Sometimes grillages were used in combination with other forms of foundation. When I was discussing them before, I pointed out that the American Surety Building has grillages over concrete caissons; if I return to my favorite, the Park Row Building, it has grillages over masonry pile caps.
The picture above is the full foundation plan of the building, courtesy of the UK journal Engineering. If it seems odd that Park Row would get international attention, remember that it was the tallest occupied building in the world for eight years. The building has an oddly-shaped plan because of the combination of it sitting on an oddly-shaped block (because of Park Row’s diagonal path) and the vagaries of assembling lots for a large building. Park Row is at the bottom of the plan, and we’ll call that the north side, because that’s as close as anything else. Theatre Alley*, which is a tiny street, runs diagonally at the top, and there’s a short facade on Ann Street** off to the right.
The small circles on the plan represent wood piles, driven into the sand that covers the site. The detail at the far left is, when turned 90 degrees counter-clockwise, a section through the foundation. Here’s a close-up:
There’s a layer of unreinforced concrete over the site, then a layer of granite directly over each group of piles, then pyramids of brick over the granite, and then another layer of granite to top off the masonry. These piles of masonry*** are the pile caps. The grillages then sit on top of the masonry, distributing the concentrated loads from the columns. Here’s a close-up****:
Column C3, with the smaller foundation at the top, has 42 piles and a grillage with a bottom layer consisting of six 15-inch-deep I-beams that weigh 45 pounds per foot, and a top layer consist of three 24″ x 68# beams. Column C2, with the larger foundation at the bottom, has 63 piles, 15-inch beams in the bottom layer and 24-inch beams in the top layer. One of the keys to constructing large frame buildings is the use of independent, analytically-designed foundations. Column C2 has a larger load than column C3, so it has a larger and stronger foundation. This makes the foundation pressure below each column roughly the same and so minimizes potentially-destructive differential settlement.
From our perspective, the foundation system seems a little odd. We have a modern steel skeleton frame sitting on steel grillages, on masonry piers, on wood piles. But if you look at the tools that the designers had to work with in 1898, it makes perfect sense. The sand sub-grade wasn’t terrible material, but was not strong enough to take footing loads by itself. The grillages spread the pressure from the columns so that it was low enough to be safely handled by the masonry pile caps, which then distributed it to the piles. Each piece is logical by itself, although the overall system could be – and eventually was – improved.
* Theatre Alley is one of a small handful of streets in New York that use English rather than American spelling, for some unknown reason.
** Ann Street is tiny by any rational standard, but it’s a lot wider than Theatre Alley.
*** The “piers” in the note are the masonry pile caps, not the piles.
**** The blank squares on the big plan are similar to the nearby foundations of the same size. I can hardly blame the drafter for not wanting to replicate all that detail.
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