State Building took one year and 45 days in 1931. The order never changes: test the ground, secure the hole, build the foundation, raise the core, hang the frame, pour the floors, close the skin, fit out from the bottom while the top is still bare steel. The stages overlap, so a half-built tower has glass on floor 20 and concrete on floor 40.
Borings and the foundation
A geotechnical crew drills borings to find how deep competent rock sits. That answer picks the foundation, the one part nobody can fix later. Where bedrock is shallow, as in Manhattan, columns bear on it. Where it is deep, piles carry the load down: steel or concrete shafts driven to rock or held by soil friction, with caissons the same idea at larger diameter. Over them a raft spreads the weight across the piles. Burj Khalifa rests on 192 bored piles 1.5 m across.
The core goes up first
The concrete core holds the elevators and stairs and resists most of the wind, so it is built ten or more floors ahead of the highest slab, climbing inside its own formwork. In jump form, steel panels clamp to the hardened concrete below, a lift of wall is poured, and once it cures the panels are jacked up a level, adding a floor every three or four days. Slip form is continuous, creeping up on jacks while concrete leaves the bottom already set. On Burj Khalifa the pump reached 606 m, working at night because the heat set the mix.
The frame and the floors
Steel arrives as columns and beams cut in a shop, bolted on site and welded where the design calls for it. Concrete columns damp sway better, so most supertall towers use both: a concrete core with composite columns. For the floors, a crane drops bundles of corrugated steel decking onto the beams, mesh goes on top, and a pump covers the bay with concrete. The decking is the formwork and stays in place.
Closing the skin
The outer wall carries nothing but itself and the wind, the point of the metal frame William Le Baron Jenney introduced in Chicago in 1885. That makes it a curtain wall: prefabricated units of glass and aluminum, each one floor tall, clipped onto anchors at the slab edge. Burj Khalifa has 26,000. Glazing follows the structure up from the bottom, so interior trades work dry while ironworkers are thirty floors higher.
The cranes, and how they come down
A tower crane stands beside the building or in a shaft in the core, and it raises itself: a climbing frame slides up the mast, lifts the crane's top, and a new mast section is pinned into the gap. Coming down is the part people wonder about. The crane lowers its own mast sections out, a derrick on the roof dismantles it, and a smaller one dismantles that, until the last fits in a service elevator.
Wind is the governing load
Above forty floors, wind matters more than weight. A tower moves, and people feel acceleration long before anything is in danger, so the design is tested on a scale model in a wind tunnel. Tapering and a twist break up the vortex shedding that makes a plain prism sway in step with itself, and what the shape cannot absorb a damper takes: Taipei 101 hangs a 660 tonne steel ball on cables.
Miami: limestone and groundwater
South Florida sits on porous oolitic limestone with groundwater a few feet down, so the hard part in Miami is water: an excavation fills from the rock. Crews mix cement into overlapping columns around the perimeter and base to form a plug, then dewater inside it. Auger cast piles drilled through the plug carry the tower down and hold the base slab against the water pressure below.
Dubai: building on sand
Dubai has no usable bedrock near the surface, only weak cemented sandstone under the sand, and salty groundwater that attacks reinforcement. Friction along long bored piles carries the load. Above ground the answer is the buttressed core: a hexagonal concrete core with three wings pressing against it, each propping the other two. It uses about half the steel the Empire State Building needed, for twice the height.
Japan: designed to move
Japanese towers assume a magnitude 9 earthquake. Base isolation puts the building on laminated rubber bearings with lead cores, or sliding bearings with oil dampers, so the ground moves under it while the structure moves much less. In the tallest buildings the frame absorbs energy too: Abeno Harukas in Osaka carries 516 oil dampers that turn shaking into heat. A Japanese skyscraper is deliberately flexible, and the engineering controls how it bends.
Where this fits
All of it rests on one idea from 1880s Chicago: let a metal frame carry the building and the wall becomes a skin. Louis Sullivan worked out what that meant for a tall building's look, and the twelve buildings run from Jenney's 12 stories to today's record holder.