The Great Chicago Fire of 1871 destroys over 17,000 buildings in three days — almost all timber and unreinforced masonry. The reconstruction that follows isn't just a real-estate opportunity: it's an unprecedented technical laboratory, where architects and engineers experiment with fireproof construction systems on an urban scale, with the metal frame taking the lead almost out of necessity before aesthetic choice.
Why Chicago
The fire explains the opportunity, not the form. The skyscraper was born in Chicago because four conditions met there. The first was economic: at the end of the 19th century the city was the railway and commercial hub of the Midwest, and in the Loop, the small business district hemmed in by the lake, the river and the railways, land values rose so fast that the only way to exploit them was to build upwards. The second was technological: the safety elevator, presented by Elisha Otis in the mid-19th century, made the upper floors usable, whereas before they had been the least desirable because they had to be reached on foot; the telephone and telegraph made it possible to concentrate offices and business within a few blocks.
The third condition was geological, and it makes the story even more interesting. Chicago stands on a deep layer of soft clay, and a heavy building tends to sink. Every kilo saved in the structure reduces settlement, which pushed engineers towards systems lighter than masonry. John Wellborn Root developed grillage foundations, made of crossed layers of rails or steel beams embedded in concrete, which spread the load over a wide area without digging deep. The fourth condition was cultural: a new city, without an entrenched academic tradition, where engineers and architects were willing to experiment. The result was a generation of office buildings that, for the first time, grew out of structure and economics rather than historical styles.
The Home Insurance Building and the birth of the frame
In 1885, William Le Baron Jenney completes the Home Insurance Building: ten floors (later extended to twelve), conventionally regarded as the first tall building with a structural iron-and-steel frame that carries the entire vertical load, freeing the perimeter walls from any load-bearing function. It's a conceptual leap, not just a technical one: until then, a tall building needed ever-thicker walls at its base to carry its own weight — Burnham & Root's Monadnock Building (1891), sixteen storeys of load-bearing masonry, has walls about 1.8 metres thick at the base, and would remain one of the last skyscrapers built this way. With the metal frame, wall thickness becomes independent of building height: you can go up without the foundations having to carry additional tonnes of load-bearing masonry that serves the structure no purpose.
The frame of the Home Insurance Building is made of different materials, and this very mixture tells the story of the transition under way. The columns of the lower floors are cast iron, a material that resists compression well but is brittle and breaks without warning if bent or struck; the beams are partly wrought iron and, on the upper floors, Bessemer steel. The Bessemer process, developed in the mid-19th century, made it possible to produce steel in large quantities by blowing air through molten pig iron, and within a few decades it slashed its price. Steel combines the compressive strength of cast iron with the ductility of wrought iron, the ability to deform before breaking. As early as 1889 Burnham & Root's Rand McNally Building was built with an all-steel frame.
The way the pieces were joined also changed the history of construction. For decades frame connections were hot-riveted: a red-hot steel rivet was tossed by one worker to another who inserted it into the hole and hammered it home, while the metal, as it cooled, contracted and clamped the plates together. Four-person gangs worked like this tens of metres up, often without protection. From the 1930s and above all after the war, riveting gave way to welding and high-strength bolts, which were faster and safer. It is an evolution that can still be read in historic buildings today: the regular rows of rounded rivet heads are the unmistakable sign of an early 20th-century structure.
Sullivan and form following function
Louis Sullivan, Dankmar Adler's partner, doesn't just use the new construction system: he theorizes its aesthetic. In his essay "The Tall Office Building Artistically Considered" (1896) he formulates the principle "form follows function," which will become the mantra of structural modernism: a skyscraper's façade, Sullivan argues, should honestly express its frame structure — base, repetitive shaft of office floors, crown — instead of disguising itself as a Renaissance palazzo with historicist cladding stuck onto a completely new construction system. The Wainwright Building in Saint Louis (1891) applies this logic: the frame's vertical columns are emphasized on the façade with continuous pilasters, while the horizontal beams stay more discreet — a visual hierarchy that communicates the building's real structural behavior instead of hiding it.
There is, however, a detail that makes the Wainwright a surprising precedent for the Seagram. The vertical piers articulating the façade are twice as many as the structural columns: every other one corresponds to a real column, while the rest are simple brick casings that double the rhythm to make the building look more slender. Sullivan too, then, uses the façade to represent the structure rather than to show it literally. His "form follows function" is not an engineering equation but a principle of expression: the façade must make clear what the building is, even at the cost of retouching the constructional reality.
