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Short-Term and Long-Term Changes

A close-up view of a connection in the lowest level of an old mill. (More on this project to come…) There’s something interesting going on here, but it’s a little convoluted. First, an explanation of what you’re looking at:

The green arrow is pointing at one of the main floor beams, supporting the ground floor of the mill and running continuously the full width of the building. The red arrow is pointing at a post that runs from the foundation to this level only, roughly halfway across the width. And the purple oval surrounds a cast-iron post cap, meant as a connection between the post and beam…except that there’s a two-inch gap between the underside of the beam and the top of the cap. You can’t see it in this picture, but the beam is fairly straight, with no significant deviation down (sag) or up in the middle of the span.

The first thing this condition tells us is that the beam is not heavily loaded relative to its capacity. If it were, it would be sagging and in contact with the post. The amount of sag in a beam varies based on geometry, load, and the elastic modulus of the wood species, but seeing three or four inches of sag in a heavily-loaded beam of this span would not surprise me, and that’s more than it would take for the beam to touch the column cap and start transferring load there rather than continuing to span the full distance. Second, the gap appears to be an error in construction, as the similar posts and post caps at the other beams are in contact with those beams, which also show no significant sag.

Third, something here has been altered. The mill was built in the first decade of the nineteenth century and these iron post caps are a detail generally used in the US after the Civil War – that is, after 1865. Maybe the posts were added to reinforce the ground floor as new machinery was installed over the decades. Maybe there were always posts there but the caps were added later. Or maybe there were old posts there that rotted in the damp cellar and had to be replaced, and were replaced with contemporary caps. So long term, something changed here and it would be helpful for me to know why. (This is a new project and it’s entirely possible that other people involved already have the answer to this question.)

Back to the first point for the last one: if this were still an operating mill, with loads that changed as materials were brought in and product shipped out, the floor would move up and down on a regular basis, in sync with the load changes. If the loads at any time were heavy enough to make the beam sag until it made contact with the post cap, the extra support would help greatly. So in the short term, the structure will change load path depending on the load, which is cool and not something that old buildings were – as far as the general literature goes – designed to do.