At Least Dead Load

Another of Ellen’s pictures, this time from an abandoned fort:

This fort upstate was built starting in 1842 and designed by two quite famous military engineers, Montgomery Meigs and Joseph Totten. In other words, any limitations on the design were ones of budget and the available materials and labor, not of skill or imagination.

The parapet¹ walls are, as you can see, mostly built of stone, which I assume is native to the Lake Champlain location. Those stone blocks were cut and assembled with some real skill, but cutting voussoirs for arches is significantly more difficult: if you get the angles wrong for the side of the blocks, you can weaken the arches. All of the arches at the interior the fort, whether for doors, casemates, or vault roofs, are brick.

Abandonment has taken its toll, and the arch at the center of the photo has lost close to half its material at the center. The brick that’s left is mostly damaged, and the mortar joints are badly eroded. On the other hand, this arch was never supporting much load. There used to be a stone running from the empty space on the left above the arch to meet up with the stone above the arch on the right side. And that’s about it: there was nothing else in the empty space above that stone.

I suspect the purpose of this arch-and-flat-stone combination was to serve as a compression link between the wall to the left of the opening and the wall to the right; to keep them from moving independently. Load sharing between different segments of wall is a great idea if you are, for example, expecting people to be firing heavy cannonballs at high velocity into them.

So the arch was, in terms of gravity, carrying its own weight and the weight of the stone above, and that stone is now gone. As bad as the arch is, it can carry it’s own weight, which is provable by looking at it: if it couldn’t, it would collapse. If left alone, there will be continued small-scale material failures – for example, continued mortar loss will lead to more bricks dropping out – until that triggers an overall arch failure because there will no longer be a viable load path. Hopefully the project that we are involved with will improve conditions before any of that happens.

1. “Parapet” in the military sense: the enclosing walls of the fort. ↩︎