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2x6 span 16’

A

asmith1923

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Joined
Dec 20, 2023
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9
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Haymarket
I’m building a 2 story apartment inside my existing pole barn and am very restricted on height. I can only use 2x6 for the floor joists. Most of my spans are 10’ or less but a couple spots I need to span 16’. Is there anyway by doubling them or a tighter spacing than 12” oc that will work? I could do LVLs in these areas but haven’t been able to find a span chart for floor joists. I’ll have a live load of 40 psi in one area and 30 in the other.

Any help would be greatly appreciated.
 
Even for a 30 psf live load, the best grade of the best species only spans about 12 feet with 2x6s at 12" on center. Comparing the maximum spans for joist spacings of 24", 16", and 12", you don't get anywhere near double the span by reducing the spacing by 50%. Since there are no tables for 8-inch joist spacing, it will have to be calculated. Imy state, that's engineering so, even if you know how to do the calculation, you can't submit your calcs to a building official unless you are a licensed architect or structural engineer.

Without crunching any numbers, I'm going to say you might be able to do it using Doug Fir or Southern Pine at 8 inches on center for the 30 psf area, but I don't think you have a prayer of doing it for 40 psf.
 
Yankee Chronicler said:
Even for a 30 psf live load, the best grade of the best species only spans about 12 feet with 2x6s at 12" on center. Comparing the maximum spans for joist spacings of 24", 16", and 12", you don't get anywhere near double the span by reducing the spacing by 50%. Since there are no tables for 8-inch joist spacing, it will have to be calculated. Imy state, that's engineering so, even if you know how to do the calculation, you can't submit your calcs to a building official unless you are a licensed architect or structural engineer.

Without crunching any numbers, I'm going to say you might be able to do it using Doug Fir or Southern Pine at 8 inches on center for the 30 psf area, but I don't think you have a prayer of doing it for 40 psf.
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Thank you. I appreciate the help
 
asmith1923 said:
I’m building a 2 story apartment inside my existing pole barn and am very restricted on height. I can only use 2x6 for the floor joists. Most of my spans are 10’ or less but a couple spots I need to span 16’. Is there anyway by doubling them or a tighter spacing than 12” oc that will work? I could do LVLs in these areas but haven’t been able to find a span chart for floor joists. I’ll have a live load of 40 psi in one area and 30 in the other.

Any help would be greatly appreciated.
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What species and grade of wood are you using?
 
AWC's span calculator says that 2x6 Select Struct Douglas Fir carrying 10 psf live and 5 psf dead can span 16' 5" at 16" o.c. https://awc.org/calculators/span-options-calculator-for-wood-joists-and-rafters/

So for that grade and species, for 30 psf live and 15 psf dead, you could use (3) 2x6s every 16" o.c.; and for 40 psf live and 20 psf dead, you could use (4) 2x6s every 16" o.c. Not very practical, you'd be much better off using 2x8s.

Also, you'd probably need to get an engineer to sign off on it. You could try referring to IRC Table R802.5.1(2), on ceiling joists spans. It says that Douglas Fir Select Struct 2x6s at 12" o.c. carrying 20 psf live and 10 psf dead may span 16' 4" with a deflection limit of L/240. That means double 2x6s @ 12" o.c. could carry 40 psf live and 20 psf dead with the same deflection limit. But that deflection limit is too low, you need L/360; moving up to triple 2x6s @ 12" o.c. would further decrease the deflection limit by a factor of 2/3, to L/360 as required. And 3 @ 12" o.c. is the same as 4 @ 16" o.c.

Again, not very practical. Your floor system would be 37% solid wood.

Cheers, Wayne
 
I seen a couple of cases where "homemade" beams were made sandwiching a piece of 3/8" steel plate between 2 wooden beams and bolting through the assembly. Depending on how many beams you're going to need this may be a good approach however you probably should get a engineer to sign off on it or else the inspector is going to have a fit when he does a inspection.
 
asmith1923 said:
Southern Yellow Pine No.2
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Yeah so Yankee Chronicler is pretty spot on, I looked at the span charts, but, depending on the amount of 2x6s that have to span 16ft the building inspector may be a little forgiving if it’s just a few. I definitely think they would probably say put them on 8” centers. But if that’s a no go you will have to get a letter from an engineer on it. Not sure if this is possible but there is no way of putting a beam in the floor system where they are going to be 16’, this would allow you to cut the span in half.
 
Look into steel joists but not sure you'll get to 6" deep. Stressed skin panels - sips - might also get you there. 6" deep for 16' in any material is going to be bouncy for sure.
 
