Historic Structural Detail: Prefab

by Don Friedman on August 25, 2016

A column in the same 1920s concrete building as yesterday [click to enlarge]:

IMG_1771

The column has been stripped of its original paint, exposing the rough surface of the concrete. The form marks are clearly visible on the capital, and a little less distinct on the shaft.

In theory, the columns could have been formed in wood, as the floor slabs were. (Form marks from wood plank can be seen on the sale underside at the top of the picture.) Cylinders and cones – the column shafts and capitals – are “developable surfaces” which means they can be formed from a series of straight lines. A hemispherical dome, for example, is not developable and therefore extremely difficult to form using timber. But, critically, it would be quite time-consuming for carpenters to form the round columns and tapered capitals.

As reinforced-concrete construction became popular for industrial buildings between 1900 and 1920, the use of prefabricated steel forms also became popular for elements that were difficult to form. The seams of the form seen on the capital show that one piece of form covered the capital and column top (vertically) and a 45° sweep (around the circle). To create that form in wood would be finish-quality cabinetry; to create it in steel was an industrial process.

One interesting result of the use of steel forms was that columns were created in a limited number of sizes. Why fabricate and store large numbers of different size forms for use in industrial buildings where no one really cared about the appearance or minor loss of space from inefficient column sizes? Despite concrete’s reputation of being able to be formed into any conceivable shape, the reality of economics and logistics forces the material into rather predictable patterns in most buildings.

 

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