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Post tension design
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(Structural)
(OP)
7 May 13 10:19Hi everyone I'm a fresh employee.I'm learning to become a post tension(PT) designer.Usually my company do two-way PT slab with drop panel and Banded slab(PT beam +PT slab).In Designing these slabs and beams what should be kept in mind?And In continuous slab and beam,how should I balance the load to determine the prestressed force.I don't know too much about PT.And I really appreciate you answers thank you all.
(Structural)
7 May 13 20:06You can't expect to learn all you need to know about post-tensioned concrete design by asking questions on the internet. Your company should have mentors who can assist you, or else send you for training. You should have a basic knowledge of the subject from your University education, but you will have to now obtain the required training, some of which can only be gained by working under the supervision of experienced engineers. The major post-tensioning companies have excellent resources which are freely available. Suggest you contact the representative of a prestressing company in your area. The following publication is a starter:
(Structural)
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7 May 13 22:40There is a lot that goes into post-tensioning and as hokie said, there is no way you are going to learn it here. From your post, I'm not sure I understand if you are speaking of suspended slab/beam systems or foundation systems?The Post Tension Institute is a good place to start as well.
(Structural)
7 May 13 22:48I think it is clear that the OP is talking about suspended slabs. Flat slabs and banded slabs are two of the most common framing systems which employ post-tensioning.
Just a precaution on starting with the PTI information...it really only applies to North America. Most of the world does post-tensioning differently.
(Structural)
8 May 13 06:07Check out suggested span to depth ratios for a start. Bands are around span/25 and slabs are span/40.
Find the minimum duct cover, generally about 1" cover for 2hr fire rating and 1.5" cover for 4hr. Best to put in tendons to reverse the bending moment diagram and try to have 1.4 to 2.0MPa prestress. Keep tendons about 800-1200 centres but can be closer spacing in bands or column strips.
I always place reinforcement over the columns to help with punching shear and peak moments, generally 16mm bars at 200mm centres.
Keep tendon lengths to about 30m otherwise losses are large and tendon loses it's effectiveness, also 30m x 30m is a good size for a concrete pour (200-250 cubic metres).
I generally like to size the slab thickness with 1.4-2.0MPa prestress for deflections and shears first. When designing for flexure the tendons will normally have capacity to do it by themselves except over columns or other supports.
To study up on it find a good PT publication, I think there is one by concrete society which is good and also read through some of rapt's post that he has put on this website because they know PT design better than most.
(Structural)
(OP)
8 May 13 11:00thank you all for your advice now I know I have to learn it step by step.I was just anxious to become a pt designer.
(Structural)
8 May 13 16:49"...it really only applies to North America. Most of the world does post-tensioning differently."
'Hokie66' speaks the truth!
(Structural)
8 May 13 17:08The main difference being the use of unbonded tendons, right? I don't think it is used extensively in this part of North America any more, but I could be wrong.
BA
(Structural)
8 May 13 17:27In buildings, UNbonded PT is still the predominant PT system in NA.
From a design perspective, ACI-318 has some peculiarities that appear somewhat unique to NA re banded and uniform tendons in flat plates and average moments across full slab panel.
(Structural)
9 May 13 00:00I agree that unbonded tendons is a big difference, and also the differences pointed out by Ingenuity. These systems are not permitted where I am, in Australia.
(Structural)
9 May 13 02:28Interestingly, UNbonded tendons ARE permitted in Australia for slabs on the ground, however, I have never seen one constructed in Australia with UNBONDED tendons.
(Structural)
9 May 13 07:07Yes, that is an anomaly, and I don't know why it is there. I would never design a post-tensioned slab on ground with ungrouted tendons. When restraint cracking occurs in a slab like that, the slab can just slide relative to the tendon, opening the cracks wider.
(Structural)
9 May 13 17:13Ingenuity,
I think one group in South Australia was doing them. Agree with both of you that the method does not work well. Both the lack of crack control mentioned by Hokie66 and also the very low levels of P/A used. The combined effect basically results in an unreinfored slab.
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There are two reasons why builders used post tensions slabs during the construction of a home or building.
A post tension slab is reinforced with stranded steel cables that are tensioned (tightened) after the concrete hardens. The cables are slid inside a plastic sheathing, like a wrapping. This prevents the cables from touching the concrete. When the concrete has sufficiently cured, the plastic sheathing is stretched. Stretching the post-tensioned cables applies significant force to the concrete system, lifting the slab into a compressed state, which reduces shrinkage cracks and cracks caused by difficult soil conditions.
One of the biggest problems with a post tension slab is cracks. The cables laying in the concrete are generally not tightened until at least 7-10 days after the concrete is poured. Since the cables are not stretched or elongated, they cannot provide any crack control in those first few days.
Another reason for cracks is when a vehicle or another large object is placed upon the structure, and its concrete slab undergoes tension. To solve this problem, post tensioned steel tendons are placed when the concrete is poured and tensioned after with conventional reinforced bars.
A second issue with post tension slabs is that the footings anchor the foundation in place. The tension required to bind the additional concrete in the footings plus the dirt between the footings can’t be met.
Defects such as slab cracks can be costly to repair after construction is finished. Picture ripping up flooring or carpet to repair a small crack you may never notice. Trying to fix cracks can be time-consuming and expensive. Depending on the extent of damage, a family may have to leave their home while the work is being done. Talk about prices adding up.
Learn more about – Concrete slab cracks in Florida home.
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