Author Topic: Post and Pier question  (Read 7820 times)

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Offline Abbey

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Post and Pier question
« on: April 04, 2013, 07:20:26 AM »
I've been thinking about a post and pier foundation but I don't like the look of concrete or of the pressure treated posts. What I've found is some pictures of stone foundations that I think look nice and I was wondering about some ideas that I've had and would like to get some advice.



Instead of using a PT post like in the picture above or sona tube I was thinking about using a steel rod welded to a flat steel plate that would be the base. The rod and base would be painted with rust inhibting paint. I would line the hole with lanscape fabric and fill with gravel that would be compacted. On top of the gravel I would stack and mortar stones in a circle around the rod and when I got close to the height I need I would place a threaded J-bolt for the sill beams.

Something like this but with a larger circle of stones for the pier.



My land has a very high clay content, lots of ledge, and is wet. I haven't dug any test holes where I'm thinking about bulding so I don't know how deep I'll be able to get. In some places it may only be a foot before I hit ledge. What I'm thinking is the fabric will allow water to enter and exit and the gravel will allow any frezzing water to expand without shifting all of the gravle and causing the stones above to shift. The rod and base will protect from any uplift.

Is the rod and base needed? Would the stones be enough weight to help protect from lifting?

Offline Squirl

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Re: Post and Pier question
« Reply #1 on: April 04, 2013, 08:34:06 AM »
You are probably going to get a lot of posts about the pitfalls of post and pier foundations.  They have their downsides even when they are solid concrete and rebar all the way to the beam.

You are building in clay.  It has a very low load bearing capacity.  You will need to calculate the load of the structure and make adjustments for loading and then calculated if the piers can translate that to the soil.  A full foundation doesn’t have to do any of that.

The half masonry piers.
Typically at the time of the pour if joining concrete footings to masonry rebar is put in place before the pour.  The concrete can also be drilled and the rebar epoxied in but it is less than ideal.

A lot of the strength of masonry comes from the bond pattern.  This takes loads and evenly distributes them over a larger area.  That is why when it comes to rubble stone masonry (unevenly sized/shaped stones not in pattern) code requirements dictate extra thickness (16”).   Rebar helps add strength, but it is hard to quantify with the type of masonry you are proposing.
 
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Would the stones be enough weight to help protect from lifting?

No.  The house will weigh far more than the stones.  Weight is regardless over frozen water.  It can move mountains (glaciers).  Typically if you hit bedrock and it goes below the frost line, then it probably isn’t moving.  Normally people will drill and epoxy rebar into the rock and build the footing on top of that.  No water, no frost heave.

Allowing water into the gravel without it draining will likely cause some frostheave.


The picture of the cabin is aesthetically pleasing, but a closer look with some basic principles shows some weaknesses.  The foundation looks to have already shifted even on such a small structure. The pier that looks like it sunk on the front corner looks far overloaded.  It would have to expand to a very large footprint underground to hold the load of that it appears to be holding. The porch beams also don’t appear to be adequately sized and look like they are sagging.

Offline Abbey

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Re: Post and Pier question
« Reply #2 on: April 04, 2013, 09:57:42 AM »
What about a floating foundation?

Offline Squirl

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Re: Post and Pier question
« Reply #3 on: April 04, 2013, 10:49:32 AM »
This what I know of a floating foundation. 
http://www.nachi.org/forum/f23/floating-foundation-11184/

It is usually fine because as long as you have a continuous perimeter frost wall and a roof with good drainage, there should be no water under the slab.  No water, no frost heave.

Offline Abbey

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Re: Post and Pier question
« Reply #4 on: April 04, 2013, 11:28:29 AM »
No, it's closer to what I talked about in my first post. I read an article by an architect and he said he was at a 100 year old house that had been built on large stones that were just on the groun. He thinks the house has had no problems because it's small and any lift from the stones in the winter simple lifts the entire house too, without any damage. I'm trying to find the article, maybe tomorrow.

Offline John Raabe

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Re: Post and Pier question
« Reply #5 on: April 04, 2013, 02:53:28 PM »
Quote
My land has a very high clay content, lots of ledge, and is wet. I haven't dug any test holes where I'm thinking about building so I don't know how deep I'll be able to get.

I hate to say it, but in really poor soils (expansive wet clay) you probably shouldn't build a home at all. You are best advised to build on solid, stable ground that has good air and water drainage. Anything less might be nice for camping in certain seasons but is probably not a good investment for a long term home. Sometimes the smartest decision is to find a better site.
None of us are as smart as all of us.

