It appears that as the building had increased in height from the initial seven floors of February 19, 1884, to the final ten stories of April 28, Jenney became concerned about the size of the masonry piers in relation to the need for daylighting (just as Baumann had likewise identified) and most probably began to experiment during the latter part of this period with embedding iron sections within the masonry piers that would support the floor beams in order to keep the piers’ cross section to a minimum. This concept of embedding iron sections within a masonry structure to augment its capacity was consistent with an article that Jenney had published in the preceding year, in which he revealed his understanding of iron framing just before he had received the Home commission: “Educated architects in Europe… have been working with and writing on the combination of stone, brick and iron, in the street elevations of buildings.” In other words, he did not mention the concept of an iron frame supporting its exterior masonry curtain wall but spoke instead of the structural combination of brick and iron. This combination clearly reflected his French training and familiarity with French theorist Viollet-le-Duc’s ideas about the use of iron and masonry.
The building’s interior structure was the by-then standard iron frame protected with Wight’s terra cotta casings. Also standard were the two masonry bearing party walls on its north and east that ran the entire height of the building and provided much of the building’s lateral stability.
In fact, even the first two stories of the street fronts also were loadbearing rock-faced granite piers, battered in thickness from 4′-0″ at the base to 2′-10″ at the third floor. Constrained by the building code’s requirement for masonry exteriors, the only detail in which Jenney had departed from standard Chicago construction of the early 1880s was his insertion of a rectangular, concrete-filled cast iron section within the exterior masonry piers in only the upper eight stories of the two street fronts. In a sense, Jenney had inverted the structure of the Opera House Block, except that Cobb & Frost had also included spandrel beams (see below) and used the exterior iron columns in the first two floors to support all, and not just a portion, of the weight of the floors above.
The iron sections were story-high, hollow cast-iron sections that supported the floor beams. These sections were set on the top of the granite piers at the third floor and were bolted one on top of another, helping to support the upper seven floors and roof. These sections were cast with shelf brackets to support two 12-inch deep wrought iron floor beams. These beams were loosely bolted to the column by a single bolt that passed through the beam webs and connected them to a spacing bracket that was also cast with the column. As tolerance was needed for erection, the holes were larger than the bolt, leaving the connection with a considerable amount of play. Therefore, Jenney had incorporated a clamp that was a one-inch diameter wrought iron rod that was bent at one end and placed into a notch cut into the top flange of both beams. The rod on the other end was threaded, allowing it to be connected to the column by a nut placed inside the column, thereby pulling the beams tight to the column face, after which the iron column was filled with concrete. The concrete-filled iron column was then surrounded with masonry that at times exceeded twelve inches in thickness, creating a solid cross section in the building’s exterior piers. In the first article he published on the building in the December 1885 issue of Inland Architect, rather than describing this technique as wrapping or enclosing the iron column with a masonry skin, Jenney stated that he embedded the iron column within the masonry pier: “a square iron column was built into each of the piers in the street fronts.”
So far, so good, but now Jenney’s structure becomes more “complicated” (I choose my words carefully). Jenney was very concerned about the potential of differential settlement in his first tall building. (This was evident in the title of a paper he delivered at the October 1885 A.I.A. convention, “The Construction of a Heavy Fireproof Building on a Compressible Soil.”) He was not at all reassured by contemporary reports of the foundation problems in all three of Chicago’s new large public buildings, the Post Office, the City Hall (not yet completed!), and the Board of Trade’s tower (still under construction) that surely would have made him overly-cautious in how he would detail the structure to minimize any problems. With these reports fresh in his mind, the last thing that Jenney would have wanted to detail in the Home Insurance Building would have been a rigid framed structure.
