Another Timber Sizing Question...

I'm attempting to size beams for a loft in a cabin that's being built and I want to make sure my calculations and assumptions are correct. I'm an engineer (geotechnical) with little structural experience. I blew the dust off a few books and did a little research. Here's what I've come up with.

The loft will be ~12 feet deep by 22 feet wide. I'm trying to size beams to span the 22' width with no support. I assumed 4 beams spaced 3.5 ft c/c using eastern white pine. I analyzed the beams for a live load deflection (30 psf live load and 1/360th of length for an allowable deflection), combined load deflection (LL of 30 psf and dead load of 10 psf and an allowable deflection of 1/240th of length), and performed a bending evaluation.

For eastern white pine, I assumed E=900,000 psi, a density of 30 psf, an an extreme fiber stress of 575 psi. I'm not sure how wood is graded, so I think I'm conservative with these values. Our sawyer indicated that there's virtually no knots in the wood. I ended up calculating that and 8 x 12 will be acceptable.

I want to be certain that the beams are sized appropriately before they're cut. I think my calculations are correct, but the bending evaluation has me worried a bit. I'm also concerned about the working stress analysis; should a factor of safety be incorporated into the design? Any help or suggestions would be apprectiated.
Mike

Hi Mike,

Why have you chosen not to simply span across the 12 foot dimension ?

Regards

Ken Hume

The loft is being constructed over a great room and will be constructed as a cantilever. The back of the loft will be against the back wall and the front will span the width of the room. We were trying to construct the loft with no posts.

Hi Mike,

This doesn't sound like a brilliant idea !

Could you live with one substantial 12 ft beam spanning across the centre and then 11 ft axial joists running from either side of this beam to bridge the 22 ft span ?

Regards

Ken Hume

I'm not sure what you mean by cantilever? I can't picture the configuration. Are you sure cantilever is the word you meant to use. Cantilever would mean that one side is overhanging with no support at all.

I would think that the 8X12 would work but you'd only need one (bent) on either side of the 12 foot bay you are creating. An 8X12 tie beam every 3.5 feet seems like a lot of overkill. With two bents 12 feet apart, you can then add floor joists, about 11'10 inches long, between the bents every 24 inches or whatever it takes to support your load.

If my assumptions are wrong, please help us by adding a sketch so your design intent is more clear.

Oh, we used 8X12 X 22' tie beams 16 feet apart in the frame we built in the workshop I took. Of course all of your numbers need to be worked out to be sure your joinery will be sufficient. You will need knee braces for your tie beam too...

I suppose I used the wrong term. I'm simply trying to span between two walls separated by 22' to create a loft that's 12' deep. The rear of the loft will be against the rear wall and front will extend into the great room. I was trying to construct this with no posts in the center of the room.

Would it be possible to use trusses to span the 22' distance and hold up the loft floor? Seems to me that you might be able to design 2 trusses (one at the back wall and one at the front of the loft) over the loft area, and look better than a lot of 8x12 (heavy) floor joists over the back half of your great room. If the trusses carried the load, you could reduce the size of your loft floor joists, as they'd only be 12' long, running from truss to truss.

From what you describe, it doesn't sound like any problem for a kingpost bent or maybe even a queen post. You wouldn't need any center post. The tie beam will likely be between 8X12 and 8X16. The larger depth is often needed to accomodate the floor joist mortises. You then have a number of options for floor joists. You can have 12 foot joists all the way across or split it up with larger beams and then smaller shorter joists. It depends on what materials you have to work with or what look you want. If you are not supporting a roof with this loft, well that is even easer with less load. You'll stil want beams in proper proportion though so they look right. Have you learned joint stress analysis? A good start to understanding joint stress analysis and sizing of beams is Steve Chappell's book "A timberframers workshop". Always have your worked checked by someone qualified. I'm an engineer too, but I always want another good set of eyes to catch my mistakes.

Porcupine,
Take a look at the photos in this thread. http://tfguild.org/forums/ubbthreads.php?ubb=showflat&Number=16209&page=1&fpart=2
The bents on my frame are 4' on center, clear span is nearly 20' and the anchor beams are 8" x 14" white pine. I'm not saying these dimensions would work for you but it will give you an idea of the proportions.

Karl.

If you are worried about the bending of the simply supported beam than you could make it a truss (as Brad and DKR suggested). I guess this depends on the roof and wall design and how far into the build you are. The king post and queen post in their respective trusses work in tension thereby negating some of the bending stress you are worried about. It all depends on what is done and what you have designed already. If you have a sufficiently supported ridge a cable might even work anchoring the center of your beam. If you don't want to support the load from the bottom with a post, the other option is to pull it up. Im not an engineer, just some thoughts. Good luck with your project.