Let us first examine Jenney’s Manhattan Building, as it was more traditional in its exterior design. It was owned by Charles C. Heisin, who was granted a building permit on June 7, 1889. Not only did the Inter-Ocean openly question whether or not the site was too far south to be profitable for office rental, but as I noted in the last section, the site also posed two new structural problems for Jenney to resolve. The process of extending Dearborn south of Jackson Street (where the grid of Chicago changed from square blocks to rectangular blocks: three blocks per two streets) had further reduced the widths of lots on both along Dearborn, where both the Manhattan and the Monadnock were to be erected, from 100’ to 68.’
The combination of the shallow depth of the site and the planned height of over 200′ concerned both architects about the potential deflection caused by the wind that they had to respond by stiffening the buildings’ interior structures against wind loads. In addition, the Manhattan’s site had existing buildings on both the north and the south property lines. To the north was a seven-story building containing printing companies; to the south was the Como Building designed by Van Osdel that also contained a number of clients. Jenney faced the same problem that Root had the previous year in designing the Rand-McNally Building and solved it in a similar fashion. The existing masonry party walls on the north and the south of both the adjacent buildings could not be increased to the new height planned for the Manhattan, as they would need to be extended by a number of additional stories. The added weight of these floors would naturally have required the foundation of the existing walls to be increased, meaning that the basements of both buildings would have to be vacated for a number of months to allow the construction of underpinning to allow the foundations to be enlarged. Inland Architect explained this problem:
“On the north is a building occupied by printers, in the basement of which three boilers against the party wall furnishes power for the steam presses, and on the south a fine office building, the basement for rent as stores or shops. To have carried these party walls the sixteen stories, would have necessitated the removal of the boilers and the building of new foundations under each of the walls, requiring the use of each of the basements for some months., and from the necessities of the case entailing a very large expense, particularly the removal of the boilers, depriving that building of power until they could be reset.”
Jenney, concerned over these unique engineering problems hired engineer Louis E. Ritter to help design the building’s structure. The two of them resorted to the same concept that Root had first developed: they employed the cantilever to carry the loads of the new party wall away from the existing wall’s foundation to a new foundation set safely within the Manhattan’s lot. As opposed to Root, however, who had brought the loads of each of the new floors down, one on top of the next, and then cantilevered only the final column load at the basement away from the existing foundation with tall iron box beams, Jenney avoided the extreme cost of these large beams and the corresponding loss of space in the basement by cantilevering each floor out to the party wall.
As Jenney deemed that steel was also expensive to use, the columns were cast iron and the beams were wrought iron. He cantilevered each floor with double girders that spanned from the second column line in from the wall, over the first column line, that acted as a fulcrum, and ended in midair at the party wall. At this point, he constructed the new party wall with two wythes of 12” thick hollow tiles. Even though the existing building’s walls stopped at the Manhattan’s ninth floor, Jenney extended his wall one more story to the new tenth floor that the cantilevered structure easily permitted. Jenney could have continued to do this for the upper six stories with no problem, if he needed to do so. The resulting exterior form, however, would have been a massive rectangular monolith.
Instead, he stopped these upper floors at the first column line, that was the fulcrum for the cantilevers, and enclosed these stories with a wall of two wythes of 8” thick hollow tiles, the exterior wythe constructed with glazed tiles. This move created what would later be called a “setback,” or a stepback skyscraper. One can only surmise why this decision was made. Could it have been that it was more important to construct a record-breaking 16 stories, because the same amount of the building’s final floor area would have resulted if the cantilevered volume had been continued for only 14 stories. Or could it have been an overly cautious decision taken out of respect for the unknown wind loads on such an experimental structure to an unknown height, that had caused the building’s “sail area” to be prudently reduced?
Nonetheless, the wind loads in such a structure by this time were already being resisted with the help of diagonal bracing, quoting Buffington’s Cloudscraper or Gilbert’s Tower Building as two earlier examples. The Manhattan Building would be no different. Diagonal cross-bracing was located in the first line of columns in from both the north and south party walls, that were also the supports for the exterior walls in the upper six floors where they could run continuously to the top of the sixteenth floor. These braces were not in every bay of these column lines. The actual role of this bracing, however, has been called into question. First, the actual size of the braces was only a ¾” diameter iron rod with turnbuckles. This assembly simply was not sufficient in size to resist the magnitude of the forces generated by the wind on this size of structure. Second, when the Manhattan was being inspected for potential restoration, a number of these rods were found to have been removed at some earlier time, with no apparent effect on the building’s overall stability. Some of the removed braces had been located in the lower floors that completely negated all rigidity provided by the bracing located above it because the loads in the bracing then had no path to the foundation. It was speculated in an interim study published in 1978 by the Chicago Landmarks Council that the bracing was included to provide temporary bracing for only the ironwork during construction, until the building’s exterior masonry and floors were in place so as to give the building its permanent lateral rigidity.
Condit stated that lateral bracing was achieved through the rigid connections (known as portal bracing) made between the 15” deep wrought iron girders that were riveted along the entire depth of the web to an angle bolted to the cast iron columns. While the existing party walls could not support any new gravity loads without resulting in major damage to the existing buildings, these walls could easily be used to assist the resistance to lateral loads on the new building assuming they were continued to the foundations as basement walls, and were connected to the floors that tied the building together horizontally at each level. Thomas Leslie speculated that the interior hollow-tile partitions would also provide some rigidity as they would behave like the exterior hollow-tile party walls. This was, apparently, how Jenney’s structure behaved, therefore, the Manhattan’s lateral stability was achieved through a combination of its rigid-connected iron frame, the exterior masonry, the hollow-tile party and interior walls, the building’s floors, and most likely, the elevator core that is appropriately located in the exact center of the floor plan. I believe that in the Manhattan Building, we have a genuine candidate for Chicago’s first completely skeleton-framed skyscraper. Whether it was or not, the Manhattan’s framed construction proved a 16-story skyscraper could be built on Chicago’s soil: it did not suffer the excessive settlement of the Monadnock Block or the Auditorium’s tower.
Condit, Carl W. The Chicago School of Architecture: A History of Commercial and Public Building in the Chicago Area, 1875-1925. Chicago: University of Chicago Press, 1964.
Leslie, Thomas. Chicago Skyscrapers: 1871-1934. Urbana: University of Illinois, 2012.
Turak, Theodore. William Le Baron Jenney. Ann Arbor: UMI Research Press, 1986.
Weese, Harry and Associates. Four Landmark Buildings in Chicago’s Loop. Washington, D.C.: U.S. Department of the Interior, 1978.
(If you have any questions or suggestions, please feel free to eMail me at: firstname.lastname@example.org)