Framing a loft for a pole barn

[UncleBuck1]

I'm planning on building a 36'x48' monitor style pole barn using standard 12' spacing and I need some help figuring out how to frame the loft floor. Basically the loft floor will be a deck, but instead of sitting on top of posts it will hang between them.

Here's a picture I stole from another thread that illustrates what I'm trying to build:



My specific question has to do with the ledger boards that the joists will nail into; floor joist span tables are all over the place, but I can't find any guidance on how big the ledger boards need to be. As I mentioned in another thread, the loft will be used as a bunk house. I did find a porch beam table that said I would need (3) 2x12's glued and nailed for a 12' span, but then found another table that only called for (2) 2x12's- and to compound my confusion I'm not even sure if beam sizing corresponds directly to ledger sizing.

Although its probably overkill, I'm trying to stick with the standard residential 40psf live load/10psf dead load for my design; since the ledgers will be carrying the entire weight of the loft and contents and I figure better safe than sorry.

Also, what is the right way to attach the ledger boards to the posts? Do I need to notch the ledgers into the posts? If so, how deep does the notch need to be? I'm planning on using 6x6 posts, but may go to 6x8 if I need to notch them.

Any guidance will be greatly appreciated!

Brian

EDIT: Since I'm asking about ledger board sizing I thought I would mention that the floor joists will be 2x8's on 18" centers.

 

[LD1]

First let me mention that I would not use 18" centers. It is not divisible by 4 or ,8 which is the common size for flooring (plywood). Go with 16. A few more joists will save much more in the long run.

Now as to a ledger, I don't know off the top of my head. But I do know an issue is not so much the vertical load, by the horizontal. The joists sag a little and push out on the ledger an cause them to pull out of the hangers. There was s thread on here a while back about an overloaded loft failing for this reason.

 

[KennyG]

Your design using 2x8 on a 12 foot span is right on with design tables for 20psf dead load and 40 psf live load, which is common. With this design, each joist will have an average load of 960 lbs and each end will support 480 lbs. So you need enough bearing area on the ledger board to support 480 lbs. Since the wood compressive strength will be over 500 psi, even with reasonable conservatism, from a technical standpoint, a 3/4 inch ledger board would be adequate. However, as mentioned above, there are a lot of variables with wood shrinkage, etc. I would probably use a doubled 2x ledger board in this case, just for safety as far as thickness. The real difficulty here is not the thckness of the ledger board, it's the strength of it with your 12 foot spacing. The ledger board needs to be strong enough to support the floor between the posts. Each ledger board will have to support 4320 lbs distributed over a 12 foot span. Looking at the tables, I think you are looking at doubled 2x12s for the ledger boards, at a minimum. I would rather have 8 foot spacing under the loft.

 

[KennyG]

Oh yes, the ledger boards need to be through bolted to the posts. Seriously through bolted.

 

[UncleBuck1]

But I do know an issue is not so much the vertical load, by the horizontal. The joists sag a little and push out on the ledger an cause them to pull out of the hangers.

Would properly installed joist hangers control sag/pull?

Oh yes, the ledger boards need to be through bolted to the posts. Seriously through bolted.

I'm planning to through bolt with at least (3) 3/4" carriage bolts- but will that be enough vertical support, or should I also notch the ledger into the post?

 

[LD1]

Yes, properly installed joist hangers will help. But you are GOING to have sag. No way around it. On a 12' span of the joists, using L/360 deflection, (if thats the tables you used), that means the joist is ALLOWED to sag 144/360 of an inch. Or about .4 inches. Thats okay. And the hangers are designed to hold that to the ledger.

What I am saying is that if the ledger is insufficient, it may bow enough to allow the joist to comre free of the hanger. So you not only have to worry about the vertical strength of the ledger, but also the horizontal since you dont have joists on the opposite side to help keep the ledger board from bowing out.

Do you have any type of code enforcement in your area? If so, it is best to follow that. Also, a phone call to a PE in your area would probably be worthwhile.

As to the carriage bolts 3 should be fine. If kenny G's numbers are right, you will be supporting under 5k lbs with each ledger board. The shear strength of them bolts is roughly 9000lbs each.

 

[Dr Dave]

You could attach a vertical 2x, to the post down to the floor, and set your ledge board, or beam on top of it. Then you would not need to notch your beam.

DAVE

 

[LD1]

Try this

http://www.southernpine.com/pdf/ref207.pdf

Page 17 and 18 I think applys to your situation.

It calls out for 2 2x12's of #1 grade or 3 2x10's of #2 grade.

#1 grade stuff is usually a bit pricier. I think 3 common #2 2x12's like you were planning would be fine.

On a side note, If it were mine and because it wouldnt add much $$$ but WOULD add much more peice of mind, Use the 3 2x12's, but on the bottom part of the face of the ledger, add a 2x4. This will assist the joist hangers in the vertical load.

 

[LD1]

You could attach a vertical 2x, to the post down to the floor, and set your ledge board, or beam on top of it. Then you would not need to notch your beam.

DAVE

I was going to suggest that as well, cause the thread awhile ago about the loft collapsing from overload...I think that is what they did. I'll see if I can find that thread for reference.

 

[Dr Dave]

I was going to suggest that as well, cause the thread awhile ago about the loft collapsing from overload...I think that is what they did. I'll see if I can find that thread for reference.

I think, I would add 2 long threaded rods from ledger to ledger to prevent separation, as in this failure. Place them on 4' centers between post's.

I like the idea of triple 2x12's with a 2x4 below the joist hangers.

Dave