20 x 30 1.5 story loft

[BlueRidgeBuzz]

Hi folks:

This is my first of likely many many more posts.  I purchased the 20x30 1.5 story plans a couple of months ago and am trying to figure out a couple of issues before I start the permitting process.

First a little bit about the site.  It's about 3.5 acres in the N Georgia mountains sitting at 2200 ft above sea level.  The site is quite level and the soil test indicates an Evard series consisting of well drained sandy clay loam.  Code calls for footings at least 8 inches deep in these parts.

Regarding the foundation, I'm thinking about 24 in wide by 12 in thick square concrete footings at 16 inches deep spaced as prescribed in the plans.  That would give me at least 4 inches of backfill.  For the piers I'd like to go with 8 inch concrete tubes with the shorter side approx 8 inches above grade and the tall side no more than 16 inches. (there's a slight slope but I'm unsure exactly)

My questions are:
1.  Are these general dimensions OK given the structure and soil conditions?
2.  Should the piers instead be 10 or 12 inches, and if so, what is the best way to skirt them?  It seems anything larger than 8 inches will protrude past the walls, sheathing and siding.  Or do you skirt in between them?  I'm considering a two sided porch so aesthetics are only an issue on two sides.

Thanks in advance for your feedback!
-Jeff-

[John Raabe]

Jeff:  

Your footings should be fine for your soil but I would check locally to confirm. Tie your footing and concrete tube together with rebar.

To allow for skirting of the piers you can set the beam line back inside the wall line by as much as the depth of the floor joists. This is not a cantilever and should allow you to skirt straight down the outside wall (you can use PT wood, corrugated metal, plastic lath, etc.). For the height you are coming out of the soil an 8" dia. pier should be fine. Others who are building in similar areas may have other suggestions.

If you want to be more conservative, a deeper footing with a larger diameter pier will be stiffer if you could be exposed to high wind or earthquake.

[Redoverfarm]

I probably go with at least  10" pier for rigidity.  In addition you might rethink the 8" off grade for the mere fact of maintenance being able to access what utilites you may have under the house.  16" is about the least I would consider for a crawlspace.  As John stated if you could determine your offset from the finished wall to the face of the pier to incorporate 2X4 framing with a skin cover that would only be 4" or so depending on you finished skirting material.  That would allow you to attach that framing to the pier which would not be in contact with the ground.

[BlueRidgeBuzz]

Thanks guys!

I was thinking 8" was little small on the piers but I couldn't get past the skirting thing.  That's a great idea about offsetting the beam.  So if I'm thinking about this correctly, if I push the pier and beam in about 5 inches, that should give me enough space for a flush 2x4 frame with PT lattice and still line up with my siding (give or take a half inch).

The more I think of it, you're probably right on the 8" height.  I was thinking if I needed to get under there, I could get under the high side beam and still have plenty of room but it would get tight towards the low side.

One question.  You said I could keep the skirting off the ground this way.  Wouldn't I want it on or just below grade to keep critters from building a home under there?  Or use a wire mesh behind it instead?

I'm an engineer (wrong kind for this project) and by nature a planner so I have to get all of the details worked out in my head first.

Thanks!
-Jeff-

[Redoverfarm]

I guess I am old school when it comes to ground contact and wood. Decay,rot and termites come to mind.  Yes there is pressure treated but I have seen this occur to that as well.  Burrowing critters cannot be stopped unless there is a deep penetration below the surface. You would be far better IMO to make it apperance wise and take steps to protect the actual house with hardware cloth on the bottom of your floor joist.

Yes the heigth is a plus when you decide to insulate.

[Don_P]

This came up on the inspector's website last week regarding an addition on piers, not even a full house. This is from the '09 IRC, GA is on the '06. The intent has not changed, the language is now much clearer. This was one inspector responding to another-

Here is what the code says.

R602.10.7 Braced wall panel support. Braced wall panel
support shall be provided as follows:
1. Cantilevered floor joists, supporting braced wall lines,
shall comply with Section R502.3.3. Solid blocking shall
be provided at the nearest bearing wall location. In Seismic
Design Categories A, B and C, where the cantilever
is not more than 24 inches (610 mm), a full height rim
joist instead of solid blocking shall be provided.
2. Elevated post or pier foundations supporting braced wall
panels shall be designed in accordance with accepted
engineering practice.


