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Don_P
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# Posted: 4 May 2016 07:20
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"You WILL have problems with moisture", that's untrue. You will need to make sure it is dry under there. I've been under just about every kind of structure with moisture problems including rotted piers and beams. Read first the building code regarding foundations. Pay attention to ventilation, drainage and site grading. Then go to the buildingscience site and read up on conditioned crawlspaces.
Nowhere in the US can you build a habitable structure on the surface, minimum footing depth in frost free zones is 12" below grade on undisturbed soil. They want to be below the loose organic topsoil.
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bldginsp
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# Posted: 4 May 2016 09:00
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Quoting: Lawdog I read that if you build a full perimeter footing with blocks, you will have problems with moisture
Hmmm... wonder where this odd rumor got started. With plenty of underfloor ventilation and good grading outside you should be fine. That's how the majority of houses are built, though most with concrete stem walls, not block wall. but that doesn't matter.
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old greybeard
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# Posted: 4 May 2016 15:03
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Man. I hope the guy that built mine knew what he was doing. 24x28 on block piers. With a steel beam running thru the center. Stood 18 years so far with no sign of settling or movement. But I think it helps we are on top of a mountain with the piers on bedrock.
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KinAlberta
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# Posted: 4 May 2016 16:49 - Edited by: KinAlberta
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Quoting: bldginsp I would NEVER build a 24x28 habitable structure on piers alone, unless I didn't care if it was still there in 30 years. That's just me.
I really think it depends on the site and an understanding of where and what you are building on. So to be absolutely safe, yes, a full foundation down 6 or 8'.
However, as I said, our main 24x30 is around 60 years old and just requires a bit of work now to level it up, but has essentially been untouched for 50 of the 60 years with about a 1" sag (twice maybe) at one end where we let eavestrough water dump onto the corner for a decade without putting back the drainpipe after removing the water barrels. (Soften clay with water and the building sinks.)
It sits on concrete blocks, open air underneath it, and has survived -40 to near +40 degree weather over the years.
If it had been built with larger beams, more of the load would have been carried by blocks further under the building where it is perpetually dry. While our building sits well above the lake level, I'd guess that some improved drainage around the building would absolutely guarantee zero frost heaving (which we have never experienced to date).
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bldginsp
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# Posted: 4 May 2016 20:13
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Quoting: KinAlberta I really think it depends on the site and an understanding of where and what you are building on.
Agreed. A larger building on tall piers in a strong earthquake would almost certainly fall off the foundation or topple over. The same thing could happen with wind shear, but I bet it would take a hurricane to do it. My grandfather built a 30x30 on piers to bedrock in the 1930s and it's still there. That cabin is in a low earthquake and wind area.
So a lot of it is just personal preference. A full perimeter spread footing is permanent- or about as permanent as you can get. But it is a hurdle both in terms of cost and effort, and for recreational use I can see that people just don't want to do it. I just think people should always be made aware that it's an inferior method, so they make their own informed decision.
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Don_P
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# Posted: 4 May 2016 21:41
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Quoting: KinAlberta I really think it depends on the site and an understanding of where and what you are building on. So to be absolutely safe, yes, a full foundation down 6 or 8'.
When you design a pier foundation you're practicing geotechnical engineering. Some people understand things better than others. I've dug in enough dirt around this country to know I wasn't born with that natural talent. The prescriptive foundations work, if a client wishes me to build something other we'll bring in the engineer, now it works again. It's not that I don't believe the homebrew engineers can't make something that'll work. It's just the knowledge that if it does work it is pure luck, there really is no understanding of geology or structural mechanics and no desire to find and research failures, ever see them discussed on these forums? In OH I'd bet the footings could be about 2' deep. I would not be surprised if the engineer comes back with a pier that is 6-8' deep... more of why I don't practice at that.
The older pier and beam houses here were typically grounded on the upper part of the slope. That sill, that was quickly in ground contact, provided the lateral anchor for the taller piers through the floor system. Generally by the time I get to them the naturally decay resistant sills have decayed and the piers have overturned. Jacking a building back up takes a toll and generally some things have taken a permanent set. I don't recall any, jack it up, shim it and go out for lunch jobs.
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bldginsp
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# Posted: 4 May 2016 22:19 - Edited by: bldginsp
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Quoting: Don_P The prescriptive foundations work, if a client wishes me to build something other we'll bring in the engineer, now it works again.
Even the prescriptive foundations can fail, because soil conditions can be so unpredictable. But a prescriptive full perimeter spread footing stands the best chance of long term success, because it is big and it is a single monolithic unit. A pier foundation is a dozen or more separate foundations, each of which can go its own way. A pier foundation puts a minimum of concrete surface area against the weight bearing soil, a perimeter footing is overkill, putting a maximum of concrete area to the soil, particularly with smaller buildings. Since soil conditions are so variable and unpredictable, that's where I'd prefer to go a bit overkill to protect my investment. All of the rest of construction is exactly calculable, soil performance over time is not.
But it's a crap shoot. Many buildings on piers will last a very long time, some won't. More buildings with spread footings will last even longer. Which side of the risk equation do you want to be on?
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KinAlberta
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# Posted: 23 May 2016 10:42
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In my experience, people today don't appreciate anything old. So if you build anything to last, it's probably a waste because the next owner may just come along and tear it down.
Around here in Alberta any home or building over 40 years old really isn't prized by anyone anymore, especially if it's small and lacking a lot of flexibility. That said, sad as it is, people really need to think about how much they spend on their structures.
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