How skyscraper elevators work
A steel frame lets you build high. The elevator is what makes the height worth anything, because a floor nobody reaches quickly is a floor nobody rents.
What Otis actually demonstrated
Hoists for freight existed long before 1854. What did not exist was a reason to ride one, because a cab on a frayed rope falls the full height of the shaft. Elisha Otis built a spring-loaded ratchet that gripped notched guide rails the moment rope tension went slack. At the New York Crystal Palace exhibition in 1854 he stood on an open platform, had the rope cut with an axe, and dropped a few inches. His invention was a brake, and the brake is the part that mattered.
Before that, a building was about six stories tall because that is how many flights of stairs a tenant would climb, and the top floor was its cheapest address. The first safe passenger elevator went into the E.V. Haughwout store in New York in 1857, and within a generation the rent arithmetic flipped: the high floor had the light and the view, so it became the expensive one. The penthouse exists because of a ratchet and a notched rail.
What is in the shaft
A traction elevator hangs the cab on steel ropes that pass over a grooved sheave at the top of the shaft and down to a counterweight of roughly the cab plus 40 percent of its rated load, so the motor moves only the difference. Guide rails hold the cab square, the safety gear grips those rails, and a governor trips the gear above rated speed. Hydraulic elevators top out around six floors.
The design problem in a tall building is the hole. Every shaft takes the same footprint out of every floor below the top one it serves, so a tower with one bank running from lobby to roof would reach its upper floors with a core and almost no rentable space around it. Elevators are how a tall building pays for itself, and they eat the thing they pay for.
Sky lobbies and express service
The answer is to stop running every shaft the whole way. A building gets divided into vertical zones, each with its own bank of local elevators. Express elevators carry people from the ground to a transfer floor, a sky lobby, where they change to a local cab. The upper zones' local shafts never reach the ground, which frees the floor plate below.
The original World Trade Center towers put sky lobbies on floors 44 and 78, and the saving made everything above the 80th floor pay. TheBurj Khalifa, at 828 m and 163 floors, has them on 43, 76 and 123, with double-deck express cabs to the first two. Taipei 101 transfers at 35 and 36, then again at 59 and 60.
A double-deck cab is two cabs in one frame, one above the other, loading from two lobby levels at once, so a single shaft carries close to twice the people. Destination dispatch gets the same effect in software: you enter your floor at a lobby kiosk, the controller puts passengers bound for the same zone into one cab, and the stops per trip drop. There are no buttons inside to press.
How many a building needs
The Empire State Building opened in 1931 with 73 elevators for 102 floors, run by operators with hand controls, and it still has roughly that many. The Burj Khalifa runs 57. A rule of thumb puts one elevator per 40,000 to 45,000 square feet of rentable office space, and missing it means queues in the lobby at nine every morning.
Speed, and why down is slower than up
The Shanghai Tower holds the record at 20.5 m/s, about 74 km/h, on a run of 578.55 m from the basement to the observation deck, the longest continuous elevator run anywhere. Taipei 101 managed 16.8 m/s in 2004 and held the title for a decade. The top of the Shanghai Tower's 632 m arrives in under a minute.
Those are the ascent figures. Descent is set lower, around 10 m/s in the Shanghai Tower, because of your middle ear. Going up, pressure falls and the eustachian tube vents outward easily; coming down, the tube has to open against rising pressure, which is the painful direction. High-speed cabs bleed their internal pressure ahead of the altitude change, and the descent rate is still capped. The binding constraint is the passenger.
The rope is the limit
Steel rope weighs enough to become its own problem. Around 500 m of travel the ropes for one elevator outweigh the cab and its passengers, so the motor spends most of its work lifting rope, and a taller run needs thicker rope that weighs more again.
Kone's UltraRope answered it with a carbon-fiber core in a high-friction coating, roughly a seventh the weight of steel at equal strength and rated for a single run of 1,000 m. The Jeddah Tower is designed around it. Carbon fiber also sways far less, so the cab keeps moving in wind that would once have idled it, which matters in a building that moves at the top.
Where this fits
The elevator is one of the two inventions the skyscraper needed. The other is the steel frame, which belongs towho invented the skyscraper, while the tallest building in the world is where this problem is hardest right now.