3.26. THE EVOLUTION OF EXTERIOR SKELETON FRAMING

Burnham and Root, The Rookery. Exterior Lightcourt. (Author’s collection)

The only solution to overcome this inherent limit to Chicago skyscrapers was to invent a system of fireproof construction that weighed less per floor than did traditional masonry construction, so that the foundation pressure remained below 3000 psf as skyscrapers grew taller and taller.  This is the true genius behind the invention of “Chicago construction:” taller skyscrapers that actually weighed less than their shorter, bearing-walled brethren.  While Wight’s system had solved this problem with interior iron framing, it would take another six/seven years following Wight’s triumph in the Boston Architects’ test in 1881 before iron columns finally replaced a skyscraper’s exterior masonry bearing walls.  Because of the complexity of engineering and in constructing a ten+-story building, in addition to the municipal building codes that required masonry walls all along a building’s exterior perimeter for protection from an adjacent fire, the change from “boxed construction” to “framed construction” would be a cautious trial-and-error, step-by-step process during the years 1885-1890.

Post, Equitable Building. Banking Hall as it appeared in 1889. (Landau, New York)

We will see the first tentative experiments in exterior iron framing will occur in the exterior walls of the interior lightcourts of skyscrapers that had no such restrictions (remember Post’s detailing in the lightcourt walls of the Equitable Building). This legal loophole had simply reinforced the practical reason for employing iron framing in these locations: these exterior walls were supported above a major space of the ground floor that wanted to be as open as possible.  Therefore, the weight of the lightcourt’s walls could not be supported by carrying a bearing wall all the way to the ground floor but had to be transferred at the second or third floor to a series of beams that were then supported by columns.  So if the weight of the lightcourt’s exterior walls was going to be transferred to a series of beams, why not try to make the walls as light as possible in the first place, and simply carry the iron framing of the first and second floor up, into the exterior walls of the lightcourt? This was rather straightforward to detail and easier to do than immediately experimenting with iron framing in the exterior walls because the weight and rigidity of the exterior masonry walls was still needed to resist the force of the wind (the wind is a horizontal force, so this quality of a building’s structure is referred to as lateral stiffness or bracing).  Nevertheless, it eventually had to be done if the building’s overall weight was to be sufficiently reduced so that it could be built taller than 10 floors in Chicago.  In solving this problem with the skyscraper’s exterior construction, Chicago again invented, or rather, perfected, a new material to reduce the weight of the exterior wall: architectural terracotta (this is a different product from fireclay tile).  These are the reasons for this type of construction during the 1880s being called “Chicago construction.”

3.27. THE NEED FOR LATERAL BRACING

Once the skyscraper’s weight had been reduced by “Chicago construction,” the skyscraper could resume its upward growth.  As soon as it did, however, it immediately ran into a new structural problem that had to be solved.  As long as a building was constructed with load bearing masonry walls, the building’s weight, while being a negative with regards to the foundation issue, was a positive (its inertia) in resisting the wind’s pressure.  The weight and the stiffness of the masonry simply meant that there was little, if any horizontal deflection in the upper floors, even on the windiest of days.  Once that weight and stiffness was removed with the use of iron framing, things began to move rather noticeably.  Making the building taller not only made it more flexible, it also added more wind pressure, i.e., it made a bigger “sail.”  One of my favorite anecdotes from this period that pertain to the significant increase in building deflections in the early framed skyscrapers to which we can no longer relate, were the problems that people had keeping accurate time in the upper floors, because the excessive deflections wreaked havoc with the pendulums of the clocks.  Another was the problem office workers experienced with their ink wells on windy days.

Burnham & Root, Masonic Temple, Chicago, 1890. Cross section showing the configuration of the diagonal bracing. Note the total height at the top of the building reads: 302’ 1.” (Engineering Record, January 21, 1893)

The problem of stiffening a framed building with regards to wind deflection was only highlighted by the great Charleston earthquake of August 31, 1886, still the largest quake ever recorded in the southeast.  It was large enough that it was felt even in locations as far away as Chicago. The earthquake gave architects and owners alike, pause to reconsider the ramifications of removing those heavy, stiff walls…  The solution to the deflection problem of the iron skeleton framed skyscraper was two-fold.  First, builders incorporated diagonal bracing to impart a geometric stiffness (triangulation) to the frame.  Second, the connections between the beams and the columns would be made stiffer through a combination of replacing bolts with rivets, and the use of gusset plates at the connection (that was referred to as portal framing).

Holabird & Roche, Old Colony Building, Chicago, 1893. Diagrams of Portal Bracing. The left shows a Phoenix column at each end with steel gusset plates riveted between the columns and beams. (Condit, Chicago School)

(If you have any questions or suggestions, please feel free to eMail me at: thearchitectureprofessor@gmail.com)

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