cabin is started

[sbishop]

Hi, just an update that i've started my cabin, I will be finishing the sills this weekend (8"X8") and i'm going to be ready for my floor joist soon and had a question. my cabin is 22'X26'. I will have a span of 10' between by outside sill and my center beam.

What would you recommed for a joist? 2X8? would I be pushing it if I used 2X6?

i'm leaning towards 2X8 but I won't want to overkill!

THANKS

PS. Will post pics after this weekend!
PS2. Thanks to PAPPY for all the great info!

[Jim_Rogers]

What spacing between joist were you planning on?

What type of wood?

Will you have anything heavy in there like a cast iron stove?

Jim Rogers

[sbishop]

I knew I was missing some info!

16" OC
Spruce lumber...not rough..getting it from a mill

Nothing really heavy except myself  ;D


Sbishop

[Jim_Rogers]

Ok, well when you say not rough then I'll assume you mean standard framing lumber.
So a 2x6 is really a 1 1/2" x 5 1/2", and a 2x8 is really a 1 1/2" by 7 1/4" SPF (spruce/pine/fir) grade of standard lumber.

Now lets use some of the tools we have on hand here on the Forestry Forum.

If we go to the tool box on the left hand side bar and click on it, we'll be taken to a menu of extra tools. If we select DonP's calculators we'll be taken to another menu.
Here we would select a "uniformly loaded beam calculator".

This will open and we now can read "how the calculator works".

This will tell us we need to figure the floor load for the joist.

We know that the length is 10' and the spacing is 16" so each joist will hold up 13.33 sqft of floor. So now we need to decide on what number of pounds each sqft will be. Code puts a first floor at 60 lbs, and a second floor at 40 lbs, live load (that is furniture and people).
Most decking and other flooring could add 10 to 15 lbs per sqft.
But this is just a cabin so lets say 50 live load and 10 lbs dead load to get a combined load of 60 lbs per sqft. So 60 x 13.33 = 800 lbs. Now each floor joist will be responsible for holding up 800 lbs of floor space.

This is the first number we add in the calculator.

Next we need to enter the span of the joist in inches (10' x 12" = 120 inches).

Now we enter the width of the joist 1.5" and the depth of 5.5".

The next box requires us to look up the design values for the type of wood we want to use.

Using the chart under the link "click here for wood design values", we see a row for "Spruce-Pine-Fir" and as these are horizontal pieces we'll use the number 2 value for "beams and stringers". (this chart is really for timber, but we'll see how things come out first, before we look up other values).
There the values are 575, 1.1, and 65.

We now enter these into the calculator and click "show results".

Wow.... every test fails......

Ok, now what?

Let's just change the depth from 5.5" to 7.25" and see......

Ok well now only two fail....

Now what?

Well we either change the depth of the floor joists until they pass.
Find correct values for standard SPF, or reduce the load on the floor, and maybe decrease the spacing to 12".

These are some of the options.

Lets assume, as it's just a cabin we can call the total floor load 40 lbs per sqft.

40 x 13.33 = 533.2 lbs

Let's try that.....

Well ok, now we've got two passing but one still failing.

We can now reduce the spacing to 12"

So 10' x 1' = 100 sqft x 40 lbs = 400.

Enter 400 and click show results again....

Now all three pass.......

If you want me to look up the proper values for SPF standard lumber to verify this I will.
Just let me know....

I hope this helps you to understand how to use the tools here on the forum.

Jim Rogers

[Jim_Rogers]

For fun, I looked up SPF in the NDS book of values and there are many classes or grades of lumber. Such as Utility, standard, construction, stud, No.3, No1&2., and Select Structural being the strongest. Utility is the lowest and each grade has different values.
In order to properly figure this out, you'd need to go to the place where you're going to buy the lumber, and first look at the grade stamp and see what grade it is.
Then I could look up the values for it and use DonP's calculator to see if this would be ok.

Let me know if this is something you want to do....


Jim Rogers

[Raphael]

I usually use the span calculator at AWC (American Wood Council) when dealing with framing lumber.
Graded dimensional framing lumber benefits from a better set of design values vs. larger timbers (the values for timbers are adjusted to allow for hidden flaws).  Some inspectors want rough sawn 2x material calculated using timber values.

www.awc.org/calculators/span/calc/timbercalcstyle.asp

Looks like No.2 or better SPF will do it for you assuming 40psf live and 10psf dead loads with a moderate L/360 deflection.  For 16" OC you'll need 2x8s with a minimum bearing length of 0.64 in. required at each end of the member OR for 2x6s you can go with 12" OC with a minimum bearing length of 0.4 in.  My preference would be for the larger space to swing my hammer in.  ;)

[Don P]

Yup, their's is the best one to use for standard dimensional lumber.
There you'll find that a #2 SPF 2x8 can span 12'3". I'll try to explain how they are getting to adjusted design values that could be used with my calc to give very similar spans to their results. I think this is where Jim was headed.

1st is using the table for the dimensions you'll be using- the one on my beam calc is for timbers 5x5" and larger. Like Raphael said, big timber has a table with lower design values than the table for 2-4" thick dimensional lumber . The thinking is that we're making more assumptions about the insides of a large timber than a 2x8. As an example, #2 SPF Beams and Stringers 5x5 and larger, has an allowable Fiberstress in bending (Fb)of 575psi. That same base design value for 2-4" thick dimensional #2 SPF is 875psi, a big increase already.

Next a size factor correction (Cf) is applied. For a 2x8  we multiply the 875 x 1.2 = 1050 psi.

Next a repetitive member use correction is applied, whenever members are in contact or spaced not more than 24" apart. Multiply by 1050psi x 1.15 = 1207.5

For a #2 store bought SPF 2x8, used as a repetitive member in conventional framing, we can use Fb=1207.5 psi, E=1.4 million psi, Shear=135 psi.

That has very big implications when you're thinking of whether to use a built up vs. solid sawn girder for example, Fb of 575 vs 1207 psi.

[sbishop]

ah my head hurts  :D

Just kidding...great info guys!

i'm going with 2X8. will be able to install them in 3 weeks! floor before winter....cover with a tarp and that's it for this year