The first two weeks in May 1885 had been, indeed, a heady time for Burnham & Root. On May 1, the opening of the Board of Trade, not only did Burnham & Root’s Insurance Exchange quietly open, but Armour, Kent, and Bensley proudly used the occasion of the Board of Trade’s opening to announce that finally, after more than two years of false starts, construction of the Rialto Building, located immediately to the rear of the Board of Trade and proclaimed to be the world’s largest office building, was about to commence.
Immediately to the east of the Board of Trade, their ten-story Phoenix Building was preparing to start construction. And to finish listing the office’s workload, Root was still working on the final design for the thirteen-story Monadnock to be erected just one block further east of the Phoenix, at the southwest corner of Jackson and Dearborn. The staff at Burnham & Root seemingly had their work cut out for them. And then twelve days later it was announced that E. C. Waller’s bid for the old “Rookery” site had been chosen. Root would, within days, be designing an office building for this site that was planned to be even larger than the Rialto.
Unfortunately, precisely at this moment, the owners of the stone quarries at Lemont and Joliet chose to refuse what apparently was a just demand on the part of their workers for an increase in their daily wage. On May 3, only two days after the announcement for which Burnham and Root had been waiting somewhat impatiently for the past two years, that construction was finally to begin on the Rialto, the quarry workers struck, denying Chicago builders the material traditionally needed first for any large construction project: the cut stone for a building’s pyramidal foundations. The timing of this event, coinciding with his office’s upcoming construction projects, cannot be easily ignored in understanding Root’s development, precisely at this moment, of a new type of foundation that required no stone at all, thus allowing construction of the Rialto to proceed on schedule.
As discussed earlier, the size of a stone pyramid required to transfer the weight of a ten-story office building to Chicago’s relatively weak soil could easily exceed ten feet in height. The combined bulk of all the pyramids in the basement of a ten-story building took up the majority of the usable space in this level, preventing any significant use of this potentially valuable floor area, just at the moment when such space was becoming more of a necessity for the growing amount of machinery being incorporated in tall buildings (elevator, electric generators, boilers and ventilating fans, etc.) With the advent of electric lighting in the mid-1880s, commercial buildings were now being equipped with their own power generators, a location for which had to be found. The basement seemed to be the appropriate space, if only the Egyptian-like monuments located there could be reduced in size, or better yet completely eliminated.
Whether it was the lack of cut stone due to the strike, or a desire on Root’s part to open up the basement for the location of mechanical equipment, or a combination of these two issues, necessity proved to be the mother of invention: Root invented the modern iron-reinforced concrete pad footing. Three years earlier, he had used a layer of iron railroad rails in a number of foundations in the Montauk Block to reduce their overall height by one course of stone, so that the foundations would still fit within the basement without either punching their way into the ground floor or forcing Root to locate their bases deeper below grade than was thought to be safe in Chicago. This was simply a “one-off” solution to this specific problem because Root did not use iron in his foundations for the next three-plus years,
Now, almost four years later, the idea of using iron rails in a foundation, albeit at a much larger scale, had returned to him. Instead of using a ten-foot high stone pyramid allowing gravity to gradually distribute the load at 45° in compression to a sufficient area of ground, Root used the inherent tensile strength of the iron sections to directly transfer the same load in two-directional bending, completely eliminating the need for the 45° angled stone pyramids. Apparently, Root had experimented with concrete sufficiently by this time to be confident that this relatively new material would stand the test of time in such a critical location, for he eliminated all cut stone, opting instead to place a leveling base of concrete on the ground. On top of this, Root laid a line of iron rails or T sections, each one immediately next to its neighbor. These were embedded in a layer of concrete in order to hold them in place, and apparently in an attempt to prevent them from rusting (which, although slowing the onset of oxidation, does not necessarily prevent it). Upon this was laid another layer of rails, running perpendicular to the lower line of rails. After also embedding these in a layer of concrete, a third layer of rails were laid in the same direction as the original layer, and so on, usually stopping after four layers of rails had been laid, creating a two-dimensional grillwork or criss-crossed stack of rails similar to how rails are still stored in railroad yards.
The resulting thinness of this multi-layered assembly of iron grillwork encased within the concrete pad permitted it to be located directly below the basement floor slab, but still safely within the thirteen feet deep soil strata that had sufficient bearing capacity. Upon this concrete and iron pad could then be placed either a brick pier or an iron column section that was no larger in section than the part of the above-grade superstructure it would eventually have to support. This reduced the size of the structural elements in the basement significantly, resulting in a greater amount of usable space available for mechanical equipment. The elimination of the cut stone pyramid also made such a substantial reduction in the weight of each space-consuming footing that also had to be supported by the ground, that meant that either the overall area of the footing could be correspondingly reduced, or that the building could be made even taller (i.e., heavier) without exceeding the capacity of the supporting soil. Either way, so successful was the new footing that in a short time, cut stone would be entirely eliminated in the construction of building foundations.
Hoffmann, Donald. The Architecture of John Wellborn Root. Baltimore: Johns Hopkins University Press, 1973.
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