An Iron Appendix

by Don Friedman on January 16, 2018

That’s a photo of an areaway in London. The wall on the right is the building and I’m standing at the corner of the areaway. The black coping along the left is on top of the curb that’s an extension of the areaway’s outboard wall, extending above the sidewalk.

If I had to guess, I’d say that the house was built in the middle of the nineteenth century, although I could easily be wrong by fifty years either way. In any case, both the areaway wall and the house are unreinforced masonry. The material matters because the areaway wall is a retaining wall holding back the soil and water below the sidewalk, and unreinforced masonry does a lousy job of working as a cantilever retaining wall. You can see that the top of the wall is a little wavy from movement.

The two black arches are cast-iron struts to help stabilize the wall by transferring lateral earth pressure to the building. I like them very much: the iron is elegant and far lighter than a masonry brace would have been. On the other hand, their shape makes no sense. These are compression struts that are effectively pinned on the ends, since there’s no way the bolts from the iron to the masonry are capable of preventing any movement in tension. The normal shape for a compression strut has the same cross section for the whole length; an idealized form would be bigger in the middle to reduce the likelihood of buckling. The shape here is backwards, narrowest at the middle.

I strongly suspect that the reason for the arched shape is a carry-over from earlier construction forms. Before iron was readily available, this type of compression strut was usually made as a masonry arch. The arch’s own weight wold serve to counteract the tendency of the arch to buckle upwards under axial compression. If that type of arch sounds a bit peculiar, it sounds even stranger when called by its usual name: a flying buttress. The material changed, but the shape didn’t, putting this in the category of forms that survived past their usefulness.


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