I'd be interested if for 40 psf LL and 10 psf DL on a 16' span, if you could get an engineer to agree to them 1 1/2" on center. Deflection mostly.
 
bill1952 said:
I'd be interested if for 40 psf LL and 10 psf DL on a 16' span, if you could get an engineer to agree to them 1 1/2" on center. Deflection mostly.
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Post #8 demonstrates, in 2 different ways, that for an excellent 2x6 (DF SS), 4" o.c. suffices.

Cheers, Wayne
 
bill1952 said:
For deflection? 1/90? 1/180?
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For the calculator, I used L/360, as appropriate for a floor system, which I neglected to mention. For the IRC tables, as indicated, also L/360.

bill1952 said:
And forgive me but are you a registered design professional?
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No, so I couldn't sign anything official. But I understand the math that the engineering and the AWC calculators use. There's a strength check and a deflection check. There's also a bearing area check, which I did not consider, but as the IRC tables do not call out anything extraordinary in that regard, standard IRC details would apply.

To understand my IRC table based answer, there's just two fairly simple engineering principles you need to know, and you don't need to get into the details of the relevant equations.

1) All the relevant checks are based on the load per member. So if you double the load, and double the member frequency, the allowable span is unchanged.
2) The deflection equation is linear in the load per member. So if you halve the load per member, you halve the deflection.

In the above, I say "double" or "halve" but those are simple to understand proxies for "any multiplicative factor". I guess there's also an assumption about the IRC tables, that the span table values for bending members depend only on the specified parameters (member species, grade, spacing, loading, and deflection limit) and not on the member name (floor joist, ceiling joist, etc).

Cheers, Wayne
 
2x6 Doug Fir Larch with a 16' span at 8" o.c. is inadequate by 176.9% so it fails miserably. This will require engineering, as it is not prescriptive and will problem end up with LVLs or steel. This is impractical.
 
jar546 said:
2x6 Doug Fir Larch with a 16' span at 4" o.c. is inadequate by 38.5% so it fails too. Again, impractical. Add a girder below.
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You haven't specified the grade, so the above statement is imprecise. Post #8 indicates that with SS grade, DF-L 2x6 at 4" o.c. may span 16' with 40 psf live and 20 psf dead and an L/360 deflection limit.

[I failed to specify Douglas Fir-Larch in post #8, which was imprecise, as I see Douglas Fir (South) has lower design values than Douglas Fir-Larch and Douglas Fir-Larch (North).]

Cheers, Wayne
 
jar546 said:
For the 10' section, the 2x6 Doug Fir Larch would have to be 8" o.c. in order to be adequate.
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Huh? IRC Table 502.3.1(1) says that for 10' span and 30 psf live, 10 psf dead, L/360 deflection, you could use 2x6 @ 16" #2 or better for any of the species listed, DF-L, H-F, SP, or S-P-F. For 40 psf live, 10 psf dead, Table 502.3.1(2) any of those would work at 12" o.c.; at 16" o.c. you'd need DF-L SS or SP SS.

For 20 psf dead, you'd need to double check the tables. Most of the #2 grade options are out, as they are strength controlled; all the SS options are still OK, as they are deflection controlled; and the #1 grade options are mixed.

Cheers, Wayne
 
wwhitney said:
You haven't specified the grade, so the above statement is imprecise. Post #8 indicates that with SS grade, DF-L 2x6 at 4" o.c. may span 16' with 40 psf live and 20 psf dead and an L/360 deflection limit.

[I failed to specify Douglas Fir-Larch in post #8, which was imprecise, as I see Douglas Fir (South) has lower design values than Douglas Fir-Larch and Douglas Fir-Larch (North).]

Cheers, Wayne
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#2 by default makes it precise.
 
wwhitney said:
Huh? IRC Table 502.3.1(1) says that for 10' span and 30 psf live, 10 psf dead, L/360 deflection, you could use 2x6 @ 16" #2 or better for any of the species listed, DF-L, H-F, SP, or S-P-F. For 40 psf live, 10 psf dead, Table 502.3.1(2) any of those would work at 12" o.c.; at 16" o.c. you'd need DF-L SS or SP SS.

For 20 psf dead, you'd need to double check the tables. Most of the #2 grade options are out, as they are strength controlled; all the SS options are still OK, as they are deflection controlled; and the #1 grade options are mixed.

Cheers, Wayne
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I'm not using the tables and I am assuming a 10# dead load and a 40# live load. These are being calculated.
 
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