The Loop's Structural Grammar: How a Square Mile Taught the World to Build Tall

The Chicago Loop's first generation of architects did not invent the skyscraper as a height contest. They assembled a structural alphabet. Walk into the Loop from the south along Michigan Avenue past Grant Park, and the first building on a structural-grammar tour stands at the corner of Congress and Michigan: ten stories of granite and limestone with a tower bay rising on the south side. It is the Auditorium Building, finished in 1889 by the partnership of Adler and Sullivan. From the sidewalk it looks like a Romanesque mass, vaguely medieval, modeled on Henry Hobson Richardson's Marshall Field Wholesale Store a few blocks away. The walls are doing the work. The exterior is the structure. This is the older grammar at its crown. Hold it in your eye. Three blocks north, the building changes everything about how that older grammar was thought to work.

The story usually told about Chicago architecture is that the city invented the skyscraper. That sentence is half true. Iron-framed mills had been built in England by the 1790s. New York's Equitable Life Building of 1870 had a passenger elevator. Cast iron commercial façades had been routine in lower Manhattan since the 1850s. What Chicago invented, between roughly 1880 and 1900, was something more useful than any single building type. It was a structural alphabet. Five or six elements that, once assembled, let a building's outside be designed independently of how the building stood up. Once you had that alphabet, you could write any building you wanted with it. The literature calls the cohort that assembled the alphabet the First Chicago School. The buildings they left behind are still in the Loop, still teachable, still working.

The four problems and their letters

The first problem was load. A masonry-walled building can only be so tall before the wall thickness at the base eats the rentable floor area. The Monadnock Building at 53 West Jackson, finished in 1891 by Burnham and Root, is the case study. Sixteen stories at 215 feet, walls of solid brick six feet thick at the base, tapering to eighteen inches at the top. Walk to the entrance and look into the door reveal where the cross-section of the wall is exposed. Six feet of brick. The architectural literature is exact about what that means. The 215-foot height was calculated to be the highest economically viable for a load-bearing wall design. Go taller and the wall would consume the rentable floor area. The north half of the Monadnock is the tallest load-bearing brick commercial building ever constructed. It is also the announcement that the load-bearing wall had nowhere left to go.

The solution was the metal frame. Iron at first, steel from about 1890 onward, organized as a complete skeleton on which the entire weight of the building rested. The wall stopped being structural and became cladding. William Le Baron Jenney's Home Insurance Building at LaSalle and Adams, finished in 1885, is the conventional first-skyscraper attribution. It carried its ten stories on a fireproof structural steel frame on both the inside and the outside. The building was demolished in 1931, but the grammar it opened did not go with it. The Sullivan Center at State and Madison, finished in 1899 by Louis Sullivan, shows the move at street level. Look past the bronze-plated cast-iron Art Nouveau ornament at the rounded corner. Above the second floor a regular grid of vertical piers and horizontal spandrels runs all the way to the cornice. The grid is the steel skeleton, expressed through its cladding. The wall is no longer doing the work. The frame inside is.

The second problem was fire. The 1871 fire had taught Chicago's underwriters that any tall building rebuilt downtown had to be fireproof or it would not be insured. A steel frame is strong but it is not fireproof. Steel softens at around 1100 degrees Fahrenheit and collapses at around 1300. A typical office fire reaches 1800. So the steel had to be sheathed in something that would not burn, would not conduct heat fast, and would be light enough not to overload the frame it protected. Terra cotta, a glazed architectural ceramic, was the answer. Fired clay tiles, hollow, light, fitted around the steel members like a sleeve. The Reliance Building at 1 West Washington, finished in 1895, is the prototype. John Wellborn Root drew the lower floors in 1890. Root died in 1891. Charles B. Atwood, working under Daniel Burnham, completed the upper floors after his death. Fourteen stories of white terra cotta and plate glass, the structural steel sheathed inside, every bay almost entirely window. The architectural record names it the first skyscraper to have large plate glass windows covering the majority of its surface area. The terra cotta was also believed, in 1895, to self-clean every time it rained.

The third problem was vertical movement. Elisha Otis had demonstrated his safety elevator in New York in 1854, on a wooden platform, in front of a crowd, by cutting the rope while standing on it. The brake clamped to the rails. The platform stopped. By the 1860s and 1870s the steam-driven safety elevator was in commercial use. By 1880 it was standard. The economic consequence is the inverted floor stack. Before the elevator the top floor was the cheapest because nobody wanted to climb. After the elevator the top floor was the most desirable because the view, the air, and the prestige all improved with height. Once that inversion happened, every Chicago architect could think about tall buildings as full real-estate stacks rather than as expensive caps on cheap warehouses. The Auditorium Building had thirteen Otis hydraulic elevators when it opened in 1889. The Sullivan Center had a full bank. The Reliance was designed around the assumption that every floor would be a destination.