Offline Remington760-308

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Re: Post and Pier question
« Reply #6 on: April 04, 2013, 03:53:26 PM »
I priced everything from cement post 5' tall too sona tubes - too frost walls and found that I could buy and install 2'x2'x2' cement block first  and 2'x2'x4' on top cheaper and have a solid POST AND PIER system that will last my life time and my sons also.... the blocks are going to cost me $550.. JMO

Offline Don_P

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Re: Post and Pier question
« Reply #7 on: April 04, 2013, 07:34:35 PM »
An engineer is cheaper than misplaced optimism or abandoning the site... and it becomes legal. And as one engineer puts it, you just picked up the insuror of last resort, his. I agree with John but I've also built on more clay than you can shake a stick at, it depends on the clay, and I leave that judgement up to them that know.

Offline Abbey

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Re: Post and Pier question
« Reply #8 on: April 05, 2013, 05:16:51 AM »
I guess I can move a little farther up the hill and build directly onto ledge, the only thing is then I'll need to use a water pump where as my first choice for building would allow a gravity fed system from the spring. There's a big glacial erratic, maybe I could build on top of that and cantilever my floors out over the edges of the boulder. When I first saw it I thought about building on it, but I was figuring the house would be tippy sitting on top of a boulder.

Oh well, guess I'll have to do some more walking and thinking.

Thanks for all of the input.


Offline Squirl

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Re: Post and Pier question
« Reply #9 on: April 05, 2013, 06:55:16 AM »
I have what sounds like a very similar site.  I had an engineer and the inspector (also an engineer) out to test my soil for septic as part of the purchase process.  Both gave recommendations about the soil.  I have a "fragipan layer of shale clay."

http://countryplans.com/smf/index.php?topic=10998.msg140774#msg140774

It doesn't drain.  At all.
 http://countryplans.com/smf/index.php?topic=10998.msg142925#msg142925

 With no secondary drainage system because of the slope of the land, it eliminated a few foundation options.  No wood foundation.  Not recommended pier foundation.  Neither individuals were fans of concentrating loads especially not with my soil.  Rather than having to deal with possible basement water issues I went with a crawl space.  Another option open for me was the frost protected shallow foundation.  I would have used that one if the mini-excavator couldn't get through the shale clay.

In my conversations with engineers, there is a way to accomplish anything (for the right price).

Offline Abbey

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Re: Post and Pier question
« Reply #10 on: April 05, 2013, 08:27:52 AM »
Thanks for the link to your build, Squirl, I had read it a few weeks ago but it's always good to re-read things because everything starts blending into one gigantic build after reading so many projects.

What I'm hoping for is a foundation that is very simple and inexpensive, but will last and be safe. I know I'll be making some compromises and this is probably too much to ask for in a simple and cheap foundation. Since I'm only looking at building a one story around 600 square feet I don't think I need such a highly engineered foundation.

What I'm thinking now is going up on the hill near the glacial erratic and digging down some, once the snow is gone, and see how deep I have to dig before bedrock. In some places it's on the surface so I might not have to dig all that much. My hope was to build near the base of the slope where I’ve got a field for solar electric, hot water and can have a gravity fed water system. It’s actually prettier higher up on the slope with some sweet exposed ledge, the erratic, and some big old sugar maples. It’s also over 500 feet from the road or my closest neighbors, so it’s even more private, which is nice when you enjoy showering outside in the summer.

I’ve got options, I just need to narrow it down to one and then move forward. All of the information I’m getting here is helping.

Offline Squirl

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Re: Post and Pier question
« Reply #11 on: April 05, 2013, 08:50:11 AM »
Yes.  There are many options.  Almost all involve some type digging.  I don't remember if you posted a location.   A general location may help in describing alternatives. 


By the way, I have posted this a lot.  Post and pier isn't necessarily always the cheaper option.  It is common for people not to add up all the costs.  A full perimeter foundation was very little difference in cost from post and pier and I am in a 4ft deep frost depth area.
Rather than repost my usual diatribe.
http://countryplans.com/smf/index.php?topic=12895.msg167945#msg167945

Offline Abbey

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Re: Post and Pier question
« Reply #12 on: April 05, 2013, 09:40:46 AM »
I saw that post, with the pictures.

I'm in central Vermont.

Offline Adam Roby

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Re: Post and Pier question
« Reply #13 on: June 04, 2013, 02:29:20 PM »
Yes.  There are many options.  Almost all involve some type digging...

