Author Topic: Foundation Debate help if you will ?  (Read 2876 times)

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Offline Rensmif

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Foundation Debate help if you will ?
« on: February 13, 2012, 09:41:09 AM »
I am still in the beginning stages, ordered and recieved the 20x30 1.5 story plans but want to get this right so I want to start with the foundation.

I will be building in GA in sandy type soil, the guys who did the cores for the septic tank hit sandy brown soil all the way to around 4.5 feet and then it was red and starting to clump but still not brown clay (they stated that would be awful for septic and foundations) and they went as far as 6 feet down.  Frost line is 12 inches and there is a slight grade in elevation maybe 2 feet in a 30 foot length.

I dont particularly want to do a slab foundation, and I now realize that the concrete perimeter foundation with crawlspace is the strongest next option, and have not been overly confident in the post and beam foundations with PT wood.  I was thinking of filled concrete blocks being stronger but had read they dont offer the lateral support of a wood sytem done correctly.

SO - my long question - based on you guys past expieriences, is a concrete pier a better way to go, or a well braced PT wood sytem ?

I am confused and asked after finding what "I" thought was one of the nicer wood foundations here  http://brucemcdonald.com/MainPages/Foundation/Found1.html

I would love opinions on my question as well as the bracing he uses on his build, it is new to me but seems very nice

Offline Squirl

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Re: Foundation Debate help if you will ?
« Reply #1 on: February 13, 2012, 11:25:42 AM »
I was thinking of filled concrete blocks being stronger but had read they dont offer the lateral support of a wood sytem done correctly.

 ???  That is the second strongest type of foundation behind a solid rebar and concrete wall, and pretty close to that.

If you are concerned with bearing capacity of the soil a full perimeter continuous footing is best.  It distributes the weight over a larger area.  The wider the footing the more soil the weight of your house bears on it.  Sandy Loam is usually a good soil to build on as long as it isn’t back fill.

I would KILL to have a 12” frost line.  I could probably excavate the whole foundation with a walk behind trencher. 1 day, 1 person, $150, done. I never understood why I see a lot of people on this site opt for a deep pier foundation to a full perimeter in a shallow frost line area.

I generally don't go with pictures of on the find on web.  First rather than using a 4x6 or 6x6 post, this person used a 4x4 post with a 2x4 nailed to it.  The 2x4 is not in a bracket. And there are no brackets connecting the girders to the posts.  Most PT 2x4s are not ground contact rated, but these could be, we just don’t know.  The plywood across creates a nice diaphragm and is good for a pier foundation.  Based on views of the pictures, my best estimate for sizing the setup, the piers are spaced randomly from 6-8 ft with a girder built of 3 – 2x6s.  This violates girder sizing provisions in the building code. Some of the spices don’t fall over piers, which is not only a good practice, but is also required for code. In addition the house is 1 story, which may not always translate to other buildings.  This could all be engineered to work, but I just can't tell that from the site.

 I was curious as to why he chose this route.  Under the earth moving section he states that 75% of his site is backfill (what a nightmare).  He had to get back to solid undisturbed soil.  It wasn’t bad, I just would have done it this way. 

Offline Squirl

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Re: Foundation Debate help if you will ?
« Reply #2 on: February 13, 2012, 11:36:05 AM »
I believe you are also in Georgia.  I would be more concerned with ground contact wood.  You are in a wetter, humid, warmer environment with termites.  I would be more comfortable with concrete and a termite shield than ground contact wood.  Under the southerpine guidelines for PWF and lumber than is even treated to PWF standards, parts of the gulf coast, Florida, and Georgia are listed as severe wood deterioration zones

Offline Rensmif

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Re: Foundation Debate help if you will ?
« Reply #3 on: February 13, 2012, 11:57:46 AM »

I generally don't go with pictures of on the find on web.  First rather than using a 4x6 or 6x6 post, this person used a 4x4 post with a 2x4 nailed to it.  The 2x4 is not in a bracket. And there are no brackets connecting the girders to the posts.  Most PT 2x4s are not ground contact rated, but these could be, we just don’t know.  The plywood across creates a nice diaphragm and is good for a pier foundation.  Based on views of the pictures, my best estimate for sizing the setup, the piers are spaced randomly from 6-8 ft with a girder built of 3 – 2x6s.  This violates girder sizing provisions in the building code. Some of the spices don’t fall over piers, which is not only a good practice, but is also required for code. In addition the house is 1 story, which may not always translate to other buildings.  This could all be engineered to work, but I just can't tell that from the site.

 I was curious as to why he chose this route.  Under the earth moving section he states that 75% of his site is backfill (what a nightmare).  He had to get back to solid undisturbed soil.  It wasn’t bad, I just would have done it this way.


Thanks for the reply and ths SOLID advice,  That is an impressive "looking" home he is building and I figured he may have reinvented the wheel with his style of bracing, I guess I should have been on alert when he said he "filled the hole with concrete" and mentioned nothing about rebar or gravel at the bottom of his holes, if not that perhaps his method of attaching the brackets.  That is why I am trying to read as much as I can so I can proceed in the best method. 

It is remarkable how much I have learned since coming here, I have visited new construction sites recently with licenced contractors building and seen things that I was quite surprised by.  As for the trencher advice , I am going to have to rent one to run my water line, but never thought about using it for the excavation as well, brilliant advice.   Thanks again

Offline MountainDon

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Re: Foundation Debate help if you will ?
« Reply #4 on: February 13, 2012, 12:02:00 PM »
With a 12" frost depth I think a perimeter trench at 12 to 18" with a continuous reinforced concrete footing would provide a base superior to anything else. Then wither a short concrete block wall, or a formed and placed concrete mix wall on that. With such a shallow depth in the ground there are minimal lateral ground forces on the blocks. That gets you a strong foundation, no doubts, no engineer needed. It's all prescribed in the IRC. Link Hereto the Foundation chapter

Just because something has been done and has not failed, doesn’t mean it is good design.

 

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