The fourth problem was the ground. The Loop sits on a layer of soft blue clay running more than a hundred feet deep, above firmer hardpan, above bedrock. Any heavy thing built on the clay would sink, and would sink unevenly, unless the foundation distributed the load. Each of the buildings on a Loop walk solves this problem differently. The Auditorium Building's engineer, Dankmar Adler, working with engineer Paul Mueller, designed a massive raft: crisscrossed railroad ties, a double layer of steel rails embedded in concrete, the building floating on top of the clay. The Rookery at 209 South LaSalle, finished in 1888, sits on John Root's grillage foundation: iron rails and structural beams arranged in a crisscross pattern, encased in concrete, distributing the weight wider than the footprint above. The Reliance, finished in 1895, sits on concrete caissons sunk roughly 125 feet down through the clay to firmer soil. The Inland Steel Building on Monroe, finished in 1958, sits on 450 steel pilings driven 85 feet deep, the first of their type at scale in a Chicago skyscraper. Four buildings, four answers to the same clay. The variety of solutions is itself a kind of literacy. There is no single "Chicago foundation." There is the problem and the family of moves that solved it.

What survived the half-century pause

By 1910 the First Chicago School had stopped producing major commercial buildings. Sullivan was working on small-town banks. Burnham was working on the 1909 Plan of Chicago. Root was dead. The country's commercial-architecture energy moved to New York, then briefly to Detroit, then to the West Coast. The Chicago Loop kept its buildings but stopped adding to them.

The restart came in 1938. Ludwig Mies van der Rohe arrived from Germany and took the chair of the architecture department at the Illinois Institute of Technology. He held it until 1958. Working in parallel with the local firm Skidmore, Owings and Merrill, Mies and his students restated the entire First Chicago School grammar in postwar materials. The Federal Plaza complex on Dearborn (1960 to 1974) is the canonical specimen. Stand at the center of the plaza with Calder's red Flamingo behind you and the Kluczynski Federal Building rising to the south. The structural system is the steel skeleton from sixty years earlier, restated. The exterior columns are steel I-beams painted flat black. The horizontal mullions are also I-beams painted flat black, projecting from the façade so the structure can be read. The wall between them is bronze-tinted glass. The whole envelope is curtain wall, a term first earned by the Reliance Building. The vocabulary has not changed. The execution has.

The Inland Steel Building at 30 West Monroe, finished in 1958 by Bruce Graham and Walter Netsch of SOM, holds the resolution beat. Nineteen stories of green-tinted glass curtain wall. Stainless-steel exterior columns visible at every corner. SOM moved the structural columns from inside the floor plate to the outside of the building so the interior could be a clear-span office floor uninterrupted by columns. The smaller stainless-steel tower behind the main mass is the mechanical core: elevators, stairs, ductwork, everything that supports the building pulled out of the office floors and sealed into a separate vertical volume. Both moves became standard practice in every American corporate headquarters built after.

What the alphabet became

The reason this small set of buildings carries so much weight is that the alphabet they assembled is not local. Every glass office tower built in any American downtown since 1955 (and most of the office towers built in London, Frankfurt, Tokyo, Shanghai, Singapore, Dubai across the same period) is a sentence written with the same letters. Steel skeleton. Terra cotta or glass curtain wall. Safety elevator stack. Foundation deep enough to carry the weight. Mechanical core. Clear-span floor plate. The five or six terms travel. The architectural record names the explicit line of descent: the Reliance Building, the architectural literature notes, is a direct precursor to the all-glass Friedrichstrasse skyscraper Mies proposed in 1921 for Berlin. Mies carried that grammar forward into IIT. SOM carried it forward into Inland Steel. The Sears Tower, finished 1973, applied the same alphabet at 108 stories. The structural ancestors of every contemporary tall building stand inside one square mile bounded by Congress, Michigan, the Loop's elevated tracks, and the river.

You can walk the alphabet in about ninety minutes. The walk is short because the city is dense. After you have walked it, every glass office tower in any downtown reads as a structural argument rather than as a stylistic preference. That is the gift of the Loop. The buildings teach you to read them, then they teach you to read every other building you will ever see.