And from the commentary.
Elevated post and pier foundations
Elevated post and pier foundations generally provide little
lateral support to the structure above. Lateral support
instead typically comes from diagonal bracing or other
elements of the foundation; however, there are ways of
cantilevering the posts of the foundation by burying the
ends deeply into the ground, so that they develop lateral-
load-resisting characteristics.
It is difficult to accurately and fully present all of the post
and pier lateral support methods in one set of prescriptive
provisions. For this reason, post and pier foundations supporting
braced wall panels must be engineered in accordance
with the IBC or referenced documents, as stated in
the Section R602.10.7, Item 2.

[John Raabe]

If you want to avoid all these code issues build the crawlspace or slab  foundation instead (page 2 or 3 in the plan set).

[Don_P]

Although this article is geared towards porches it describes the thinking behind the code section I quoted above;
http://www.ulteig.com/uploads/Ulteig_Porch_Lateral_Bracing_Article._0910pdf.pdf

With engineering this can be done. With engineering in accordance with accepted practice a standard could be proposed as an exception.

[BlueRidgeBuzz]

Your replies have certainly given me pause.  I think the concept of piers gives many of us DIYers hope at the possibility of a foundation that can be done in incremental steps.  However most plans and documentation do not address the racking issues especially when the need for a deep footing due to frost is not there as in my case.  If I had 4 foot deep piers with 18 inches above grade, I don't think it would be as much of an issue.  But if I'm pouring that much concrete, I might as well pour a continuous more shallow footing.

The concrete stem wall in the plans seems a little tough given the amount of concrete required and the difficult access to my site.  If I poured a 16 inch wide and 8 in deep continuous footing below frost depth, let's say 12 inches deep, and then built an 18 inch above grade block wall (8 inch block) with some rebar reinforcement, would that be sufficient?  I know cinder block is not as strong as a solid concrete wall but it's much more do-able for me.  What do you guys think?  Thanks!

[Don_P]

Chapter 4 of the IRC is on foundations, GA basic codes are not online, I'd bet mine, VA are very close.
https://www2.iccsafe.org/states/Virginia/Residential/Res-Frameset.html

A 16" x 8"" thick footing would be standard... and all depending you don't want to go narrower, I've seen folks barely hit that with the blockwork. A block wall will be many times stronger than most braced piers.You're also saving girder material to help defray cost and functionally the floor just picked up solid perimeter support, only the joists are deflecting, not the joists and the girders. The minimum block wall would be piers spaced around it made of 8" block or larger with as small as 4" thick block or brick between, pier and curtain wall. This is the lowest prescriptive standard detail that reliably provides adequate bracing for normal lateral loads in most situations. You can go further with engineering.

For most places the ability to get a concrete truck in is pretty much the kind of road you need.

My foundation is a block crawlspace and we did get hit by design wind speed just after I was rough framed (Hugo), no problems with the foundation. It got greater than design snow load a few years later.

Edit,
Here is the SC code cite for a pier and curtain wall foundation
http://publicecodes.citation.com/st/sc/st/b4v07/st_sc_st_b4v07_4_sec004_par006.htm?bu2=undefined

[BlueRidgeBuzz]

I check the VA code pretty regularly.  I wish GA would post theirs.  All I can find in GA are the amendments but most are not foundation related.  I would guess VA and GA are very similar other than maybe depth requirements.

The road into my place is good and my gravel drive is fairly steep but accessible to a concrete truck.  Well, the last 50 feet are pretty steep (my F150 with positive traction can't make it with an empty bed) but my AWD Honda Pilot takes it without a problem.  So I feel pretty confident a loaded concrete truck can handle it although it will be tight up top.

Good point about the savings on the beams. That's pretty significant and makes the crawl space look even better.  We don't get hurricane force winds through here very often but every once in a while they're still at tropical storm force by the time they get to N GA.  I don't want to take my chances and skimp on the foundation.  Thanks Don!

[Don_P]

He'll have alot more sticking those tires down than an F150. We've been running it up 20%+ grades here that I have to lock my posi Dodge in for when empty.

Pier and curtain is probably minimum. I don't think it'll really be much cheaper than just going with all 8" block.

[BlueRidgeBuzz]

I think all 8 inch block is simple and straightforward.  The pier and curtain just seems like unnecessary complexity.

We're getting 90 mph gusts around here right now with some nasty thunderstorms passing through the Atlanta area.  I'd feel a lot better sitting through this on a solid foundation!

[John Raabe]

Here is a good overview of the steps to building a block foundation wall.

http://www.ehow.com/how_6510975_build-concrete-block-foundations.html