2x6 span 16’

[jar546]

If you want to use doubled up 2x6 #2 SPF, you can span the 10' at 16" o.c. You are adequate by 29.4% at a 1/360

 

[wwhitney]

#2 by default makes it precise.

OK, but in a challenging application like this, specifying #2 would obviously be a poor choice.

But I agree that (4) SS grade DF-L every 16" would be impractical for any significant width.

Cheers, Wayne

 

[wwhitney]

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

Then your calculations do not agree with the IRC tables or with the AWC span calculator, both of which say DF-L 2x6 works at #2 12" o.c. or SS 16" o.c. If you're doing this via the NDS formulae, are you including the size factor and repetitive member factors?

Cheers, Wayne

 

[jar546]

Then your calculations do not agree with the IRC tables or with the AWC span calculator, both of which say DF-L 2x6 works at #2 12" o.c. or SS 16" o.c. If you're doing this via the NDS formulae, are you including the size factor and repetitive member factors?

Cheers, Wayne

For which span? The 10' or the 16'?

 

[wwhitney]

For which span? The 10' or the 16'?

For the 10' span, which should be clear from the context.

Cheers, Wayne

 

[jar546]

Then your calculations do not agree with the IRC tables or with the AWC span calculator, both of which say DF-L 2x6 works at #2 12" o.c. or SS 16" o.c. If you're doing this via the NDS formulae, are you including the size factor and repetitive member factors?

Cheers, Wayne

What is missing here? I even lowered the deflection limit for live load from 480 to 360.

 

[Yankee Chronicler]

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.


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

This approach doesn't work the way you think it works. The IRC includes span tables for joists spaced 24" on center and 12" on center. Compare them. Obviously, ajoists spaced 12" on center carry half the load that joists spaced 24" on center carry (for the same pound-per-square-foot loading). But the 12" spacing doesn't result in anywhere near twice the span as the joists at 24" on center -- it's more like 1.25x.

 

[wwhitney]

What is missing here? I even lowered the deflection limit for live load from 480 to 360.

I'm not quite sure--is your software calculating the deflection limit based on the total load instead of the dead load? Try changing the dead load to 5 psf and see if the result you get changes. If it does, then I think that's the problem. My understanding is that the span tables apply the deflection limit to the live load only.

Cheers, Wayne

 

[wwhitney]

This approach doesn't work the way you think it works.

It does; you've misapprehended what I wrote.

The IRC includes span tables for joists spaced 24" on center and 12" on center. Compare them. Obviously, ajoists spaced 12" on center carry half the load that joists spaced 24" on center carry (for the same pound-per-square-foot loading). But the 12" spacing doesn't result in anywhere near twice the span as the joists at 24" on center -- it's more like 1.25x.

Of course it doesn't work that way; that's why we have tables instead of a simple linear formula we could use. But I never said it would work that way, nor have I applied any such inference. [BTW, when the span is deflection limited, then the deflection/span varies as the span^3, so doubling the joist frequency would let you increase the span by cube root of 2 = 1.26, so your 1.25 is a very good approximation.]

Rather, my method only involves finding entries in span tables that are at least as great as the desired span (10' or 16' in this discussion) using the desired member size; seeing what the corresponding member frequency, loading, and deflection limit are; and then inferring what increased member frequency is required for the desired loading and deflection limit.

For example, let's consider 12" o.c. 40 psf live 20 psf dead, found in the IRC floor joist tables with an L/360 deflection limit, to 24" o.c. 20 psf live 10 psf dead, found in the IRC ceiling joist tables with an L/240 deflection limit. The per member loading is identical in each case. As the deflection limits differ, we will only get identical allowable spans when both cases are strength limited. And indeed, if you look at DF-L #3, for 2x6, 2x8, and 2x10, the floor joist table gives spans of 8'-1", 10'-3", and 12'-7" (again for 12" o.c., 40 psf live, 20 psf dead). Likewise, for the same species, grade, and member sizes, the ceiling joist table gives identical spans of 8'-1", 10'-3", and 12'-7" (again for 24" o.c., 20 psf live, and 10 psf dead).

Cheers, Wayne

 

[jar546]

I'm not quite sure--is your software calculating the deflection limit based on the total load instead of the dead load? Try changing the dead load to 5 psf and see if the result you get changes. If it does, then I think that's the problem. My understanding is that the span tables apply the deflection limit to the live load only.

Cheers, Wayne

The default dead load is 15psf and I reduced it to the common 10psf. I am sure, since it failed at a very low percentage of less than 2% that it may actually work. Just tell them not to tile the floor?

 

[jar546]

I'm not quite sure--is your software calculating the deflection limit based on the total load instead of the dead load? Try changing the dead load to 5 psf and see if the result you get changes. If it does, then I think that's the problem. My understanding is that the span tables apply the deflection limit to the live load only.

Cheers, Wayne

So it is not the dead load. I see no legitimate reason to lower a dead load to 5psf when the minimum is 10psf, especially since he will need sheathing, insulation, and probably drywall for separation since this is a pole barn. I think the issue is the deflection limits I sent. I used a live deflection of 360 and a total of 360 but I then changed it to the absolute minimums of 360/240 and used SPF #2 so we have the bare bones and it works. Therefore:

#2 grade 2x6 SPF 12"o.c. can span 10' and meet a 40#LL and 10#DL and be adequate by 7.9%.

For the 16' span, however:

4 #2 grade 2x6 SPF 12" o.c. can span 16' and meet a 40#LL and 10#DL and be adequate by 5.3% with a controlling factor of deflection.

But an engineer would have to spec it out.

 

[Yikes]

Maybe use parallam engineered wood?

