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Old 04-10-2017, 01:50 AM
 
4 posts, read 8,021 times
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Hi!

we are about to start a new home in Westbury (77035). We had a soils test done (2x20ft) and I expected very clear recommendations on the "best foundation" for our soil. We had a plasticity index of 30 in the first 5 feet and then 48 at around 8-9 ft.

However, the soils report listed 4 options
1) slab with 12 ft piers resting on existing soil if the loads are not too high
2) structural slab with 12 ft piers on voidboxes if the loads are higher
3) post tension slab with removing 3-4 ft of dirt and replacing it with select fill
4) conventional re-bar slab with removing 3-4 ft of dirt and replacing it with select fill

Now, I believe those are literally all the slab on grade foundation options that exist and the soils report lists them all instead of saying "pick this one".

To make this more confusing: The builders we are considering and currently talking to feel very differently about this. One says that with this report his structural engineer will suggest doing post tension (option 3) and that he does almost exclusively post tension even on Mil dollar homes. Another builder insists that we should do a void box slab and is 90 % sure that his engineer will come back with that design (option 2). I also read that Bellaire and West U only accept this time of foundation. A third one says he has no preference and we can bring any engineer we want to design the foundation. An engineer we contacted independently for a quote said that he would never do a PT and suggests the conventional rebar (option 4).

Is this normal? Is choosing a foundation type in Houston really a matter of personal preference / believes of the engineer? And are we indirectly by choosing the builder or engineer making the choice about which foundation we end up getting?

Would appreciate some thoughts from structural engineers or other foundation experts!
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Old 04-10-2017, 09:59 AM
 
1,717 posts, read 1,669,356 times
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Are you going with a planned community and builder? They have their own way of doing things and I'm sure they have a tried and true way to do the foundation.


You could call a few of these planned communities and ask how they do their foundations. Do more research online. I'm blank here but wanted to contribute.


I do know the Katy area was rice fields. There's a lot of clay. If you don't water around your foundation this climate and heat vs heavy rains means shrinking and contracting = Foundation cracking.


When we had our home built we had the timbers set into the slab treated to detour termites.
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Old 04-10-2017, 11:03 AM
 
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we are building on our own lot in an existing neighborhood from the 50ies (tearing down the old home). so we have the option to choose builder and engineer ourselves.
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Old 04-11-2017, 07:44 AM
 
Location: spring tx
7,912 posts, read 10,012,660 times
Reputation: 1990
Quote:
Originally Posted by RobzHou View Post
Hi!

we are about to start a new home in Westbury (77035). We had a soils test done (2x20ft) and I expected very clear recommendations on the "best foundation" for our soil. We had a plasticity index of 30 in the first 5 feet and then 48 at around 8-9 ft.

However, the soils report listed 4 options
1) slab with 12 ft piers resting on existing soil if the loads are not too high
2) structural slab with 12 ft piers on voidboxes if the loads are higher
3) post tension slab with removing 3-4 ft of dirt and replacing it with select fill
4) conventional re-bar slab with removing 3-4 ft of dirt and replacing it with select fill

Now, I believe those are literally all the slab on grade foundation options that exist and the soils report lists them all instead of saying "pick this one".

To make this more confusing: The builders we are considering and currently talking to feel very differently about this. One says that with this report his structural engineer will suggest doing post tension (option 3) and that he does almost exclusively post tension even on Mil dollar homes. Another builder insists that we should do a void box slab and is 90 % sure that his engineer will come back with that design (option 2). I also read that Bellaire and West U only accept this time of foundation. A third one says he has no preference and we can bring any engineer we want to design the foundation. An engineer we contacted independently for a quote said that he would never do a PT and suggests the conventional rebar (option 4).

Is this normal? Is choosing a foundation type in Houston really a matter of personal preference / believes of the engineer? And are we indirectly by choosing the builder or engineer making the choice about which foundation we end up getting?

Would appreciate some thoughts from structural engineers or other foundation experts!
i messaged you.

as in my message, a few things, flood plain. that area is prone to flooding, and the flood plains have been changing the last few years. i did a foundation on the other side of 610 from that area, about a year ago that was designed so they could elevate the entire home 4' to get out of the flood plain.

bellaire and west university dont "require" conventional foundations BUT you would be hard pressed to get a post tension foundation through their permit process. they just dont like them.
youre in city of houston so that wouldnt mater.

in regards to the foundation types, it really becomes a mater of how much money do you want to spend.
a PT slab could run you $5-7/ sq ft, a foundation with piers and void boxes could run you upwards of $20/ sq ft, (though $14-16 is more realistic) a 2500 sq ft pt slab could run you $12,000-15,000 and a void box conventional could run you $50,000. you would spend $4000-7000 in soil removal and a pad being built.

