You guys are too darn fast for me, I’m still thinking and working around posts #7 - 13. I suspect that your balcony table R502.3.3(2) is pretty much on the money, but it’ll take me awhile to get to that post and think on it a bit. At the moment, I can’t think of why I wouldn’t treat the balcony essentially the same as this deck and handrail canti. Except it does appear that there is a difference in the IRC LL’s, with decks at 40#/sf and balconies at 60#/sf.
TimNY..... I don’t think most of you are engineers, but some of you have a pretty good feel for some of the engineering involved. And, what I have tried to say is you should be very careful about pretending to practice engineering if you don’t know what you’re doing; but also to do your job well you must have a fair amount of basic engineering intuition to understand, interpret and enforce the code. This is doubly true because no prescriptive code (any code for that matter) can possibly cover every condition under every situation, and yet you are being asked every day to stretch the IRC; but please don’t make me go see an engineer and have to pay him for his advice, and use him as our insurance of last resort.
GH.... I think you might want to get your calculator recalibrated, maybe even overhauled, before you do any more PEing. You do some unusual engineering, post #7.
Jeff... The OP was a good question, and my additional questions (brain teasers) were just to see if anyone knew how to start approaching this problem. The code’s admonition about the potential for uplift at the house ledger is an important one, and if you don’t have a feel for how to do this problem, how do you know when to say, ‘get an engineer to design this.’ And, I’m not trying to turn any of you into an engineer, I’m just trying to teach you how to think a little like an engineer on the questions I chime in on, or to help you know when you should stand your ground and say, “this doesn’t comply with the IRC, get an engineer’s help, then come back for your permit. That still doesn’t assure you it’ll get built right, but it’s a step in the right direct. When you switch to 24" o/c jst. spacing the uplift/ft. on the ledger doesn’t change at all, but the uplift per jst. or on each jst. connection increases by a factor of 24/12 or 24/16.
I’m a fairly conservative old guy, but the way I see the deck cantilever (canti.) is as follows:
1.) The uniform fl. ld. gives you a canti. moment of, -M = wa^2/2, or (50#/sf)(2.5')^2/2 = 156.25'#/ft. of width, or 208.33'#/jst.
2.) The hand rail at 15 or 20#/ft. gives a -M = Pa, or (20#/ft.)(2.5') = 50'#/ft. of width, or 66.67'#/jst., but this may really only be applied in a more concentrated fashion to one jst. ever 5 or 6' at a guardrail post.
3.) The 200# horiz. load on the handrail at 36" high causes another -M = (200#)(3') = 600'#, the worst case is this horiz. load at a handrail post, and this moment goes into 1 or 2 jsts. depending upon how you attach the post to the deck framing system.
4.) I would check a single jst. canti. with the sum of these three -M’s, shears and the resulting uplifts, the worst case condition, and maybe adjust my framing and attachment details if I had trouble making a single jst. work for the size jst. dictated by the simple beam backspan condition.
5.) I believe the 40 or 60#/sf LL debate goes something like this: we have the normal 40# fl. load, and we have a 40# snow load, and decks are notoriously prone to drifting off the adjacent higher roof. Now, it’s not likely that anyone would hold a dance party in a snow drift, but either the drift or some people out on the deck with some snow is highly likely. Otherwise, the difference btwn. a deck (LL = 40#/sf) and a balcony (LL = 60#/sf) might be some conservatism because the balcony is solely dependent upon canti. action off the exterior wall, while the deck is simply supported by a ledger and a beam near the other end of the fl. jsts., a more forgiving structural system. It would be interesting to hear what a real code historian had to say about this, if any one of you know some of those type people. On commercial balconies you have vibration considerations, and the potential of everyone in the stands stamping their feet after the touchdown, or the whole crowd moving at once. the wave or at exit time. This is akin to the soldiers marching in unison and bringing the bridge down.
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