Third, and most important for skyscrapers in Chicago, was the fact that the sheer weight of the massive wall would create its own self-imposed height limit. This was due to the nature of Chicago’s underlying geology. Sufficient bearing capacity (3000 pounds per sq. ft) was immediately available for the first 12′-15′ of depth below grade. Below this stratum, however, the soil had a much weaker bearing capacity, which meant that foundations in Chicago had to be located within this first layer of good bearing. This posed an additional problem for builders in Chicago to which I will return to later. The self-imposed height limit for a masonry bearing wall was determined by the limited capacity of the upper layer of soil. At 3000 psf, a masonry bearing wall could be built only to a height of 10 stories, without resulting in excessive settlement. I said excessive because all buildings in Chicago at this time settled over time. The walls were built on pad foundations, that once loaded with the weight of the building, would put 3000 pounds per square foot of pressure on the ground that would eventually compress the ground accordingly, and the building would lower or settle over time. The settlement was a fact of gravity and physics. It couldn’t be eliminated, but the goal was to have even settlement across the length and depth of the building, or the building would experience differential settlement and the masonry would crack. I’ll return to this issue at a later time as well. The settlement was an accepted fact in Chicago, so much so that the sidewalks adjacent to a tall building under construction were typically built with a one to two inch slant up to the building, so that as the building completed its settlement over time, the sidewalk would settle with the building and would end up being level.
How this phenomenon created its own 10-story height limit was simply the fact there was also a limit to the area that a footing (a building’s foundation comprised of a number of footings) could be made. Prior to Root’s invention of the iron reinforced concrete pad footing during the winter of 1885/6, footings for buildings were built as stone pyramids, a shape that spread the concentrated load of the wall or column above to the weaker soil below. As gravity disperses force (the weight above) in a material at a 45° angle, the height of a conventional pyramidal foundation grew taller as the weight it supported corresponding increased. As the depth of the good bearing layer in Chicago in which this footing was located was limited to under 15’, the overall height of the pyramid was also limited to under 15.’
That is, if the footing was to be kept within the basement, without shooting its top into the ground floor. This area/height limit equated to being able to safely transfer the load of a 10-story bearing wall. Therefore, once the maximum size of the footing had been reached, any extra load which resulted from additional floors would simply increase the bearing stress beyond the 3000 psf limit and result in significant settlement (the 17-story tower of the Auditorium Tower settled almost 28”) unless the pyramidal footing was made sufficiently large in area to keep its bearing pressure at 3000 psf. Of course, as the pyramid grew in area, it also had to correspondingly grow in height. As it could not be buried any deeper than the 15’ limit, this would require that the pyramid would project out of the ground floor and into the first story that, of course, was unacceptable. But owners still needed or wanted to build taller buildings in Chicago than ten stories. The taller buildings meant that the long-tem settlement of the building would be excessive. The worst example was the tower of the Auditorium whose eventual settlement was such that one would have to walk down three new steps to the ground floor that had to be added at a later date. (This excessive settlement was partially the result of a number of issues, including the addition of extra floors after construction of the foundation had already begun.)
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