i dont agree with the soils report that you need to remove 3-4' though, generally 1-2' down and a 1-2' up is the norm.

any builder that tells you their engineer will only design a XXX style foundation isnt the builder i would choose though. every builder has options, both in engineers and in design. they are trying to point you in a direction that is their preference IMO.
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Old 04-11-2017, 11:51 AM
 
4 posts, read 8,021 times
Reputation: 10
Thanks for all the information, I really appreciate it! I'm just outside of the flood zone so a slab on grade is fine (would still like to go up 1-2 feet from current grade though). Slab size is 2900sf, two story home. We plan on being in the home for 30+ years, so we don't want to go cheapo on the foundation and deal with expensive repairs in 20 years, but also of course don't want to waste money on something that is truly not needed.

What would be the cost per sf if we did a rebar slab on grade (option 4) and just beef it up to be on the safer side (e.g. deeper and thicker beams, thicker slab, more rebar, etc.) Would that get me somewhere in the middle between the PT and the piers with void boxes price wise and performance wise? Or would you say the only way to do a conventional like this is to support it with piers?

I have tried to do my own research reading on many forums and blogs. The opinions on Post Tension are all over the place. Taking out the rants and listening to the more logic based criticisms I got these two main conclusions:
1) Post Tension works well on paper (better than rebar), but installation is more difficult and there are more things that can go wrong during the installation. One example mentioned was that even if everything is done right initially and the pre-pouring inspection comes back perfect, the cables can move while the heavy concrete is poured onto the cables or while the workers move the concrete around and then they are not in the right location anymore.
2) If Post Tension has an issue, the whole slab is affected. Repairs in that case are very difficult. If a conventional slab has an issue it is easier to fix a localized problem.

So my conclusion was that nobody does a "perfect installation". Seeing videos of how concrete gets poured, it looks messy and is not fine art work. The risks of getting a PT wrong are higher than the risk of getting a conventional wrong. And the consequences of making a few mistakes on a PT are much more severe than making a few mistakes on a conventional.

Do you think there is some validity to these concerns?
Maybe the best question is: If you were to build your own home right now, which foundation would you put in for yourself?


PS: I read probably on 20-30 different websites and forums, here just two of the most recent ones:
http://foundationrepairhouston.com/p...b-foundations/ - from a structural engineer
conventional vs post tension slabs? | Page 2 | TigerDroppings.com
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Old 04-11-2017, 04:07 PM
 
Location: spring tx
7,912 posts, read 10,012,660 times
Reputation: 1990
Quote:
Originally Posted by RobzHou View Post
Thanks for all the information, I really appreciate it! I'm just outside of the flood zone so a slab on grade is fine (would still like to go up 1-2 feet from current grade though). Slab size is 2900sf, two story home. We plan on being in the home for 30+ years, so we don't want to go cheapo on the foundation and deal with expensive repairs in 20 years, but also of course don't want to waste money on something that is truly not needed.

What would be the cost per sf if we did a rebar slab on grade (option 4) and just beef it up to be on the safer side (e.g. deeper and thicker beams, thicker slab, more rebar, etc.) Would that get me somewhere in the middle between the PT and the piers with void boxes price wise and performance wise? Or would you say the only way to do a conventional like this is to support it with piers?
a balpark range for a conventional foundation without void or piers is going to run anywhere from $9-12 depending on what size/count of beams and rebar. this option IMO is just fine in that area and likely more cost effective.

I have tried to do my own research reading on many forums and blogs. The opinions on Post Tension are all over the place. Taking out the rants and listening to the more logic based criticisms I got these two main conclusions:
1) Post Tension works well on paper (better than rebar), but installation is more difficult and there are more things that can go wrong during the installation. One example mentioned was that even if everything is done right initially and the pre-pouring inspection comes back perfect, the cables can move while the heavy concrete is poured onto the cables or while the workers move the concrete around and then they are not in the right location anymore.
2) If Post Tension has an issue, the whole slab is affected. Repairs in that case are very difficult. If a conventional slab has an issue it is easier to fix a localized problem.

a lot of that is a bit over stated. post tension placement can shift during concrete placement, BUT this rarely to never will affect integrity of the foundation. generally speaking engineers will put in a factor for "dead cable" in their calculations. in other words if a cable fails, is broken or something of that nature, the other cables in the foundation can and do hold its load. i cant say for sure as i dont remember the exact % but i believe more PT foundations are designed to handle something like 1/4-1/3 cable failure. in addition, the post tension institute (the governing body for PT) has standards, most allow for 12" of deflection in a cable so if the concrete does "push" the cable it is not an issue as long as it doesnt push FEET. you want the cables to be as flat and straight as possible, BUT there is no such thing as perfect.
as for installation, PT is far more cost effective, both in material and labor. the install is far easier and less time consuming. generally a 3-4 man crew doing JUST install of PT can do multiple homes in a day, while the same crew would take a couple of days for a comparable rebar slab.

as for repairs, i also dont agree, PT repairs are fairly simple. the cable has a plastic sheathing with a layer of grease between plastic and cable to allow the cable to move. in a repair case, the old broken cable is simply pulled out and a new cable inserted in its place and re-tensioned. if the foundation shows signs of failing, the cables can be re-tensioned, closing cracks. this is not always the case but is common.


