Hidden Technology Transfer

When people took the idea of braced iron frames from bridges and started using it for buildings, it was clear for all to see, at least during construction. Before the metal framing was hidden by brick or terrace cotta, it was visible above the construction fences surrounding the building sites. I’ve put up any number of pictures here showing that. But another important idea transferred from bridges to buildings – the use of pneumatic caissons to carry heavy loads through poor soil to rock (or at least to better soil strata) – was literally invisible because it was entirely buried. This is on my mind because I’ll be talking about this topic with Tom Leslie on Tuesday, with Jared Green refereeing.

The picture above is the Eads Bridge over the Mississippi at St. Louis, built 1867 to 1874 and one of the masterpieces of nineteenth-century engineering in the US. Everyone’s attention is drawn, naturally, to the huge steel arches, but the foundations for the mid-river piers are worth talking about. They were, if not the first pneumatic caissons in the US, the first at a large scale. They were the largest caissons built anywhere until they were surpassed by the caissons used at the Brooklyn Bridge shortly afterwards. Two pictures give the general idea:

The hexagon on Plate VII is two half plans combined: the left side showing the roof of the caisson proper, and the right side showing the working chambers below the roof. (In rather poor graphic design, several letters are used to represent more than one thing, so “B” is both the air chamber (i.e., the space for workers to excavate sand) and the middle of the long side at the bottom of the drawing.) The short version is that this is an iron box with a top and sides but no bottom, with iron and timber girders reinforcing its roof. It was sunk to the river bed some 33 feet below the water surface, filled with compressed air, and then people went inside and excavated below. As they undermined the box and it sank, the masonry of the river pier was built on its back. When the box hit bedrock, some 100 feet below the surface, it was filled solid and left in place as part of the foundation. The compressed air kept the river water out, but the had the terrible side effect of causing the bends (i.e., caisson disease) among the workers.

Pneumatic caissons seem weird until you get used to the idea. We tend to think of foundations as immobile, but here they are being moved down, through soil, into position. Undermining a foundation element is terrible, unless you are doing it deliberately. In any case, they were used extensively in skyscraper starting in the mid-1890s because they provided a safe way to put heavy concentrated loads onto bedrock below poor soil. (Improved rules about the speed with which workers could move through the airlocks greatly reduced the prevalence of caisson disease.) They were extensively used in Chicago, where poor soil is everywhere, but they were also used on a number of tall buildings in New York (and, of course, other cities) where soil was locally bad.

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