A Weakness Masquerading As Strength

A view of the ground floor os a small industrial building in the Bronx:

The building has no cellar, so this is its bottom. The interesting thing here is the design of the column bases, but I’ll get to that in a minute. The floors are wood joists in two bays: spanning between the side walls on the left and right and a line of steel girders down the middle; the columns support the girders. (You can see one of the girder, painted green, just to the left of the big red pipe.) So, if we engage in some categorization, this is a bearing-wall building with some interior steel framing.

Those trapezoidal plates and the projecting T-shaped brackets at the bottom of the columns look like they are there to transfer moment from the columns to the (hidden) footings. They’re not there, for example, just to keep the columns upright during the 1910s construction: steel material used to be much more expensive, relatively, than steel-workers’ labor, so temporary and re-useable bracing would have been used. But moment-base columns were quite rare a century ago, largely because of the difficulty of designing footings that could carry the moments properly to earth.

There’s another, more practical reason that the columns don’t have moment-capable bases: there’s absolutely no reason for that. This is a bearing wall building, with the four exterior masonry walls acting as shear walls to resist lateral load. The columns are required to carry gravity load only, straight down from the girders to the foundation, and could be pin-ended without any loss of capacity.

Not to drag this out for too long, the brackets are there to solve a problem with the foundations. Concrete design was quite backwards in New York at that time, and the limits on allowable concrete stress meant that designing the kind of reinforced-concrete spread footings we use today was difficult. Unless you made the footings very deep, it was difficult to make a footing, even a small one, work as a cantilever. For this reason, steel-grillage foundations (whether encased in concrete or not) remained popular into the 1930s: the heavy loads were carried by structural steel. But steel was expensive, so you wouldn’t want to use more than was absolutely necessary for a little Bronx industrial building. If you used brackets attached to the columns bases to spread the load out well past the column itself, you greatly reduced the distance that a reinforced-concrete footing had to cantilever, and so made the concrete footing feasible. In other words, those brackets used less steel than grillages would have, so they were a bargain.