12x12 cabin foundation questions

[rocking23nf]

Hi all, first time poster, lots of great info here.

Anyways im building a 12x12 addition onto my existing cabin. the cabin is 30 years old and was built on concrete blocks has 3 beams with joists resting on top.

According to the building code, I have to build my addition with peirs in the ground, which is fine..

Anyways, i want this thing sturdy, so I decided to go with 6x6 posts, 3 - 2x10x12 for the beams, and 2x10x12 for the joists 16"oc, all wood is pressure treated.

On the existing side of the cabin, im going to use the Beam thats already there, and likely reinforce it. On the other end I want to put in 2 posts using the bigfoot peirs that I found on this site.

My question is, I know that 2x10x12 is more then enough for the joists according the AWC calculator, but that site doesnt say anything about beam span. I want the posts at the outer edge of the frame, with no cantalever. so the beam span will be 12 feet minus 6 inchs for the posts. Is this acceptable for that size of wood? My other option would be 2 - 2x12x12

Thanks all.

[MountainDon]

  rocking23nf

Let's see if I interpret what you are saying correctly; you want to know if placing two piers, one at each end of the beam span of approximately 11 feet is okay?

First though, beam size calculations have to take into account all the downward loads from the basically one half of the entire building. There are so many variables including how many stories high and snow loads that simple calculators won't do. Hence the reason you has trouble finding one.

You could do a span as you described but the beam would have to be very large. A more practical solution would be to use more piers. John has provided a free foundation plan for a 16 ft wide cabin. It may be found at the link below. It is wider than your 12 foot width and is sized to carry the weight of the wider building. It uses three 2x12 #2 or btr with piers at 6'6" centers. Because of your reduced width it may be possible to bring those built up beams down to three 2x10's on 6' centers.

Have you asked your building dept. if they have a suggestion that they would accept for your local conditions?


http://countryplans.com/smf/index.php?topic=6288.0

[rocking23nf]

Ok looked at that plan, and it makes sense.

Here are my thoughts, 3 6.6 posts for a 12 foot beam, with the 2 outside posts flush with end of beam.

However I would like to use 3 - 2x10 instead of 3 2x12, the cost is much more for a 2x12 PT board. any thought on that?

[MountainDon]

...Because of your reduced width it may be possible to bring those built up beams down to three 2x10's on 6' centers.

I think it would work for you application. That's not an engineered solution, just making mental extrapolations.