Retrofitting Foundation on 20x30 1 1/2 story cabin

[rfwoodvt]

Hi!

I'm looking at a 1 1/2 story 20x30 cabin that sits on a railroad tie perimeter foundation with railroad tie center beam support.

It is over 40 years old and the building is suffering from the degredation of the ties including some sill rot.

Anyhow, I'd like to replace the RR ties with a pier-and-Beam foundation and that will mean jacking up the building and there won't be that much room to work under the building for digging.

Soil, if you can call it that, is primarily glacial till/gravel with interloping sand.  Organic topsoil is about 3 inches thick.  We'd be building a frost protected, shallow foundation.

All I really need to know now is what is the maximum span between piers for such a building using either beams made up of 3 2x8's or 3 2x10's? (pressure treated lumber)

I assume that I can support each set of joists (one set each side of center loadbearing wall) with two beams  (total of 4)  but that seems like over kill and over work. That said, can I just use three beams? one for the center, load bearing, wall and the other two spaced just inside the outer wall to cantelver the floor joists?

All thoughts and help welcome!

Wishing you His Peace!

[glenn kangiser]

Welcome to the forum, rf.

If it is on RR ties and there is one center one then the center beam would have to go there wouldn't it?

That would require temporary beams on both sides wouldn't it?

Is access crawling or are you planning on removing some floors?

[rfwoodvt]

Hi Glenn,

I'm planning on raising some floor boards to access  the joists.  would be a lot easier than "tunneling" under the ties.  Though the cabin hasn't settled, in 40years there has been a lot of ground creep due to vegetation and the fact that the ties only kept the bldg off the ground by about 8 inches.

As for lifting, I suppose that if we would have to either span all joists on either side of the center beam or actually jack up the center beam I'm hoping that the center beam is really a beam and we can just set that onto the piers. but we'll have to wait and see what is really under there.

In any case I kinda figured that I'd need at least one along the center beam and at least one out near each box-header on the outside of the joists.

I really want to cantelever the joists over the outside beams though for several reasons, most of which I have seen discussed here and in other sites.

What  still remains though is how much span between piers?  

[glenn kangiser]

A 20 x 30 ---1 1/2 story I have plans for has 5 piers on each side at 7'6", plus 1 at the center of each endwall.   Since you will have intermediate piers also you would probably be fine with similar spacing on your 1.5 story with a concrete or PT base under it.

If you are looking for an estimate that would probably work but to be really accurate you may need a local professional to do a load check for you.

[rfwoodvt]

Glenn,

How would that look in a sketch?

If I understand correctly, each beam on the long side of the building is supported by 5 piers at 7.5 feet but the center beam is only supported on the ends (on the short side)  That adds up to 12 piers.  is that correct?

When you say intermediate piers does that refer to the ones along the center load bearing wall?

Also, do your plans show 2x8 or 2x10 for the carrying beams?

[glenn kangiser]

Correct on the above.  There is not a load bearing wall in the above example so I would add the 3 center piers under your center bearing wall for a total of 15.  

It uses built up beams of 3 - 2x12s and a strip of 1/2 plywood for thickness.

A difference for yours would be that you have a center bearing wall carrying the load so an architect may design lighter beams due to the weight being spread out differently.  How could depend on how your roof load is supported also.  I understand the concepts but am not qualified to make the calculations.  I'm just a lowly steel guy.