Construction History: Grillage Foundations

In any rapidly-changing technology, there are sometimes branches off the main stream that are critically important, highly developed, and short-lived. People work very hard to perfect something that they need, only to have it replaced by something better a few years later. Structural engineering is not now changing rapidly, but 125 years ago it was. Grillage foundations were part of the change to steel-frame technology in the 1890s and remained in use into the twentieth century, but are now so rare that the majority of engineers have never analyzed one.

The idea behind grillages was simple: there was little guidance available for analyzing reinforced concrete, so concrete was used mostly as a mass unreinforced material, effectively as geometrically flexible masonry. Steel was well understood and strong, but vulnerable to rusting. Given those materials, how do you design a spread footing, where a concentrated load from a column, pier, or wall had to be distributed over a large area to get the pressure low enough that the soil could resist it? The answer was a grillage, with two or more layers of steel beams at right angles to spread the load. The beams were either sitting on top of a layer of unreinforced concrete or embedded in a concrete footing. When the beams are embedded in concrete, it’s easy to mistake for a reinforced-concrete footing, but the structural action is different: in a grillage the beams are doing all the work rather than working compositely with the concrete, as in a modern reinforced-concrete footing. Improvements in reinforced-concrete analysis and materials are what eventually killed off grillages.

The picture above is from the construction of the American Surety Building in New York in 1895, and shows the steel columns sitting on bare grillages. In this case, the grillages weren’t on a concrete pad but rather on top of unreinforced concrete caissons. Here’s the grillage plan, showing the criss-crossing beams at each column and beams spanning between the perimeter grillages to support the exterior wall base. There are none of those wall beams on the Broadway side of the plan because there is a sidewalk vault there. North is to the left; the photo seems to be looking at the southeast corner.

The Wilkes Building, three blocks away, had a reasonably advanced system using inverted knee braces and plate girders to spread the load above the top layer of grillage beams:

The Spreckels Building in San Francisco, which had an advanced frame with some attempts at seismic bracing, appears to have had a grillage with a continuous mat of steel beams:

The Rand McNally building in Chicago used rails for the grillage, which is a throwback to the mid-1800s, before more slender I-beams were available:

The St. Paul Building, a block from American Surety, had as typical a grillage as existed:

Interestingly, two buildings with pre-steel-frame designs, the World Building in New York and the Drexel Building in Philadelphia, had the less-advanced inverted-arch type foundations:

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