according to it, if i use 2 2x12 syp to make the beam, i can place posts 12' apart if the joist span in no more than 9'. if the joist span is 18', the posts should be 6'6' apart. of course i will be using fewer joists and carrying a lighter load so who knows how much more i can go - i was hoping someone might have that information.
design loads are for pounds per square foot and are designated with a symbol in the tables. to minimize serviceability related problems, such as cracking of finishes, deflections are typically limited to span/360 for plaster finish, and span/240 for drywall finish.
a double 2x12 beam can span 12 feet; a 2 2x10 can span 10 feet and so on. need pergola rafter span advice : diy you can use 50 x228 2x8 pine timber will work over that span.
your question leads me to believe that you intend to make a beam with two 2x12 nailed together to span a 16 foot wide area. that would make the floor joists on either side of this beam 10 feet long. there are quite a few factors to consider in determining whether this is a correct solution.
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tomw702 >> yes, the 2x12s are being used as a ridge beam. the span of the ridge is 23 feet. i live in the pacific northwest, and we see snow yearly, but generally not a lot and not for very long. at most the roof would probably see 2 or 3 feet of snow, and that is a *very* rare occasion.
the joists under my house are 2x12, max unsupported span is 12 feet. i think you will be ok to go 13' - 14' using them overhead, assuming you don't build above that. your local building inspector can tell you what code calls for.
it seems 2x12 fir will span a 28'4' space. regardless, i'd be looking at one of the commercial parrallam pvl style products rated for outdoor use for this span. you can clad it with whatever your material of choice is afterwards and you don't have to worry about welding or attaching wood cladding to metal afterwards.
i have an issue im trying to build a pergola with 2x12x24 the total unsupported distance is 18.5' but that is from post to post im thinking about doing some 4x4 bracing in as well to shorten the span with that it brings the unsupported distance down to 13 feet ish thats the long side its 4 beams. one on either side of a 6x6 that are pocketed.
re: 18' open span, doubled or tripled 2x12's? ok, just to clarify, i am talking about doubling or tripling the 2x12's that span the 18' direction and then hanging single 2x12's spanning appx. 8' between them. thanks for the input.