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A first time project, I need to build a 3’x3′ cantilevered landing onto an existing structure, and then attach stairs with 8’10” rise and 7’6″ run. The landing would be in contact with the structre on two sides (inside right corner) and free on the other two (outside left corner) Are there standard practice for a customization like this? For example, might a ledger lag bolted to the wall with bird’s mouthed braces work, and does changing the angle of support(thus changing the length of the supports) significantly alter the landing’s load bearing capacity?
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Dave, it doesn't really sound like a cantilever detail to me, more like a small deck that will be attached to an existing house/structure. Any reason that you can't just drop a post to grade at the outside corner? This could easily be made part of your stair handrail detail, and should be much less of a problem to design and finish than the kneebrace or corbel that I think you have in mind. Posts are typical, and usually humdrum, so this may be why you didn't mention them.
*The idea to use a corbel-type brace was to eliminate that post dropped to grade because it falls in the worst place. I need to open up the space below because it is a high traffic cut through/bottle-neck. The post would be Oh so much simpler, providing for handrail support, but that's another factor in the engineering.
*run the brace under the stair side of the landing, or whatever side keeps it out of the way of the line of travel... it's going to be a head knocker.or, run it to bisect the corner. That will keep it out of the way of the heads.30/60/90 degree triangle would be good on the angles. No less than 45, or the landing will try to pull away from the building...custom metal buckets are not so much money, provide reasonable shop drawings to a metal fabricator to attach the post to the deck, and the post to building. The building attachment should be done with a bucket that saddles the post, and attaches it to the corner framing that is strengthened to resist the push of the load. This angled saddle bucket should be nailed to the corner with large hardened hanger nails, or lagged in with small lags. A guess for the corner framing would be 4 2x6 nailed together well, and the 1 1/2" sides facing the post bracket. A 4x6 dfir post would do well too, with the 3 1/2" side facing the post. The lower you drop the post against the building, the more of the load is transmitted down. The higher up the building, the more is transmitted laterally into the wall.Or, set the post at grade next to the building, against the foundation, in a post saddle set in concrete. Let the foundation do the pushing against the post.tons of ways to go, how much work you will do depends on how nice you want it to look.
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Dave:
Nathan has some good advice. Here's a few added things: If possible, always use galvanized thru-bolts (a nut, bolt, and washers) rather than lag bolts.
Secondly, you'r gonna have trouble with those stairs that you are proposing ("8'10" rise and 7'6" run"). You do not have nearly enough run. To get more info about this, read < Obsolete Link > this thread on stair building and this article.
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A first time project, I need to build a 3'x3' cantilevered landing onto an existing structure, and then attach stairs with 8'10" rise and 7'6" run. The landing would be in contact with the structre on two sides (inside right corner) and free on the other two (outside left corner) Are there standard practice for a customization like this? For example, might a ledger lag bolted to the wall with bird's mouthed braces work, and does changing the angle of support(thus changing the length of the supports) significantly alter the landing's load bearing capacity?