Floor Joist Sizing Help

Greetings…

I’m in the planning stages for a small timber frame shed I would like to build to store fire wood. I’ve actually built the Sobon shed from the book a couple of years ago, and was thinking of utilizing the same basic size, shape, and dimensions. After thinking through the process, and reading Bensen’s book where he discusses sizing beams for weight loads and joint selection, I quickly realized that if I planned to fill my new “wood” shed with firewood, I might run into some structural issues with the floor. Below are my estimates on what the “math” tells me the proper floor beams sizes should be. And based on these projections, I guess I’m questioning the need for such large beams, basically would I be building this shed overkill?

Shed Dimensions: 12’ x 24’
Building Materials: Easter White Pine.
Foundation: 1 ft granit Block pillars; 3/corner + 3 at the middle of the sill plates. (All blocks will be flat on the ground, not stacked).

Fire wood weight: Green Oak: 4886 lb/cord (per Engineering data Web Site); Oak is most prevalent in my area for firewood and would be typical.

The weight is roughly 305 PSF for a cord of fire wood stacked 4 x 4 x 8.

So at this point, my spread sheet full of formulas tells me the following:

Floor joist sizes, 16” OC should be 10 x 6.5 to achieve < 1/360 in deflection. It also tells be to utilize a housed dove tail joint to connect the joist to the plate and ensure I have at lease 8" of baring surface to prevent shear failure at the joint.

My question to all of you is this, if I were to build the shed per the calculations, am I going overboard? Since this is not a house, do I really need to be concerned with having less then 1/360 in deflection. I realize that at any given time, the weight will vary depending on the wood’s moisture content, so the 305PSF might be excessive.

If anyone has any comments or suggestions on what a reasonable floor joist size should be in this situation, please let me know. Also, I have a spread sheet showing my calculation if anyone is interested.

Thanks,
-David

How about adding a summer beam down the middle that's supported at several spots like the sills. This will cut your span in half but you'll need to cut more joints.

Hi,
Good thinking... With a summer beam spaning the length of the building (24 ft), I can use 6 x 5 joists, with a fully housed pocket in the sill. Yeah more of them, but that's OK.
Thanks!
-David

It is possible your load numbers are off.
If a cord of oak weighs 4886 lbs it's footprint is 4'x 8'= 32 sqft. 4886 / 32 = 152.6875 lbs per sqft, doesn't it?
If the joists run 12' and are 16" apart then each joist hold up about 16 sqft. So 16 x 152.6875 = 2443 lbs.
The floor joist calculator I use says a 6x9 will do it.
But, maybe I'm wrong.

Jim Rogers

Hi Jim,
You really got me thinking on that one... Yes, you're right that a cord of firewood is 32 sqft, but the third dimension for that single cord of fire wood is only 4 feet high. I double checked my spread sheet and realalized how I came to the 305 PSF - I'm fitting two cord of wood in that 32 sqft of floor space stacking the wood to ceiling of shed for a total of 2 x 4886 lbs in the same 32 sqft.

I was rethiinking my last post too about using a summer beam length wise, maybe the thiing to do would be to divide the width into three using 2 large beams (supported with block) and then use floor joists between. I haven't done the math yet, but my thinking is each joist would then see a load of 8' x 16" oc == 12 sqrt feet. I think I'll play with those numbers and see what gives.

Thanks!
-David

Fletch,
another thought might be to omit the flooring system all together. Build the shed using the structural sills, white oak would be best for the sills, and have a gravel/stone floor. The rest of the frame could be the same. This approach might alleviate the need to have ‘huge’ floor joists and also prevent any potential, or eventual, insect/rot damage to the flooring system.

Good luck
pete

Hey Pete,
Yes, I've thought of going without a floor too. I think the benefits to that would also include not having to carry all of the wood up into the shed before stacking. :-)

You mentioned "structural sills". I'm not sure what those are. Do you have an example or something I could see. One thought I had was to use the cement sonnet tubes and put the posts onto those, but digging holes is not an options due to rocks I have. I think my biggest question is how the post of the center bent, in the front of the shed will connect to a "foundation"?

Thanks for your help!
-David

Not sure how much rock you are dealing with? Could you pour a cement slab for floor with raised perimeter w/footings beneath to accept the sills. On one end omit the perimeter wall to allow easy access to the wood ( e.g. sort of like you'd do for a garage).

A lot more work but much more pemanent.

Another thought. Three sided dry stone foundation for sills (the way they used to do it (stones are free : ) ). Omit the sill on the open end and used gravel/crushed rock for floor, get some old pallets to stack the wood on.

Hi,

Yeah, sills on three sides is an option. My only question now is how to handle the post for the middle bent coming down to the ground where the sill has been omitted? Do I just made a pillar of stone + some sort of block of wood (to act as a sill), and then put the post on top of that?

I think if I could figure that out, that will be the way for me to go.

So, you're saying you want the 24' side to be fully open?
I think MP was thinking you would load the wood from a gable end. That way you'd have two long sills and one short.
If you want the 24' side to be open then use two short sills and one long or make three short sills. One for the center bent to stand on, and just stack over it.
There are all kinds of options....

Jim Rogers

Jim,

I thought I wanted the 24' totally open, but I'm thinking now that I will use two shorter 8' sills on the front and leave the center 8' open. Great idea!!! Thanks!

For the rest of design, I will use 4 bents, spaced 8' OC (to flank each side of the door way. To address my concerns with providing stability to the posts at the entry of the building to the sill, I will add a couple of struts between the post and sill.

I think I'm now ready to draw up my plans. Thanks so much to everyone for the help. With your help, I was able to problem solve the building, making it better then what I had come up with on my own. Not to mention I won't be needing 10 x 8 floor joists :-)
Best Regards,
-David