Historic Structural Detail: Grillages

by Don Friedman on March 14, 2016

Just stating the facts of a piece of technological history can point up curiosities. Steel skeleton-frame construction dates to 1890, but reinforced concrete construction lagged behind; the details of steel framing developed into recognizably modern forms much faster than their concrete counterparts did. So…what did the columns of those steel frames sit on, if not reinforced-concrete footings or reinforced-concrete pile caps?

An early solution was to continue the use of masonry footings and pile caps as had been used for centuries before steel framing was possible. (For the rest of this post, wherever I say “footings” you should read “footings of pile caps.”) Some of the solutions make sense, like pyramidal brick footings to gradually spread the load; some not so much, like inverted arches. The problem with the continued use of masonry details was that the loads that the steel columns were carrying grew rapidly quickly as the new steel technology enabled designers and builders to create larger and taller buildings.

Faced with a materials palette of wood, masonry, and steel, the logical choice for engineers of the era was to make steel foundations. The column would be supported on a steel beam, which would rest directly on a group of beams at right angles. Multiple layers of beams could be used if necessary, each layer extending the spread of the beams wider. All of the steel was encased in concrete to provide a continuous bearing surface against the earth and some measure of waterproofing.

Grillages are not reinforced concrete. The steel is doing all of the structural work of interest and could more or less continue to do so if the concrete were removed. They are, however, a step towards reinforced-concrete footings, and were used well into the twentieth century because they perform so well.

The nature of grillages is such that they are difficult to photograph: they are a bunch of steel beams buried inside concrete. A picture of one glob of concrete looks much like a picture of another glob. This may be the best we can do:

IMG_4612

This is an elevator pit, hence the big springs – AKA buffers – at the bottom. The pit is immediately adjacent to a column footing, so we’re seeing the ends of the beams at one level of the grillage just barely poking through the wall of the pit.

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