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Old 11-07-2011, 10:58 PM   #1
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Post-Tensions Slabs


Which is the better slab-on-grade for a site built home in a seismic zone?

I was told that post-tension slabs is the way to go but I also heard that they are more expensive and that a 4,000 psi reinforced/rebar slab is just as strong as post-tension.

What are the pros and cons of post-tension? I know one CON is that you can't drill or cut post-tension slabs.

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Old 11-07-2011, 11:56 PM   #2
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Post-Tensions Slabs


Actually you can drill and cut post-tensioned slabs, you need to be very careful where you do cut or drill so as not to cut a tendon though.
My contractors usually get the slab x-rayed before cutting.
Lots of PT slabs here in So. Cal.
The PT slab can be thinner and is supposed to be more seismic "resistant", fewer cracks and whatnot. Much more expensive though.

Andy.

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Old 01-14-2012, 09:35 PM   #3
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Post-Tensions Slabs


Here are some photos of a PTS prior to the pour:






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Old 01-15-2012, 03:37 PM   #4
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Post-Tensions Slabs


thanks for the photo's
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Old 01-29-2012, 07:53 PM   #5
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Post-Tensions Slabs


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Actually you can drill and cut post-tensioned slabs, you need to be very careful where you do cut or drill so as not to cut a tendon though.
My contractors usually get the slab x-rayed before cutting.
Lots of PT slabs here in So. Cal.
The PT slab can be thinner and is supposed to be more seismic "resistant", fewer cracks and whatnot. Much more expensive though.

Andy.
Out there in Cali do they consider post-tension slabs stronger/better than re-bar reinforced slabs for seismic?
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Old 01-29-2012, 08:00 PM   #6
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Post-Tensions Slabs


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Out there in Cali do they consider post-tension slabs stronger/better than re-bar reinforced slabs for seismic?

That will completely depend on the PT slab engineering.

Andy.
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Old 01-29-2012, 09:25 PM   #7
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Post-Tensions Slabs


Slabs can be built as standard reinforced concrete, pre-tensioned concrete slabs, or post-tensioned concrete slabs. The performance differences under seismic loading is different, but each slab performs acceptably if properly designed and constructed. Post-tensioned slabs are sometimes used because in practice they can be built thinner than an equal strength ordinary reinforced concrete slab due to the use of stronger steel cable than standard rebar, but for residential use the minimum thickness of concrete slabs due to the need for protective covering of the steel limits the cost advantages of post-tensioned slabs.
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Old 01-30-2012, 01:22 PM   #8
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Slabs can be built as standard reinforced concrete, pre-tensioned concrete slabs, or post-tensioned concrete slabs. The performance differences under seismic loading is different, but each slab performs acceptably if properly designed and constructed. Post-tensioned slabs are sometimes used because in practice they can be built thinner than an equal strength ordinary reinforced concrete slab due to the use of stronger steel cable than standard rebar, but for residential use the minimum thickness of concrete slabs due to the need for protective covering of the steel limits the cost advantages of post-tensioned slabs.
Out here in Phoenix with this builder, he does ALL of the residential tract homes with post-tension, as seen in the photos. When I asked why, they stated that the post-tension gave them better results over the reinforced concrete, as they were getting call-backs and warranty claims due to cracked slabs. They don't get those now with the post-tension.

The soil out here is VERY EXPANSIVE and the clay will swell and shrink rapidly when we do get rains. I wonder if that is why post-tension is preferred out here, due to the expansive soils?

Also, as you can see in the photos, they don't lay down any "AB" or aggregate base, it is raw earth. They do compact it but that is native soil. In other areas where they do a reinforced concrete slab, they will lay down around 4" of AB, then compact it, then lay the re-bar down and pour the slab.

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Old 01-30-2012, 01:58 PM   #9
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Post-Tensions Slabs


Both post tensioned and pre-tensioned concrete slabs put the concrete under permanent compression, except under highly unusual loading conditions such as an earthquake, where it is possible that parts of the slab may go into tension. Due to the constant compression of the concrete, the slabs resist cracking better than a standard slab, where the concrete is not under compression until moment is applied to it by loading. Concrete which is not under compression tends to crack as it cures, which is primarily a cosmetic issue, but some homeowners do not like it.

With expansive soils, the slab can go into tension on the top side very easily if the soil becomes wet and the center of the slab lifts more than the edges, or the bottom of the slab can go into tension if the center of the slab settles due to shrinkage of the soil. Since expansive soil is notorious for its variable settlement/expansion over short distances, a post-tensioned or pre-tensioned slab makes sense because it puts the entire slab into compression, and if properly designed the tension forces created when the soil moves can be negated by the pre-compression of the concrete. This minimizes cracking of the slab under loaded conditions, which is why post-tensioned or pre-tensioned slabs are popular.

