Post and Beam Adaptations

[jcsforager]

I'm building the 20x30 1.5 story and am just about set on my decision to do the Post and Pier foundation. I'm attempting to limit my use of concrete and stay as local with my lumber as possilbe. I'm considering either using Telephone poles or lumber from the property to oversize my Posts. I have some 12" logs in the round that I'm considering for the posts, (making sure to treat or guard against termites at base). If I was to oversize the beam would it be possilbe to eliminate two of the Piers?  I would then be spanning 15' with my beam. Rather than the 3 2x12's could I use a larger single beam for this span? I'm considering this also with the open ceiling rafter ties. Rather than 2 2x8's having cut for me a single 4x8x20?. Just checking to see if this makes sense structurally?
john
Quilcene, Washington

[jraabe]

I would be very careful about eliminating the posts as your beam can get large fast as the span increases. Also the load on each post then gets larger as well and the footing load goes up. A 15' span carrying all roof and wall loads would be huge. IE: a bridge timber or deep and wide glu-lam. I just did a quick calculation and it looks like an 8" x 18.5" Doug Fir beam would be about right.  8-)

The solid 4x rafter ties will work fine and look better than the double 2x's. No problem there.

[jcsforager]

I will stick with the Posts every 7'6". I rather be over on my size than under. 8"x18.5" Wow, now thats a piece of wood!  Do you have a table for these calulations? So if I go with the 7'6" I can see how to build up with the 2x12's. Would my size for a single beam of Doug Fir with post every 7'6" than be about a 6"x12"?

[glenn-k]

I would consider sticking with commercially treated posts if in the ground.  I have seen 12" round log posts go to 8" or less in a short period of time when conditions are bad.  Checking (cracks) will not allow you to treat them real well by hand.  Well treated telephone pole ends should be OK but if you saw them off be sure to treat the sawed end.

Hate to see you have to try to figure out how to repair this in a few years.

[jraabe]

When you vary from the plans you need to have a local beam calculation done with a county plans checker or local engineer. Not a big project but it varies from one location to another. Mainly in the snow and wind loads.

Glenn is right about the posts.