Failure Portrait: It Doesn’t Quite Fit

I haven’t done one of these in a while, partly because of the Covid-19 lockdown and partly because of the luck of the draw in which projects I’ve visited. I like this photo because the failure is quite subtle but also completely visible.

That’s a piece of a stone-masonry facade, visible up close from a scaffold. Obviously the corner of one stone has failed, but that’s not very rare or interesting. I believed, but could not prove, that there was overall movement of the facade; seeing this close gave me my proof.

The piece of metal protruding from the stone on the right is a “cramp” – basically a big staple made of out of steel and (in this case) rustproofed with paint. Ordinarily, I’d assume that the failure of the corner of the stone on the left was caused by rust-jacking, by pressure entered by the cramp as it got bigger from rusting, but the cramp is in good condition and has not lost nearly enough material to explain the damage.

The horizontal portion of the cramp is, as is usual, in a groove cut into the top of the two stones. The vertical legs are in squared-off holes at each end of the groove. If you look closely, you can see what’s left of the hole on the left:

The back side of the hole is partially still there, a piece of the left side, and even a small piece of the right side. What’s interesting about the remnants of the hole is that the end of the cramp doesn’t line up with them. The left edge of the cramp is to the right of the right side of the hole, so it could not have been installed like that. The misalignment is easy to explain if the stones have moved since construction, as I suspected they have. And the corner break is also easily explained in this case: if the overall movement of the masonry was causing these two stones to move away from one another and the cramp was the only thing preventing the movement, it would have exerted a lot of pressure locally at its vertical sections in the holes. And locally-concentrated pressures like that are an easy way to split stones. The corner broken off and allowed the cramp to slide out of what was left of the hole.

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