So my conclusion was that nobody does a "perfect installation". Seeing videos of how concrete gets poured, it looks messy and is not fine art work. The risks of getting a PT wrong are higher than the risk of getting a conventional wrong. And the consequences of making a few mistakes on a PT are much more severe than making a few mistakes on a conventional.

Do you think there is some validity to these concerns?
Maybe the best question is: If you were to build your own home right now, which foundation would you put in for yourself?


PS: I read probably on 20-30 different websites and forums, here just two of the most recent ones:
http://foundationrepairhouston.com/p...b-foundations/ - from a structural engineer
conventional vs post tension slabs? | Page 2 | TigerDroppings.com
ill reply in bold as its easier.

ultimately PT slab is very cost effective, and as long as the soil under the foundation is "good" soil will work perfectly. that said, from the PM/EMAIL conversations, my suggestion would be a conventional foundation. piers may be an option, though if your pad is built property, then piers are not likely necessary.
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Old 04-12-2017, 12:31 AM
 
4 posts, read 8,021 times
Reputation: 10
Quote:
Originally Posted by rigas View Post
ill reply in bold as its easier.

ultimately PT slab is very cost effective, and as long as the soil under the foundation is "good" soil will work perfectly. that said, from the PM/EMAIL conversations, my suggestion would be a conventional foundation. piers may be an option, though if your pad is built property, then piers are not likely necessary.

Thanks! If we did conventional and used the standards of this video, what would be your guess for cost per sf?
https://www.youtube.com/watch?v=stOvOxIDRYo
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Old 04-12-2017, 06:33 AM
 
Location: spring tx
7,912 posts, read 10,012,660 times
Reputation: 1990
Quote:
Originally Posted by RobzHou View Post
Thanks! If we did conventional and used the standards of this video, what would be your guess for cost per sf?

https://www.youtube.com/watch?v=stOvOxIDRYo
Tough to say. I don't like some of the things he does. He's using that yellow plastic only because they clearly sponsor him. It's expensive and thinner than standards in Houston. (As an example, not to nit pick)

Most of what he's doing is typical to Houston as well, but the price range has so many factors involved. Size of rebar, size and number of beams, thickness of pad. He looked a bit shallow , 12x24" beams likely, I'd guess in the 8-10/sq ft range but really hard to say.
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Old 04-12-2017, 07:21 AM
 
226 posts, read 303,320 times
Reputation: 326
Quote:
Originally Posted by RobzHou View Post

1) Post Tension works well on paper (better than rebar), but installation is more difficult and there are more things that can go wrong during the installation. One example mentioned was that even if everything is done right initially and the pre-pouring inspection comes back perfect, the cables can move while the heavy concrete is poured onto the cables or while the workers move the concrete around and then they are not in the right location anymore.
2) If Post Tension has an issue, the whole slab is affected. Repairs in that case are very difficult. If a conventional slab has an issue it is easier to fix a localized problem.

So my conclusion was that nobody does a "perfect installation". Seeing videos of how concrete gets poured, it looks messy and is not fine art work. The risks of getting a PT wrong are higher than the risk of getting a conventional wrong. And the consequences of making a few mistakes on a PT are much more severe than making a few mistakes on a conventional.

Do you think there is some validity to these concerns?
Maybe the best question is: If you were to build your own home right now, which foundation would you put in for yourself?
As a structural engineer in another life, I was present during the foundation building of our current house. I didn't design it or anything like that since we went with a MPC builder and like others have said, they have their set methods. Ours was a PT slab design like most these days (residential and commercial). I was there when they placed the cables and checked the spacing per their drawings. I made sure cable chairs were present at critical areas to prevent sagging too low, but in reality everything was fine. During pouring the guys did a good job at avoiding the cables (they aren't spaced out ~8-10", more like 18-24" at least) which isn't difficult for them.

It is cost effective, and unlike what you stated, it isn't hard to do or mess up really. On commercial design where you are designing 2-way flat plates with PT for multi-story high-rises, then yea, cable sag, chair placements, etc all matter and are critical. However, for 1-2 story residential, not so much. As long as the cable isn't 1/4" from the surface of the slab and has the risk of "popping out" after curing then you are fine. Those foundations are way over designed anyway.
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