Start for post tensioned structure design

[hmehrdad]

Hi,

I am going to learn the design of post tensioned structures. I am a structural engineer and this is my first time in pt. I am going to use ADAPT Builder & ETABS. I know there should be more detailed guidelines than those exist in this forum. Can any one help me by providing complete design samples for real projects?
I like to have a complete package containing the plan and geometry, ADAPT design File, ETABS model for columns and/or walls and of course a detailed description for design procedure such as gravity and seismic loading, assumptions and ... !!
I think this would be a suitable starting point to learning pt design using ADAPT+ETABS. can any one help me on this matter by providing a complete package to me?

Thank you all

 

[hokie66]

You should get yourself a good concrete design text and all the material available from VSL or whichever of the major post-tensioning specialty companies which operates in your area.

 

[JAE]

I'd second what hokie states...certainly learn the concepts of post tensioning before using a software product.

 

[structuresguy]

You defintely need to get all of these references, and read them thoroughly, specifically look at odd conditions that may exist in your design.

"Post Tensioning Manual" by Post-Tensioning Institute

"Design Fundamentals of Post-Tensioned Concrete Floors" by Post-Tensioning Institute. Written by Bijan Aalami, who is the developer of the Adapt software.

"Design of Post-Tensioned Slabs using Unbonded Tendons" by Post-Tensioning Institute

ACI 423.3R-03 "Recommendations for Concrete Members Prestressed with Unbonded Tendons"

You should also familiarize yourself with the products typically used in your area. Most PT products are very similar, though some offer differing corrosion protection levels.

 

[structuresguy]

Also, when looking at span/thickness ratios for your floors, try to stay on the thicker side of the typical ranges. This will make your life a lot easier when doing punching shear calculations. And be very careful about the end bays, as most of the punching shear problems will occur at the last column. Try to keep your end bays not more than 2/3 span of typical interior bays, or you may need drop panels at your end columns.

Basically, my opinion is that PT slab design is all about punching shear. If you get the PT design wrong, well, you might get some cracking or a little deflection, but it won't fall down. If you get the punching shear design wrong, it WILL fall down, in a sudden way without warning. So make sure you are fully up on punching shear design requirements.

I would contact DECON and get whatever literature they have, and their design software, for stud rail design. Personally, I put stud rails at every column, even if the calcs say they are not required. The only exception might be if I had a safety factor of 3-4x for my punching shear, then I might not put rails in the slab. The cost is not that high, and it gives me peace of mind.

Also, get a good amount of money in your contract for site inspections. You will want to go out and verify the tendon, stud rail, and rebar installation prior to pouring each slab. I go out and do very thorough checking of tendon profiles, to make sure they have it right, as much as I can. I even do this when there is a special inspector on the job, who should also be checking this.

And one last thing, typically you will see recommendation to spec tendon profiles to 1/8" increments. Not reallistic. At best, they can place them to 1/4" accuracy in the field, in my experience. So to give even better tolerance, I design my profiles to 1/2" increments only, though I even try to stay at 1" increments, whenever practical.

 

[hokie66]

You should use reference manuals etc. which are specifically written for practice in your country. Some of the suggestions by structuresguy apply in the US, but not in some other countries. For instance, in Australia, we don't believe in unbonded tendons, so all tendons except for slab on ground construction are required to be grouted.

 

[rapt]

I agree about the comments on learning how to do it first by hand before using software. Personally, I would stay away from most of the texts mentioned. Get a good TEXT book on PT design related to the country you are designing for.

RE profiling comments from Structuresguy, if they cannot do 1/8" (5mm in metric) they should not be in business. 1" is laughable!

RE Stud rail suggestions, avoid punching shear reinforcement of any type where possible. If you can use concrete to make it work. This will depend on the country and area you are working in. Some countries like flat plate slabs and punching shear will be a problem and require reinforcement. Only put it in if you need it.

Do not believe everything the software tells you when you do start using it. Do your own checks and make sure that all assumptions are logical (based on sound engineering principals, not on what a software salesman says). The software is simply providing a set of calculations that the designer has to check through and sort out the design from. Depending on assumptions made both by default by the software and possibly by the designer, those results could be absolute rubbish.

And do not believe the deflection calculations is the stresses are relatively high and/or banded/distributed slab arrangments are being used.

 

[hmehrdad]

Hi all

Thanks for all your guidelines. I have different references about PT design. Thanks structureguy for your reference list. I don't have all of them.

I don't have problem with slab design. when I think to overall design procedure I see that I have to design columns/walls with ETABS and have to design PT slab in ADAPT or similar software.
please help me which of following statements are not correct:
-> In ADAPT I exert gravity loads into slab and design slab and beams (if any) for induced shear/moments due to gravity.
-> In ETABS we model the columns/walls and model the floor with a shell element or even just model their gravity forces into vertical elements.
-> we introduce lateral forces into floors and constraint all the joints i each floor to have a rigid diaphragm.

Now I have a question that in lateral load analysis the moment resisting frame should have to bear a ratio of lateral force (or even all of it, if walls does not exist). How this is possible when in ETABS model we have no beams between columns ?! because the floor model which the beams are part of it, would be designed in ADAPT. but the frame is modeled in ETABS without beams. How the beams are going to be designed simultaneously for gravity moments and moments induced from lateral EQ/Wind forces?

If I ask for a computer model, it is to check that how all these questions are answered in practice. I thank you if you can provide me a little multistory project, but a complete one.

 

[structuresguy]

Your 3 statements on the design procedure are correct, and is how we analyze slabs/frame using Adapt and Etabs. The only difference is I use semi-rigid diaphragm, and let Etabs work out the stiffness. Usually not much difference, unless long skinny slab.

Regarding your question on lateral force distribution on the moment frame. I would only consider doing a flat plate and columns frame in one or maybe two story building, in low seismic areas, without any shearwalls or frame beams. Then you would need to model the floor plate and columns in Etabs, work out the stresses due to the lateral load case, then add those stresses into Adapt to design slab reinforcing and then also add them into your punching shear calcs. Or you could work out how much mild reinforcing you need in your column strip for the frame action, then add that to the reinforcing you get from Adapt from just gravity loads.

If I have shear walls, then the assumption is that the shear wall stiffness is so much higher than the lateral stiffness of the columns, that all the lateral resistance will come from the shear walls. Of course, this may not be the case, truely, depending on your shear wall configuration. That is why I like using semi-rigid diaphragm. The results will show me which elements will take how much of the lateral loads. Then I can add some load at the columns into my floor design as necessary.

If you have frame beams, then the approach is similar. You will need to add the stresses (or reinforcing) from the lateral analysis and gravity analysis. There just is not an easy way to do this with Etabs and Adapt, yet.

I have not used it, but RAM is supposed to have the ability to combine the lateral stresses from their frame design module (RAM Frame) into their floor design module (RAM Concept), so that the PT slab design accounts for the stresses due to the lateral loads. They also do the punching shear design in the same module. From what I have seen, it looks very promising, and quicker than using Adapt for the PT reinforcing design. The the punching shear design looks infinitely superior to Adapt's very poor treatment of stud rails.