In the same years the Chicago School developed an element that would become universal: the Chicago window, a large fixed central pane flanked by two narrower opening windows for ventilation. It was the direct response to the possibility of freeing the façade from load. The Reliance Building (1895), completed by Daniel Burnham's firm with Charles Atwood, took this idea to its limit: a façade almost entirely of glass, framed by thin bands of white glazed terracotta, anticipating the modern curtain wall by half a century.
Terracotta as fireproof armor, not decoration
A technical detail often overlooked in the Chicago School narrative: the steel frame of the early skyscrapers is systematically clad in terracotta or brick, not left exposed. The reason isn't aesthetic but safety: unprotected steel rapidly loses strength at a fire's high temperatures — at around 600°C it retains roughly half its room-temperature strength — and the ceramic cladding, with its low thermal conductivity, delays the heating of the load-bearing structure just long enough for evacuation and firefighter intervention. The "honest" aesthetic of exposed structure, which will become the hallmark of European modernism a few decades later, is therefore actually a later achievement — made possible by more sophisticated fire codes, not by a simple change in taste.
Protection did not concern only the columns. Floors were built with hollow clay tile flat arches, blocks of perforated terracotta set as a flat arch between one steel beam and the next, which protected the beam flanges from below and reduced weight compared with a solid floor. It is a system found in a great many American and European buildings from the late 19th century to the first decades of the 20th, and one that anyone working on the refurbishment of these buildings today encounters constantly. Its weak point is water: if leaks reach the steel enclosed in the terracotta, rust expands, cracks the casing and can seriously damage the façades.
Mies and the Seagram Building paradox
In the New York skyscraper he built with Philip Johnson in 1958, Mies van der Rohe brings to completion the logic opposite to that of nineteenth-century terracotta — with an instructive paradox. The Seagram Building displays bronze I-profiles on its façade that seem to directly express the load-bearing structure, in the tradition of Sullivan's constructive sincerity. In reality, New York's fire code already required the true steel structure to be encased in concrete, invisible from outside: the exposed bronze mullions are purely decorative elements, applied in front of the real, protected frame, to still give the visual sense of structure that the code prevented from actually being shown. It's a case where the aesthetic of structural honesty, taken to its extreme consequences, ends up being itself a form of representation — no less constructed than a stone Doric capital, just with a different vocabulary.
The Seagram has a direct precedent in Mies's own work. On the 860–880 Lake Shore Drive residential towers in Chicago (1951), Mies welded steel I-sections to the façade, running the full height of the building in front of columns already encased in concrete. When asked why, he gave two reasons: they stiffened the façade plates during erection, and without them the building would not have looked right. It is a revealing answer: the technical function exists, but the main reason is visual. A few years later, at Crown Hall on the Illinois Institute of Technology campus (1956), Mies brought the structure genuinely outside: the roof of a single large space without internal columns hangs from four large exposed steel plate girders that span over the roof. Here the structure is not represented: it is actually outside.
After Mies: Fazlur Khan's tubes
The century of the steel frame closes again in Chicago, with an engineer. In the 1960s Fazlur Khan, of Skidmore, Owings & Merrill, realised that the Miesian frame of columns and beams became less and less efficient as the building rose: beyond a certain height, the design is dominated no longer by weight but by wind, and ever larger quantities of steel are needed to stiffen the structure. His solution was to think of the whole skyscraper as a hollow tube fixed in the ground, with its strength concentrated at the perimeter. In the John Hancock Center (1969) the large X-braces visible on the façade turn the envelope into a vertical truss; in the Sears Tower, now Willis Tower (1974), nine bundled tubes of different heights support one another. These buildings are much taller than Mies's yet use far less steel than a conventional frame of the same height would have required.
For designers today, this story leaves three practical lessons. The first is that fire protection is not a detail to be added at the end: from 19th-century terracotta to today's intumescent paints and boards, it often determines the final appearance of the structure. The second is that a frame's efficiency depends on its overall form more than on the grade of steel: the right geometry, as Khan showed, can drastically reduce the material required, and therefore also the emissions linked to steel production. The third is that the gap between the real structure and the represented structure is not a deception to be condemned but a design choice to be made consciously: Sullivan and Mies knew this well, and built some of the most influential buildings of the 20th century on that gap.