The tallest example of a masonry bearing wall structure is the Washington Monument, started in 1846 and completed at a height of 555′ in 1884. The walls at the base are 15′ thick, and the total structure weighs 90,854 tons. I will use the monument from time to time as an example of traditional construction up to the early twentieth century, especially in comparison with the Eiffel Tower that I will use as an example of the innovative experiments with the iron skeleton frame during the second half of the nineteenth century. The Eiffel Tower was being designed as the construction of the Washington Monument was nearing completion. While the Eiffel Tower is almost twice as tall, it weighs only 8,000 tons, or 8% of the weight of the Washington Monument. In other words, one Washington Monument is as heavy as twelve Eiffel Towers, even though the monument is only half the height of the tower. (So to compare apples with apples, maybe I should say that the weight of the Washington Monument is as heavy as 24 Eiffel Towers with the equivalent height…?) In addition, the Eiffel Tower also took only 26 months to construct. Tradition versus Innovation.
Masonry load-bearing wall construction had definite limitations when used in skyscrapers, the best example of these are revealed in the Monadnock Block in Chicago, designed by Burnham and Root. At sixteen stories, it remains the tallest surviving unreinforced masonry bearing wall building ever built, but it was not, contrary to local legend, the tallest bearing wall skyscraper ever built. This honor belongs to New York’s New York World building by George Post (do you see a pattern emerging in the career of Post?).
Although stone and brick have excellent compression strength, especially compared to their corresponding minimum strength in tension, even these materials have a limit to the amount of compression they can support, beyond which both will begin to crush. As the height of a masonry wall increases, its weight corresponding increases in a linear fashion, and so does the internal stress in the bricks below. To put it simply, the taller the wall, the more load that needed to be supported that required more material in the lower floors to do so. This phenomenon was codified by building codes throughout the U.S. at the time, that typically adopted the requirement that a masonry wall had to increase four inches in thickness for every two floors of height.
Using a sixteen-story skyscraper as an example: if the wall in the top two floors started with a thickness of 16,” there would be fourteen floors below, divided by two, requiring seven additions of 4″ each. Therefore, the ground floor walls would be at least 44″ thick and this massive thickness created three crucial problems: loss of rentable floor area, reduced daylighting, and the foundations.
3.23. THE LOSS OF RENTABLE FLOOR AREA
First, and foremost, these early skyscrapers were speculative office buildings (even the first one, the Equitable Building needed the rentable floors to help pay for the Manhattan real estate) in which the owner charged the renter on a square foot basis. A 44″ thick wall would eat up too much rentable floor area. A simple calculation to see just how much potential income was lost to solid brick in our sixteen-story example would be to average the increase in thickness accumulated in the lower 14 floors: seven pairs of floors would eventually grow by 4” increments to an additional 28” in thickness. The average increase in wall thickness would be 28″ divided by two is 14.” Let’s assume a building footprint of 100′ x 60′. Now multiply 14″ by the number of feet in the exterior perimeter of one floor (14″ x 320′ = 373 sq.ft.). Now multiply this by 14 floors (373 sq.ft. x 14 = 5,226 sq. ft.). The top floor has a gross area of 5590 sq.ft. The net loss of income due to the increasing wall thickness is more than the loss of one entire floor or in the case of this 16-story building, just over 6%! But this was not the only loss of rent due to the extreme thickness of the lower walls.
3.24. REDUCED DAYLIGHTING
Second, because of the wall thicknesses increasing to 44” at the ground, the amount of daylight penetrating through a window in such a thick wall would be significantly reduced in the lower floors. This reduction in environmental quality would result in a less desirable quality of interior space, with a corresponding reduction in rent a tenant would be willing to pay for a space above the desirable first two floors (best access to foot traffic).
(If you have any questions or suggestions, please feel free to eMail me at: thearchitectureprofessor@gmail.com)
The Architecture Professor 