 

[wwhitney]

So it is not the dead load. I see no legitimate reason to lower a dead load to 5psf when the minimum is 10psf, especially since he will need sheathing, insulation, and probably drywall for separation since this is a pole barn.

To be clear, I wasn't suggesting a 5psf dead load would be appropriate. Rather I wanted to see if the program was imposing a "total load" deflection limit of L/360 instead of just a "live load" deflection limit of L/360.

But looking again I see I missed the part of the report which spelled out exactly that, and which matches your further comments. So if you did try a 5 psf dead load (as a thought experiment), it should pass L/360 total load deflection.

Apparently the AWC calculator and the IRC span tables do not impose an L/360 total load deflection limit; perhaps they impose a L/240 total deflection limit, but that could only control if the ratio of live/dead load is less than 2.0.

Cheers, Wayne

 

[wwhitney]

Maybe use parallam engineered wood?

Not at 1.3E for the 16' span request. That condition is going to be deflection limited, so the higher the E the better. That's why SS DF-L (1.9E) is better than #2 DF-L (1.6E). A 2.0E LVL would only be marginally better than SS DF-L. But if there are higher E options available in a 5.5" height, like a 2.2E or 2.3E, that would be worth considering.

For the 16' span, however:

4 #2 grade 2x6 SPF 12" o.c. can span 16' and meet a 40#LL and 10#DL and be adequate by 5.3% with a controlling factor of deflection.

To reiterate, that result is deflection limited and your outcome suffers from using #2 grade. If you switch to Select Struct, you will get that (3) SS 2x6 DF-L (didn't check SPF) at 12" o.c. would work.

But an engineer would have to spec it out.

In post #8 I gave a sound argument that the IRC tables let you get to that result using only fairly obvious inferences (results depend on load per member; deflection is proportional to load per member). You wouldn't buy that argument as a code official?

Cheers, Wayne

 

[bill1952]

Going to be an expensive floor for 1.75".

 

[steveray]

Me too, I gotta sleep up there.

Can you sleep through a floor collapsing under you?

 

[Yikes]

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.

I know your question is specifically looking for a 5.5” structural solution, but are you willing to consider different height clearances either above or below the floor? Would one or two drop beams make the pole barn below unusable? Are you willing to lose a couple inches of headroom in the apartments?

What are the height limitations you’re dealing with?

 

[asmith1923]

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

May save me on having to install a subfloor. haha This is exactly what I needed, Thank you!

BINGO

I only need to span 16’ to support my cantilever second floor about 4 1/2 feet. I’ve decided to use 2x6 in joist hangers for this 16”oc and make the beam out of doubled 2x12”. This beam will be along the outside wall, so I’ll set the bottom flush with the joists and let the 6” extra go up into the second floor wall.

 

[asmith1923]

I know your question is specifically looking for a 5.5” structural solution, but are you willing to consider different height clearances either above or below the floor? Would one or two drop beams make the pole barn below unusable? Are you willing to lose a couple inches of headroom in the apartments?

What are the height limitations you’re dealing with?

I only have 82” finished height on the first floor and 80” on the second WITH 2x6 joists. The plan for the barn was NOT to put a two story apartment inside or I would’ve made the damn thing taller ‍

 

[wwhitney]

I only need to span 16’ to support my cantilever second floor about 4 1/2 feet.

This statement requires clarification. How about a drawing of a full cross section which shows the 10' and 16' spans?

The calculations are for a 16' simply supported span with only a uniform load on it. If you have a cantilever involved, or you have say a second floor ceiling joist load coming down midspan via a wall, the calculations would need adjusting.

Cheers, Wayne

 

[jar546]

This statement requires clarification. How about a drawing of a full cross section which shows the 10' and 16' spans?

The calculations are for a 16' simply supported span with only a uniform load on it. If you have a cantilever involved, or you have say a second floor ceiling joist load coming down midspan via a wall, the calculations would need adjusting.

Cheers, Wayne

This is why I said this thread scares me. I believe we have someone looking for someone to tell them what they want to hear and we don't have all of the facts. What we want to do, what we are allowed to do and what we can afford to do are often completely different things.

 

[Yikes]

This is why I said this thread scares me. I believe we have someone looking for someone to tell them what they want to hear and we don't have all of the facts. What we want to do, what we are allowed to do and what we can afford to do are often completely different things.

It doesn't scare me the same way. The OP simply said "any help is appreciated", and the help being offered ranges from structural options to no-can-do.
As a purely structural exercise, this is useful. We all know there are ways to do a very slender floor depth, including cross-laminated timber planks. so this is a useful exercise.

The OP still has to deal with the real-world realities of insufficient headroom at 80" IBC chapter 12, even if he solves the structural issue.
IRC R-305.1, IBC 1207.2.

Ironically, there are many people who live in RV's and boats where 80" of ceiling height is generous, but hey, that's the codes.

 

[asmith1923]

This statement requires clarification. How about a drawing of a full cross section which shows the 10' and 16' spans?

The calculations are for a 16' simply supported span with only a uniform load on it. If you have a cantilever involved, or you have say a second floor ceiling joist load coming down midspan via a wall, the calculations would need adjusting.

Cheers, Wayne

Thanks Wayne, here is the section that I'm most concerned about.

 

[asmith1923]

This statement requires clarification. How about a drawing of a full cross section which shows the 10' and 16' spans?

The calculations are for a 16' simply supported span with only a uniform load on it. If you have a cantilever involved, or you have say a second floor ceiling joist load coming down midspan via a wall, the calculations would need adjusting.

Cheers, Wayne

There is a Living room above this area and no roof load. The area is being built within a pole barn. I'm leaning towards tripled 2"x6" for the beam and 2"x6" @ 24" OC for the joists.