If Jenney had conceived this structure as a skeleton frame, he would have simply placed an iron spandrel beam connected at both ends to the columns that would have supported the window wall above. This Jenney did not do. One can imagine any number of reasons for his decision: my best guess is that a long iron beam at every floor in this location was too expensive, because this was exactly what Jenney did not detail. Instead of spanning this distance with a single iron beam upon which he could then place the masonry spandrel panel and paired windows, he still placed the conventional vertical structural iron mullion between the windows and then used a shallow iron pan to span between the pier and the mullion, i.e., he broke the single span between the piers into two spans, that met at, and were supported by, the intermediate iron mullion. This traditional structure of the paired window and mullion, as we have discussed before, had the inherent problem of differential settlement between the heavily-loaded piers and the lighter mullions because it was still conventional practice, even though Baumann had argued against it, to put the same size footing under both elements. This resulted in the heavier-loaded piers settling at a greater rate than the smaller mullions, transferring more and more load to the mullions, and often resulted in cracking around them. Chicago’s poor soil only seemed to exacerbate this problem, as we saw in Richardson’s American Express Building (see Vol. One).
Therefore, instead of using iron beams to span between the piers, Jenney detailed cast-iron lintels in the form of four-inch deep hollow pans, also filled with concrete like the columns, that spanned the distance between a column shelf bracket and the intermediate cast-iron mullion. The cast-iron lintel pans were as wide as the masonry spandrel walls that were constructed on top of them. As if the street fronts were still considered to be bearing walls, the spandrels, for no other conceivable reason, increased in thickness along with the piers, as required by the building code, from 20 inches in the top three floors to 24 inches in floors 5-7, to 28 inches in floors 3 and 4.
The lintel pans were not bolted to each other, to the mullion, or to the column brackets, but simply rested on these surfaces, relying solely on the supported masonry knee wall that was bonded into the masonry pier, to hold the armature laterally in place. The lack of bolts may have been a technique on Jenney’s part to impart some rotational flexibility at the column/spandrel connection to accommodate differential settlement of the piers. This flexible joint was augmented by notching the front of the iron lintel four inches back from the face of the pier, which allowed the pier’s exterior face brick to continue past the lintel so that the pier’s face brick would not be supported at any point along the lintel. This detail minimized the potential of the face brick to crack if an iron spandrel rotated due to the settlement of an adjacent pier, but it also meant that the pier’s eight-stories of brick facing was continuously self-supporting from the granite piers at the third floor and was not supported at each floor on the iron column.
The next decision was, how to bring the loads in the intermediate iron mullions to the foundation? The easiest way to avoid the anticipated differential settlement between the piers and the intermediate mullions was to transfer the mullion loads over to the main piers, thereby the mullions would never reach the ground. However, if this was done not with a single transfer beam at the lowest floor, but with a series of transfer beams as Jenney had detailed, the loads in the mullions would be relatively uniform, and therefore the mullions’ cross-section would not have to increase as the piers did, keeping the windows as large in the lower floors as they were in the upper floors. Jenney, therefore, placed iron transfer beams to carry the mullions’ loads to the piers immediately above the cast iron lintel pans at the fourth floor (four 7-inch I-beams), sixth floor (three 15-inch I-beams), ninth floor (two 12-inch I-beams), and roof (two 15-inch I-beams). These transfer beams also nominally laterally tied the iron columns in the piers together (especially at the roof), thereby creating what one might optimistically call a “skeleton frame.” However, if it was Jenney’s intention to actually create a rigid iron skeleton frame in the street fronts, these beams should have been introduced at every floor to not only carry the spandrels’ masonry, but also to laterally brace the iron columns at each floor to minimize their buckling length since the iron lintel pans were not bolted to the columns and, therefore, any bracing provided by them was negligible at best. As constructed, the iron columns in floors 6-8 stood laterally unbraced for three stories. Consequently, without the masonry and the concrete filling, the iron armature in the exterior would not only have been structurally unstable, but also have been very difficult, if not impossible, to erect it two or three floors ahead of the stiffening masonry of the piers and spandrels as some reports had claimed (the concrete filling of the multiple-storied hollow section would also have been impossible).
Larson, Gerald, “Toward a Better Understanding of the Evolution of the Iron Skeleton Frame in Chicago,” Journal of the Society of Architectural Historians, March 1987, pp. 39-48.
Tallmadge, Theodore E. The Origin of the Skyscraper-The Report of the Field Committee. Chicago, 1934.
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