A post-tensioned slab can be tensioned in the field after the slab is poured, a pre-tensioned slab is more suited for factory work because the tendons must be stretched prior to the concrete pour, held in place, and kept under tension until the concrete has cured around them, when the jacks are released, and the slab goes into compression. The whole process works best in a factory environment, whereas post-tensioning usually requires simply turning a nut in the field to tension up the cables.
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Old 01-31-2012, 04:51 PM   #10
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Both post tensioned and pre-tensioned concrete slabs put the concrete under permanent compression, except under highly unusual loading conditions such as an earthquake, where it is possible that parts of the slab may go into tension.
So which would you prefer in a seismic area, post-tension or re-bar reinforced?
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Old 01-31-2012, 06:16 PM   #11
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Post-Tensions Slabs


Jack -

What are you thinking of building and what part of the world are you hiding in since you did not post your location.

The cost of the concrete itself is really immaterial since most of the cost is in reinforcement/hardware and slab finishing.

Is some areas, there are differences in the construction techniques, efficiency and suitability to the local soil conditions. the cost of the concrete is immaterial since here if you are pouring a driveway, it would almost be impossible to get concrete below 4000 psi with any guarantee.

Obviously, more information would help.

Dick
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Old 02-01-2012, 04:06 AM   #12
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Jack -

What are you thinking of building and what part of the world are you hiding in since you did not post your location.

The cost of the concrete itself is really immaterial since most of the cost is in reinforcement/hardware and slab finishing.

Is some areas, there are differences in the construction techniques, efficiency and suitability to the local soil conditions. the cost of the concrete is immaterial since here if you are pouring a driveway, it would almost be impossible to get concrete below 4000 psi with any guarantee.

Obviously, more information would help.

Dick
This would be a 2-story residential home, located in Northern Arizona (Prescott). The home would be ICF.

The area is located in the 3rd most active fault area of AZ. The last earthquake was in 2011, it was a 3.6. The largest quake was in 1976 and it was a 5.1 quake. The area has a potential of seeing a 6.0 quake up to 7.0.
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Old 02-01-2012, 08:46 AM   #13
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Post-Tensions Slabs


The question of seismic performance of tensioned concrete slabs is complicated. They are generally preferred over ordinary reinforced concrete slabs in areas of expansive clays because they tend to crack less than reinforced concrete slabs as the earth moves under them slowly due to moisture changes. As I mentioned, most builders use post-tensioning rather than pre-tensioning because it is easier to do post-tensioning field work compared to pre-tensioning.

Seismic performance is much more complex because the loading tends to be very large over a short time interval. There is not a lot of research on the performance of post-tensioned slabs in seismic zones for wood framed residential use. There is a lot of research on proper techniques to connect concrete columns to post tensioned slabs, especially in multiple story buildings in seismic zones, but that is not relevant to your situation.

The important issues for wood framed construction in seismic zones include:

1. Proper connection of the foundation to the walls. You need adequate shear connections or the house can wrack in an earthquake, essentially destroying the building. This can be difficult with post-tensioned slabs because they tend to be thinner than ordinary slabs, and the connection detail is sometimes not addressed carefully.

2. Proper design of the walls. There are special grades of plywood made specifically for seismic shear reinforcement, they are worth the extra money. The plywood needs to be properly fastened to the studs, generally ring shank nails or special screws are used.

3. There are a lot of important details regarding connection of beams, joists, rafters to the walls. Usually the Simpson type connectors are very good in seismic loading.

Lastly, I would buy earthquake insurance. Most people don't have it, it does not cost that much (I have it, so I know that in MA it is pretty cheap). Most of the damage to wood frame structures comes from objects inside the house falling down, in particular tall cabinets not attached to the walls. Your slab may crack in an earthquake, but this is not a catastrophic problem as long as you have insurance to cover the repair. My sense is that a post-tensioned slab will likely perform well in an earthquake, assuming it is properly designed and constructed, and a well built wood framed house will do very well in an earthquake due to ductility of the wood members. Just make sure the connections are properly detailed and constructed, and fasten interior countertops and book cases to the walls.
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Old 02-01-2012, 01:08 PM   #14
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Post-Tensions Slabs


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There is a lot of research on proper techniques to connect concrete columns to post tensioned slabs, especially in multiple story buildings in seismic zones, but that is not relevant to your situation.
Actually, the home I am looking to build will be a 2-story ICF home. It will be 6" of reinforced concrete walls on a slab. So my home will have some concrete columns.

Unlike wood, concrete is not ductile, which I assume can be bad in an earthquake but good in high winds.
What is your view on ICF construction and it's seismic abilities?
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Old 02-01-2012, 01:20 PM   #15
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Post-Tensions Slabs


If you are planning to build essentially a concrete house, with a tensioned concrete slab, and reinforced concrete walls, you will not have columns, which are defined as relatively slender vertical elements. You will have a tensioned slab supporting reinforced concrete shear walls. This type of construction requires very careful detailing of the connection between the walls and the slab, in particular because the slab is going to have stressed cables rather than traditional bars, so the standard method of reinforcing the joint, namely bending bars around the corner with adequate overlap, is not necessarily going to be available.

You need a good structural engineer working for the architect on this one, if you want to get ductility into the frame. Certainly it can be done, however there are a lot of buildings that have been built this way that did not perform well in the Northridge earthquake, causing a lot of structural engineers to think hard about the techniques used to connecting reinforced concrete walls and columns to post-tensioned slabs. Just make sure your engineer is familiar with current research, understands the issues, and provides a good solution.

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