I'll probably get shot for this question but I think it merits a reply from the experts.  Just remember, I am a newbie here, and this is my very first post.  Welcome me...   :)  Introductions to come...

http://www.homedepot.ca/product/adjustable-pylex-50-foundation-screw/997467
 
These are foundation screws that go into the ground 50".  They say they can hold 5000 lbs.  Is it just me or does this seem like either a false advertising or a dream come true?  At 24$ a pop they sure sound a lot easier than digging a 4' hole, rounding out the bottom, lugging heavy bags of cement to the site, mixing, pouring, waiting... heck, they even come with a post anchor built in and 3" of adjustability.

Offline Don_P

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Re: Post and Pier question
« Reply #14 on: June 04, 2013, 06:02:30 PM »
 w*
Now put on your flak jacket  :)

The first thing to look for with an "alternative" product in the US, one that is not prescriptive in the building code, is an ESR number, an Engineering Services Report. A building official is not required to automatically accept this but generally they do as long as the use is consistent with the tested use. I get the feeling there is not one for this product. The next avenue is to have an engineer design a foundation using the desired product. I'll go out on a limb, I doubt this will work without some extra lateral reinforcement between clusters of these micropiles. I think I linked to an article not too long ago with a picture of a typical engineered screw pile using 8" pipe with helical flights welded to that. Lightweight screw anchors are good tension devices, to hold a building or mobile home down against wind. As a compression element it needs to be a good column and these look mighty light.

Now for the part where everyone else can change channels. A column needs something to hold it upright against toppling. Add something like a huge sail on top of the column and it really needs something significant to keep it from toppling. Mud is not a reliable bracing element which is why the building code also requires a pier type foundation to be designed by an engineer. A continuous braced wall will do a much better job of resisting the horizontal forces delivered to it by the building above. Which is why that is a prescriptive, no engineer required, method of supporting most houses.

Rounding the bottom of any foundation support is a huge mistake. Big flat bottomed holes resist toppling. Round bottoms encourage toppling. I'm certainly not saying digging 4' holes and lugging a few sacks of concrete in to make a pier is a good idea, that is still an engineered foundation or someone who is several sacks shy of a full load  ;).

Offline Adam Roby

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Re: Post and Pier question
« Reply #15 on: June 04, 2013, 06:29:53 PM »
Yeah I figured they were too good to be true.

I said "round out the hole" but I meant widen or "traingulate?" the hole.  Heck, a drawing speaks better than I can explain it.



Offline Don_P

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Re: Post and Pier question
« Reply #16 on: June 04, 2013, 07:03:09 PM »
Gotcha, that comment appeared about the same time as a round edged footing in another thread. Reminded me of a barn foundation I helped repair. The original builder had used a rounded spade to dig the footings and was too lazy to square them up properly. It took decades but the block stemwalls rolled over.

Something I've mentioned several times is a post frame building, the modern "pole barn". If the posts run from the top plate of the walls down to the footings, the walls when sheathed brace the long posts. Then the posts cannot topple.

If you have rock on the property and more time than money another option is a rubblestone foundation. They are not that expensive, look good and can be built quite strong.


Offline cbc58

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Re: Post and Pier question
« Reply #17 on: June 06, 2013, 03:20:40 AM »
someone else here from VT dug holes, dropped in bags of concrete unopened, and then put posts in and then filled it back in... can't remember who.  different soils though.  I had also posted a question about possibly using lally columns for piers - the kind that are concrete encased in metal - then you could then build a stone veneer around to make it look like the pic you posted of the old cabin.
 
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An engineer is cheaper than misplaced optimism
   This is a great quote.. and true.  The last thing you want is to put in a foundation that doesn't suffice - will cost you more in $$ and heartache in the long run.

if this is a getaway place... treehouses are cool... ever thought about building one of those?


Offline Abbey

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Re: Post and Pier question
« Reply #18 on: June 06, 2013, 04:21:20 AM »
cbc58,

It would be a primary home. I've thought about building a treehouse as well as going with the Mike Oehler PSP underground house.

Offline Squirl

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Re: Post and Pier question
« Reply #19 on: June 07, 2013, 08:58:57 AM »
They say they can hold 5000 lbs.

•Can support up to 5,000 pounds


This is probably based on the steel alone.  A 6" diameter circle will not hold 5000 lbs in soil.  No soil and even some solid rocks can bear that much weight in a 6" circle (.2 square feet).  Basic physics.

 

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