huangyhg

2 x 12 span
will a 2x12 douglas fir garage(75 psf)floor joist span 20 feet at 16 o.c. if i install 2 rows of blocking and use 5/4 t&g subfloor?

are you a structural engineer?
mike mccann
mmc engineering
no, i am a commercial structural inspector.
i haven't run any numbers, but that seems like a long span for a 2x12, and it's heavier than typical loading. also, the grade of wood will matter.
that puts bending stress close to 1000psi- too much for no.2's but okay for select structural. thats not checking deflection though which i would guess would be high.
space them at 12" o.c. and your bending stress is under 800 psi so would be okay for no 2's but still deflection may control.

not even close. your spans and spacing are near the limits for a roof rafter, and your stated loads are 3-4 times roof loads.
vteit-
just out of curiosity where did you come up with 800psi for 12" spacing? i just did a quick check and got over 1400 psi (and that's for no load other than the 75psf).
m=75*20*20/8*12=45000lb-in.
s=1.5*11.25*11.25/6=31.64in^3
fb=m/s=45000/31.64=1422psi
again, that's neglecting any other load besides the 75 psf, which he's likely considering as a ll.
op-
this tells me that the 16" spacing won't work, the 12" spacing probably won't work (depending on the grade) for strength and/or deflection.
without a 3" concrete topping that is usually present, 2x14's @ 12 might make it, df #1 & better. but, no way 2x12's.
mike mccann
mmc engineering
sorry i am answering a question with a question, but are these joists on a stamped drawing? perhaps it's worth calling the engineer with a friendly inquiry.
wow- i get 1422 also doing it over- i was using my spreadsheet earlier for a doubled up carrier beam and failed to change 'b' back to 1.5 from 3.
at least we got the thing doing it two different ways (i do my/i...guess its no different than using section modulus it just rearanges variables..

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