quote:
Ruiyi supply professional and honest service.
“ensuring a properly compacted subgrade and avoiding expansive soils are more critical in most cases.”
I build a few homes a year, but I’m not a civil engineer. I always get a bore sample and soil analysis done. Based on that I make a decision on which to use. As one poster saidThis is very true. If the site passes the bore sample – conventional. But, if it doesn’t and involves cutting down 4’ and bringing in 20 loads of select fill to be brought up in 6” lifts with compaction tested after each lift, etc., etc.. I’m going with a post tension. While doing all that dirt work is viable on commercial property, the economics don’t work on spec homes. A properly designed post tension slab can design around expansive soils, high water tables, etc. And from what I’ve seen the cost of conventional vs post tension is about a wash. Conventional uses more steel less concrete, post tension uses more concrete less steel (at least on all of my designed slabs in East Texas where expansive soil is just part of the game).
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Cast-in-place vs. Post-tensioned
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(Structural)
(OP)
13 Nov 07 14:33When would you use a post-tensioned conc. floor slab instead of a cast-in-place slab?
(Structural)
13 Nov 07 14:38Predictable loads, repetitive floors in multi-story highrise, slab on grade in residential construction, lack of penetrations(holes) adjacent to columns, minimum floor to floor heights and lesser weights for foundation design.
If you want to learn more, please visit our website post tension system solutions.
(Structural)
13 Nov 07 14:56It hard to be specific without knowing the project specifics, but in general:
Post tensioned floors allow for longer spans with thinner slabs. Typical span to depth ratios for two way slabs are in the order of:
L/30 to 33 for Conventional Reinforcing (Aci 318 Table 9.5c)
and
L/45 for PT slabs (Per PT Institute)
If your architect is looking for column spacings in the 30 foot range, I would definately consider post tensioning, as these spans would require fairly thick mildly reinforced slabs.
JMHO
(Structural)
(OP)
13 Nov 07 14:57Sounds like you'd like to use post tensioned just about everywhere!
In my neck of the woods, mid to high rise residential is usually cast in place flat plate.
What about multi-level storage building for cast in place vs. post tensioned? Live loads about 3 times residential live loads.
(Structural)
13 Nov 07 15:05No need for long open spaces in a storage building, would expect a reinforced slab with shorter spans to be cheaper.
(Structural)
13 Nov 07 15:21Me too.
Mike McCann
McCann Engineering
(Structural)
13 Nov 07 15:36Just to correct your terminology, posttensioned floors are cast in place. Pretensioned elements are not.
(Structural)
13 Nov 07 17:40Pretensioned elements are usually refered to as planks not slabs.
Composite slabs would also be worth investigating.
csd
(Civil/Environmental)
13 Nov 07 19:19LPPE,
what is the question? is it:
"When would you use a post tensioned conc. floor slab instead of a reinforced concrete floor slab?"
or does it concern pre-tensioned elements.
(Civil/Environmental)
13 Nov 07 19:25We have specified PT slabs when cracking was a huge concern (ice skating rinks, skate parks, tennis courts, ect..). My understanding is that the tension really holds the cracks tightly shut.
We have also specified them when the soils conditions were questionable (fill, ect..) and any sizeable cracking/differential settlement was a huge concern.
(Structural)
13 Nov 07 20:32A PT slab can reduce the overall height and weight of the building, and therefore will reduce the foundation, lateral system, and facade cost. But, I am sure there is a point where paying for a PT supplier/erector costs more than just laying rebar. So for a couple of stories in a rural area it may be cheaper to just use standard reinforced. When longer column free spans are needed PT is sometimes considered also. Storage buildings often have the closest column spacings of any buildings I have ever seen, I have used slab on metal deck on these with small HSS columns all over the place with steel strap bracing.
(Structural)
(OP)
14 Nov 07 07:02Thanks all for the replies.
The general question is "When would you use a post tensioned elevated conc. floor slab instead of a reinforced concrete floor slab?".
It sounds like a lot of post-tensioned slabs are used for on-grade purposes.
The typical storage building I design uses steel. My client wants to erect a building in an area where concrete framing is the norm, and have asked about the better system to use - cast-in-place (regular rebar), or post tensioned. I'm not sure which is the better system, and does the high storage live load have an impact on which system to use?Yes, columns can be closer in storage buildings, but still looking at a 20x20 or 30x30 bay. Like haynewp said, I'm sure at some point it has to be cheaper to lay rebar for a 3-4 story building than using PT.
Yes, hokie66, I understand the terminology, although I don't think I mentioned pre-tensioned as an option.
(Structural)
14 Nov 07 12:06There is a chart in this book that gives economical spans in the uk.These are in metric so multiply by 3.3 for feet.
(Structural)
14 Nov 07 17:35I'd consider a post-tensioned elevated floor slabs when I would have to control deflection say hmm.... large cantilever floors, long span floors, ... or where I need to reduce dead load or had height restrictions in relation to multiple floors, or where crack control is important say environmental structure. Not interested at all in post-tensioned floor slabs in parking structures exposed to chlorides, bonded or otherwise. Can use them to oppose forces on opposite sides of the structure, always good for anchoring to the real world, 'tie that sucker down'. All right getting late. Besides the PTI will be after me soon.
(Structural)
14 Nov 07 23:02LPPE,
Your continual reference to "case in place" is what is causing the understanding problem.
PT slabs are "cast in place" just as RC slabs are.
For what you are talking about, PT slabs will work well but I would make the following comments (repeated from another post under Post-tensioning)
"Main thing to consider with these types of loads is that the concrete tensile stresses will be fairly high and the most economical design will be to use partial prestress. Allowing for pattern loading will result in a need for both top and bottom reinforcement as well as the PT tendons.
To do this poroperly, you cannot use the banded/distributed logic and especially the "average moment" logic of the PTI and ACI. You need to consider actual stress concentrations and design for each area accordingly. Averages simply are not logical.
It is also more logical to use bonded PT to gain the extra crack control advantages post cracking.
Flat plates will normally not be the most economical solution for these types of slabs. Flat Slabs with Drop Panels are far more efficient and will give you much better results.
"
Also, someone earlier mentioned L/D ratios of L/45. Don't try it. The ACXI/PTI design methods might say it works but when you calculate it properly allowing for stress concrentrations and the actual effect of cracking on deflections you would want lower L/D ratios even for lightly loaded slabs. As the loads get heavier the thicknesses will need to increase proportionately.
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