brianVT
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# Posted: 14 Mar 2021 04:42pm
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I'm in the planning stage of a cabin with a ~16'x16' footprint and I'm spec'ing out some of the structural elements. Looking for some second opinions or thoughts.
It's going to be a single-story with a gable roof and shed dormers (like the image below, but obviously not on a trailer). In the rear half there will be a 8' depth loft spanning the full width.
At the moment, I'm a bit stuck on the "compromise" of designing around the fact that half the structure can be horizontally "tied" (i.e. the loft's floor joists effectively double as rafter ties) and the remaining half will be open/cathedral.
If the structure were completely open, I'd obviously have to rely on a structural ridge beam and/or collar ties to keep the side walls from pushing out. The ridge beam would need to bear the tributary area (16'x8' at 60psf for snow/dead load = 40lbf/in). On the other hand, if I went with a full attic and had ceiling joists, I could use a non-structural ridge board which, in principle, carries nothing. In between those two extremes, it's clear that the loft joists do significant work in reducing the load on the ridge beam.
This "half-and-half" design allows me to reduce the size of the ridge beam. However, to cover all my bases I am still planning to use a post at the midpoint of the ridge beam. That post would land on the joist that makes the front edge of the loft space (like the sketch below).The post goes a long way in terms of reducing bending stress and deflection of the ridge and can also help as a place to nail a loft railing for safety. I consider it a very simple solution to ensuring my ridge beam is VERY safe.
However, I'm a bit stuck on sizing the loft joist that will carry that post. If I pretend that I'm building a full cathedral ceiling (i.e. zero joists/rafter ties) then a mid-span post would carry ~4000lbf with the loading assumptions given above. Of course, on the other extreme of an attic/ridge board, there would be "zero" load on a vertical member.
Given that the joist supporting the post has to support the loft load (30psf) AND a mid-span point load (4000lbf), I'd have to get something like a 8x12 (which deflects ~0.4", under the L/360 criterion), but that would probably look a bit ridiculous in a small space. Also don't want to move something that large!
I kind of assume I'm splitting hairs here on a project of this size...but I obviously want it to be sound/safe. At the moment, I'm leaning towards a 6x10 which would (in theory) allow it to carry the loft load AND a 2000lbf point load right at the L/360 limit.
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