Post/Pier and Beam Questions

[cjb80]

Hello,

This is my first post here; I discovered this forum about a week ago and I have been reading through the posting history and taking notes. The content of the forum is really interesting to me so I am glad I found it. 

For the past 8-9 months or so, I have been trying to get a home project funded for construction and I have had a series of problems (not related). Originally, I was attempting to fund the project as an owner-builder project - which was our first big hurdle. The house rolled up to around $240K assuming that I was the GC.  Later we attempted to fund the house with a GC because we were running out of options.  With a GC the house cost was about $330K.  In any case, I have vastly reduced our expectations now and my wife and I are ready to build something much smaller and less expensive. This has lead me down the path of trying to build a home using our own financing.

One of the high-cost items in the home construction was the foundation.  The quote that I got for the slab on grade ~30'x70' with a garage ~26'x28' was in the area of $27K, including under-slab and perimeter foam insulation. As a lower-cost alternative, I have recently been considering pier and/or post and beam foundations.  The obvious benefits are the reduced cost and that I can easily understand how to build it by myself.

The property is located in the Willamette Valley in Oregon; in a "C" earthquake zone (i.e., it's not as bad as the coast line, but not as good as eastern Oregon). The winds on the property are not that bad, but I understand that there is occasionally a winter storm with gusts up to 40 MPH or so.  I seem to recall that the wind rating for the area was "B" (on the IRC scale..?).  The soils are described as "silty clay loam", 1500 PSF max bearing capacity, and they are "well draining" with "high water holding capacity" (here is a description of the soil: http://en.wikipedia.org/wiki/Jory_%28soil%29 ).

I am thinking that the building would be something like 24-30' x 40', probably 1.5 story.

OK, on to the questions:

1) In regard to selecting pier or post, I would prefer to use posts because it would enable me to attach braces between the posts. Moreover, I can easily build a wall around the perimeter so that I can enclose the foundation by securing the wall to the posts.  However, I don't like the idea of burying the posts because it doesn't seem like it would last for more than ~25 years or so.  So, I suppose my ideal system would be piers supporting posts supporting the beams.  If we were to use piers all the way to the beams, does that eliminate (or reduce) the need for cross bracing?  Also, with piers how would one go about enclosing the foundation (in other words, there doesn't seem to be anything to secure a wall to without pouring a footing.

2) It seems that the cross braces with posts would do a great deal to stiffen the foundation in the event of an earthquake.  Is this ability lost when running piers up to the beams directly?

3) If I were to go with a pier->post->beam approach (e.g., how "Arky" build his foundation), what are the compromises regarding the additional connection between the pier to post?  Are those compromises offset by the benefits of bracing the posts?

4) I haven't seen much discussion about permanent wood foundations that have crawl spaces similar to a concrete stem-wall foundation.  Are there issues with this type of foundation in regard to its longevity and/or strength?  Would it be possible to install a PWF on a concrete footing rather than just a gravel base? 

(Please assume that the piers are properly sized for the soil and that there is an adequate number of them.)

Thanks!

Chris

[Don_P]

If building to code a pier or post type foundation requires engineering for the reasons you are concerned about, the bracing. In a prescriptive, by the codebook house, the sheathed exterior walls form a line of bracing that creates a rigid box. A prescriptive foundation, a continuous perimeter footing topped by masonry walls also creates a rigid box. When laterally loaded by wind or earthquake they resist the  racking forces pretty well if built to code. When you build a rigid box and sit it on top of poorly braced piers or posts... well, a chain breaks at the weakest link. Obviously post and pier foundations are used and can perform well but the design should follow sound engineering.

1) piers, hinged to posts, hinged to a building is very poor construction. One engineer called it a collapse mechanism. Running piers to the bottom of the floor does not reduce the need for lateral bracing but does remove one of the hinges.

2) No, but if you are going to build walls, do so initially, this is the bracing

3) No

4) PWF is a code recognized prescriptive foundation and does provide good lateral bracing, you can build it on a concrete foundation.

[rick91351]

As far as number four concrete foundations are not that hard.  If I can do'em ....... 

[Patrick]

You should read this build, How they did it may be a solution for you also. http://countryplans.com/smf/index.php?topic=13589.msg177626#msg177626

[cjb80]

Actually, I came across that build yesterday and was reading about it.  I don't have much experience with concrete at all so I have been trying to avoid it.  My experience is limited to digging holes, sticking a post in, empty some bags of concrete in the hole and then pouring water in it.

I also discovered that insurance companies may not cover a pier and beam foundation or charge more, so this has definitely soured my thoughts on this approach.  I am now considering PWF and ICF foundations.

Just out of curiosity, does anyone know a rough estimate of what a premium increase would be for a P&B foundation?

Thanks,

Chris

[OlJarhead]

Do a search on 'Surface Bonded Cement'.  it's easier to do, stronger in many ways then traditional brick and mortar cinder-block construction and can be done in stages.

If I had the time to do a complete build from scratch today I'd start with an SBC foundation and build onto that.