Yesterday’s post about the Parachute Jump got me looking for more oddball towers. Most of the search turned up old stone forts, which are interesting in their own right, but not what I was looking for. Then I found the photo above, a circa 1910 view of a “New concrete observatory tower, Vicksburg, Miss” built as part of the Vicksburg National Military Park, so that tourists could get an elevated view of the Civil War battlefields. There were three identical towers, demolished in the 1960s.
Reinforced concrete was still a fairly new material in 1910, with no settled national code, competing ideas among engineers about how to analyze it, and used in limited forms of structure. This tower is definitely not one of the forms that people had experience with. A few details jump out at me from the picture, starting with the fact that the concrete columns are Doric style, or at least Dorickesque. Creating them was a lot of work in building formwork, and was presumably done to give the modern material a visual connection to antiquity. The circular-cut-out handrail, on the other hand, is very modern and shows off the kind of detail that would be difficult to build in stone but is readily achievable in reinforced concrete.
The structure varies from one tier to the next: The roof has six columns evenly spaced along the perimeter and three beams connecting them as diameters of the roof circle:
That star-shaped beam layout couldn’t be used lower down because of the presence of the stair. At the next level down, the beams are arranged in a rectangle that almost connects four of the six columns, with the other two connected by short secondary beams to the long sides of the rectangle:
The columns coming down from the roof land on the beams inboard of the columns from this tier. The same idea was used at the next tier down, but with the rectangle rotated 60 degrees:
Then things get funny. There are twelve columns below the next tier, arranged as two concentric rings of six, with the columns from above located between the two columns below, landing on what appear to be short radial beams:
And then there are again twelve columns at the base, with the inner ring either directly below the inner ring above or close to it, and the outer ring above again supported on radial beams spanning between the inner and outer rings below:
It’s a surprisingly complicated structure, with at least 24 and maybe 30 column transfers, which are always difficult to design in concrete. Also, unless the moment connections between the columns and beams were well-designed, there’s not a lot of lateral stiffness here. That may explain this first-hand account: “…the horror-film ascent with every step on the rattling, swaying circular staircase offering up a guess as to when the whole structure would collapse.” There are some more photos at the link, including one of the tower construction.
A design where the columns ran straight up (with each tier the same size) or with one setback, from the outer columns at the first tier or two to the inner columns from the base above, would have performed better, although perhaps it would have been less interestingly odd in appearance.