I am an amatuer and I am designing my own timber frame to begin this spring.
My question envolves joist sizes (length vs. depth). According to my calculations a floor joist that is 7x5 and 12 feet long spaced 30 inches o.c. will work. Any feedback?

I'm not sure what kind of feedback you are asking for, so I'll offer some general comments. Joist sizing is normally a relatively straight-forward procedure. The key variables, some of which you've mentioned, include span, spacing, species and grade of material, and dead load and live load magnitudes. There are a bunch of other considerations as well: duration of load, moisture and temperature effects, possibility of notching, concentrated load points, etc. Selection of the proper joist size depends on both strength and stiffness considerations. There are lots of design aids available to help you make your member selections. You'll find them in books on home building and remodeling, textbooks, and even in the Yellow Pages under "Engineer, Structural." Good luck with your project.

In general I agree that the 7x5 @ 30" c/c spanning 12 feet is adequate. This member will support a uniform load of approximately 190 lb/linear foot or 75 lb/ft\2 based on an allowable bending stress of 1000 psi. However, I don't know what you intend to support or what type of joints or wood species you intend to use so you should pay close attention to those details.





[This message has been edited by rap (edited 01-09-2001).]

my joists will be burr or red oak and they will be for the first floor. the joints will be dovetail (non-shouldered). I have numerous books and as I stated previously they calculate this to be ok I just want to be safe!
Thanks for the replies please elaborate if this new information helps.

[This message has been edited by Drew (edited 01-15-2001).]

Drew: If you are using stress-graded timbers, then a nominal 5x7 piece would be graded as "posts and timbers." The bending strength of Red Oak posts and timbers varies from 575 to 1250 psi, depending on the grade of the material. Assuming worst case, these joists will only support about 110 lb/ft, which corresponds to a distributed load of only 43 psf on the floor. Normally, a home is designed for a floor live load of 40 psf. Your floor system itself will probably add another 10 psf or so (including the joists, floor sheathing, and finishes). So technically, the joists would be inadequate for strength. However, they should be ok for deflections. You need to be sure that you are using at least #1 grade Red Oak timbers to avoid this problem. I don't have separate bending strength data for bur oak (a subspecies of white oak), but the strength vales for the white oak group are only slight above those for red oak.

Dovetail connections might lead to problems with the carrying members. If you dovetail your joists into wall plates or summer beams, then the plates or beams might suffer distress due to the notch effects.