Post tension design 1

[struggle67]

Hi 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.

 

[hokie66]

You 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:

http://www.vsl.net/Portals/0/vsl_techreports/PT_Slabs.pdf

 

[dhoward26]

There 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.

http://www.post-tensioning.org

 

[hokie66]

I 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.

 

[asixth]

Check 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.

 

[struggle67]

thank 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.

 

[Ingenuity]

"...it really only applies to North America. Most of the world does post-tensioning differently."

'Hokie66' speaks the truth!

 

[BAretired]

The 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

 

[Ingenuity]

In 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.

 

[hokie66]

I 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.

 

[Ingenuity]

Interestingly, UNbonded tendons ARE permitted in Australia for slabs on the ground, however, I have never seen one constructed in Australia with UNBONDED tendons.

 

[hokie66]

Yes, 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.

 

[rapt]

Ingenuity,

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.