During the period 1874-1885, Chicago, led by Peter Wight and Sanford Loring (of Chicago Terra Cotta), had pioneered the development of fireproofing iron structures with lightweight porous terra cotta, hence, this technique was referred to as “Chicago construction” throughout the country. Wight had been forced to initially invent such a system in the face of the threats from the insurance industry to prohibit the use of unprotected iron structural members in buildings following the second Chicago fire of July 1874 (v.2 sec. 4.6). The use of terra cotta to fabricate fireproofing encasings for iron structures had paralleled the development of fireproof floor structural elements (i.e., the segmental flat arch).
The reason for the development of terra cotta flooring systems was the reduction in the constructional weight of a building. Reducing the weight of construction had been paramount in Chicago because of the weight limitations imposed on construction by Chicago’s relatively weak underlying geology. By 1886 in Chicago, this type of construction had become standard for the interior of the city’s first skyscrapers, while loadbearing brick (and later stone) was still being used for these buildings’ exteriors. However, the upper limit to the height of these walls was ten stories because taller (heavier) walls exceeded the capacity of Chicago’s geology and the resulting settlement went beyond acceptable limits (the extreme example was the 29” in the Auditorium).
Root had showed the way to one of the solutions to these problems by being the first in Chicago (influenced by New Yorker George Post’s pioneering structures in the Equitable and New York Produce Exchange buildings) to divorce the loadbearing role of the exterior’s masonry by supporting the masonry at each floor on iron shelf angles in the Phoenix Building and the Rookery. The brick’s function in these walls was solely to fireproof the iron columns, a role that exterior terra cotta quickly assumed in order to further reduce the weight of construction.
1.4. THE TECHNICAL ISSUES OF THE IRON-FRAMED SKYSCRAPER: WIND LOADS
The constructional challenge facing Chicago’s architects in 1888 was how to incorporate the fireproofed iron skeleton frame, i.e., “Chicago construction” into the exterior of skyscrapers over ten stories in height. This problem was rather unique to Chicago (the majority of Manhattan’s soil had no such limits on taller buildings). Replacing the ever-reliable masonry exterior wall with a lightly clad iron frame would be an empirical process for which a new building design team member arose: the structural engineer. Although engineers had cut their teeth on designing iron bridges for the railroad and brought their experience to the problems in engineering a tall building, they truly had no reliable data on how to design a building’s structure to resist the power of the wind (or for that matter, earthquakes, as we saw in Burnham & Root’s recent building for the San Francisco Chronicle).
Fearing loss of life if they were wrong, architects and engineers would take their time in removing these reliable bulwarks in order replace their rigidity with a correspondingly rigid iron frame. This was done first with iron diagonal bracing and later with rigid (moment-resisting) connections between the beams and columns.
We will see this will be a process of replacing one wall in one building at a time, and praying that the building would remain standing. And then replacing two walls, etc., until eventually, someone will build a skyscraper with only an iron skeleton frame with no masonry bearing walls. In this effort they were aided by technological advances in the design and operation of elevators (Historian Lee Gray has analyzed these in his book, From Ascending Rooms to Express Elevators: A History of the Passenger Elevator in the 19th Century. Elevator World, 2002.) and the manufacture of construction materials such as steel, exterior terra cotta, and larger panes of glass. (Historian Thomas Leslie has thoroughly documented these in his recent book, Chicago Skyscrapers: University of Illinois Press, 2012.)
They would begin by simply turning the Rookery inside out, that is, putting its courtyard elevations along the exterior perimeter of the building and relocating the solid walls still needed for wind bracing somewhere within the building’s interior. This posed a new problem for the architects: how to appropriately design the elevations within which they had placed the iron frame? In other words, while the engineers endeavored to stiffen the iron frame against wind loads, the architects were struggling to evolve an architecturally meaningful and aesthetically pleasing elevation for an iron frame and glass skyscraper.
Gray, Lee E. From Ascending Rooms to Express Elevators: A History of the Passenger Elevator in the 19th Century. Mobile, AL: Elevator World, 2002.
Larson, Gerald, “The Elevator, the Iron Skeleton Frame, and the Early Skyscrapers: Parts 1 and 2,” International Journal of High-Rise Buildings, March 2020, pp. 1-41.
Larson, Gerald, “The Iron Skeleton Frame: Interactions Between Europe and the U.S.,” Zukowski, John (ed.), Chicago Architecture, 1872-1922: Birth of a Metropolis. Chicago, The Art Institute of Chicago, 1987.
Leslie, Thomas. Chicago Skyscrapers: 1871-1934. Urbana: University of Illinois, 2012.
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