01Executive summary
Four milestones in which building height stopped being limited by structure and started being limited by economics.
Three things together made the tall building possible: the elevator, the skeleton frame and relatively cheap steel. Pneumatic caissons let foundations reach bedrock through ground that could not otherwise be excavated, first used with extensive wind bracing at the Manhattan Life Building of 1894. Reinforced concrete frames became a genuine competitor for buildings of moderate height. And the Empire State Building of 1931 held the world height record for four decades.
02Three conditions, all of which had to arrive
A tall building is not a taller version of a short one. Three separate constraints have to be lifted simultaneously, and each was lifted by a different industry.
Vertical transport
Above about six storeys, floors become progressively less lettable without a lift. Hydraulic lifts served the earlier tall buildings and continued well into the twentieth century, long after electric motors became common. The safety lift is covered in the first series of this set.
The skeleton frame
In a masonry building the walls carry the load, so walls at the base of a tall building become impossibly thick and consume the floor area they enclose. A frame carries the load and the wall carries only itself — the same separation the Gothic builders made with piers and screen walls.
Cheap steel
A frame is only economic if the metal is. Everything in the preceding part of this series — the converter, the open hearth, the basic process — is upstream of the skyscraper.
The limit became economic
Once all three were in place, the constraint on height was no longer what could be built but what was worth building: land value against construction cost, lift shafts consuming lettable area, and diminishing returns with height.
That last point is worth taking seriously rather than as a rhetorical flourish. As a building grows taller, the proportion of its floor plate consumed by lifts, stairs and services increases, because more floors above must be served through the same core. There is a height at which an additional storey adds less lettable area than it consumes. That is a genuine engineering-economic limit and it moves with lift technology, not with structural capability.
03Foundations: the problem that is not visible
A tall building concentrates an enormous load on a small footprint. In ground that is sound to the surface this is manageable. In the ground beneath most of the cities where tall buildings were wanted — river deltas, reclaimed land, glacial deposits — it is not, and the material that must be penetrated to reach bedrock is frequently soft, waterlogged and impossible to excavate by ordinary means because it flows back in as fast as it is removed.
The pneumatic caisson answers this. A working chamber, open at the bottom and sealed at the top, is pressurised with compressed air to hold water and soft material out while workers excavate beneath it. The caisson sinks under its own weight and the weight of what is built on it, and when it reaches bedrock it is filled with concrete and becomes the foundation.
Finished in 1894 at 106 metres and still standing, it was the first building for which engineers used compressed air to sink pneumatic caissons in the foundation work. Fifteen caissons of various sizes and shapes carried excavations down through more than 15 metres of mud and quicksand to bedrock, were then filled with concrete and topped with iron beams supporting brick piers. It was also the first building with extensive wind bracing — added metal members in the frame specifically to guard against collapse of a tall structure in violent wind.
Working in compressed air causes decompression sickness when the return to normal pressure is too rapid, and in this era the mechanism was poorly understood and the controls inadequate. Caisson work killed and permanently disabled numbers of workers on major projects of the period. Modern practice imposes strict limits on exposure time and pressure, mandatory staged decompression, medical supervision and on-site recompression facilities. Any account of caisson foundations that presents them purely as an engineering triumph is leaving out the part the profession actually had to learn from, and the regulatory apparatus that now surrounds hyperbaric work exists because of what happened to these workers.
04Wind: the load that arrives with height
For a low building, wind is a nuisance the structure resists incidentally. For a tall one it becomes a governing action, and this changes with height faster than intuition suggests for two reasons. Wind speed increases with height above ground through the boundary layer, so the pressure per unit area is greater at the top. And the overturning moment at the base is the pressure times area times lever arm, so it grows roughly with the cube of height for a building of constant plan.
Wind bracing — additional diagonal or portal members whose function is purely to resist lateral load — therefore becomes a distinct part of the design rather than a consequence of the gravity frame. That the Manhattan Life Building was the first with extensive wind bracing is a marker of the point at which the profession recognised lateral load as a separate design case for buildings.
The problems the tall buildings of this period presented — foundations, stability under wind pressure, elevators, water supply and sewerage at height — had barely begun to arise late in the nineteenth century. Within four decades New York had more than thirty buildings over 150 metres, Chicago at least nine and other American cities ten between them. A whole class of engineering problem appeared and was substantially solved within a working lifetime.
| Aspect | Low building | Tall building |
|---|---|---|
| Governing load | Gravity | Wind, for the frame; gravity still governs foundations |
| Wall function | Often load bearing | Cladding only, carried by the frame |
| Foundation | Spread footings on competent ground | Deep foundations to rock, frequently through unstable material |
| Vertical services | Minor share of floor area | Core consumes a growing share, which sets the economic height |
| Water supply | Mains pressure suffices | Staged pumping and pressure zones, since one zone cannot serve the full height |
| Occupant comfort | Not a structural concern | Sway acceleration under wind becomes a serviceability criterion in its own right |
05Reinforced concrete enters the competition
American architects and engineers began designing many-storeyed reinforced concrete buildings partly under the influence of European builders, who used less steel. Reinforced concrete frames came to compete with steel-frame structures for buildings of moderate height.
The two systems trade differently and neither is generally superior.
The Empire State Building of 1931, at about 380 metres, topped the Chrysler Building by more than 60 metres and remained the tallest structure in the world until the early 1970s. Its construction programme is the part that continues to impress: a building of that size erected in about a year, achieved by treating the steel frame as a manufacturing and logistics problem — members delivered to a schedule and placed on arrival rather than stockpiled — which is the American System of the first series applied to a building.
06Takeaways for current practice
- Check whether every enabling condition is present. The tall building needed three, from three different industries, and any one missing prevents it.
- The binding constraint is often economic, not technical. Height was limited by core area and land value long before it was limited by structure.
- Lateral load becomes a separate design case. Overturning grows roughly with the cube of height, and bracing has to be designed for it deliberately.
- Count the human cost of a construction method. Compressed-air work injured and killed people before the mechanism was understood, and the modern controls exist because of it.
- Treat erection as manufacturing and logistics. Programme, not structural capability, is what made the Empire State remarkable.
Modern references include AS 1170.2 for wind actions, AS 4100 for steel, AS 3600 for concrete and AS/NZS 2865 for confined spaces, with compressed-air work regulated under state work health and safety instruments. Cited by number for orientation only — verify currency.