As the Leiter Building contained only one-fifth the floor area of the Shillito’s Store, there was no reason to incur the expense of duplicating McLaughlin’s use of iron girders in such a small structure. (The Shillito’s Store had a footprint of 270′ x 174′ while the Leiter Building’s was a mere 102′ x 82′. The Shillito’s Store had 567 lineal feet of six-story facade, while the Leiter Building’s five-story facades measured 184 lineal feet, over 85′ shorter than just the front of Shillito’s.) Jenney, therefore, used cast iron columns to support heavy timber beams at each floor.
The floors were constructed in a standard manner, using 3″ x 12″ wooden joists on 9″ centers that ran between primary beams upon which was placed wood decking. (Contrary to the popularly-held legend that wood construction was outlawed after the 1871 fire, wood had been used in Chicago’s buildings some eight years after the fire, and would continue to be so for another six.) The timber beams ran parallel to the Monroe Street front, meaning that the Wells Street piers would have normally been used to support the ends of the timber beams. This detail would, however, have required Jenney to increase the cross-section of the Wells Street piers to enable them to support the additional concentrated loads from the ends of the beams, and then to correspondingly increase the required section of the Monroe Street piers for symmetry (that was the apparent solution McLaughlin chose in the Shillito’s store as both sets of piers have the same dimensions). Doing this, however, would have resulted in a corresponding reduction in the amount of daylight received by the interior (Jenney’s primary concern as already seen in his design of the Portland Block).
To avoid this situation, he placed an 8″ x 12″ cast iron pilaster at the inside face of the Wells Street piers to support the end of the timber beams. The iron pilaster was not, however, continuous for all five floors, as the thickness of the piers in the fourth and fifth floors was reduced by four inches, with the iron pilasters stepping back with the interior face of the masonry. (The first person to realize the significance of this offset was historian Charles E. Gregerson.) The floor loads carried by the iron pilasters in these two upper floors appear to have been actually transferred to and carried by the lower three stories of the masonry piers, which seemingly is in conflict with the idea of eliminating the transmission of the floor loads to the masonry piers. This is evident when one reviews Jenney’s original drawings. The piers in the Monroe Street front decreased in thickness from 2′-8″ in floors one and two, 2′-4″ in the third floor, 2′-0″ in the fourth floor, to 1′-8″ in the top floor.
The Leiter Building’s two masonry street fronts were constructed in a method typical for the period, not unlike that used in the Shillito’s Store. The masonry spandrel at each floor level that spanned between the brick piers was constructed on an assembly of an ornamented cast iron window head that was bolted to two seven-inch deep I-beams. These were supported at each end by the brick piers and at third points by the two continuous cast iron mullions set between the piers Because the spandrels along the Monroe Street front supported the floor joists, this meant that some of this floor load was carried over to the piers (and Jenney may have rationalized that these loads, and the corresponding increase in their cross section paralleled the loads from the iron sections supporting floors four and five caused by the wall offsets). Not all of the floor loads carried by the spandrel beams, however, were carried to the piers because more than half of the floor loads along the Monroe Street front was supported by the non-fireproofed iron framework of continuous mullions and spandrels. However dangerous this detail may seem today, it was standard practice throughout the country for more than fourteen years after the 1871 fire.
In summary, the structure designed by Jenney in the Leiter Building was a strange amalgam of wood beams and joists supported by cast iron columns in the interior, and at the exterior by a curious hybrid of iron and masonry. As the iron framework of mullions and spandrels between the exterior brick piers had no mechanical connection or relation to the iron pilasters behind the Wells Street piers and the timber beams they supported, Jenney cannot be given credit, as he at times is, for conceiving this building as an early essay in iron skeleton framing. Some historians have claimed that Jenney’s use of iron sections in the Wells Street piers had significant architectural aesthetic consequences; i.e., the “now-slimmer” brick piers permitted a maximum of glass with a corresponding gridlike read in the elevation. This is simply untrue at two levels: first, the Shillito’s Store had no iron sections in its piers, yet it had a maximum of glass (indeed, because the building was taller, the piers were carrying more load) and its elevations were grids; and second, while the Leiter’s Wells Street piers had the iron sections, the Monroe Street piers did not, yet the Monroe Street elevation had a maximum of glass and read as a structural grid. I have to disagree: the insertion of the iron sections had no aesthetic consequence that resulted in an expression of the structural grid. McLaughlin had already proved this not to have been the case.
Nonetheless, Jenney’s use of iron sections along the building’s exterior piers represents the first example of the return of iron sections in a building’s exterior piers since the fires of 1871-4 had shown the futility of using unprotected iron columns in a building’s exterior. In essence, it marks the start of the slow but inexorable incorporation of iron columns back into the exteriors of multistoried buildings, especially in those erected in Chicago during the upcoming decade.
FURTHER READING:
Randall, Frank A. History of the Development of Building Construction in Chicago (2nd ed.). Urbana, University of Illinois Press, 1999.
Turak, Theodore. William Le Baron Jenney. Ann Arbor: UMI Research Press, 1986.
(If you have any questions or suggestions, please feel free to eMail me at: thearchitectureprofessor@gmail.com)
The Architecture Professor 