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Beginner PT Design Info

Beginner PT Design Info

Beginner PT Design Info

(OP)
While doing some research on 2-way reinforced slab design, I came across some interesting information about PT design. I would like to expand my knowledge of the design of a PT system so what references would you all recommend looking into? Remember, I need very basic stuff because I have no formal training in this type of construction.

RE: Beginner PT Design Info

You're in unbonded PT country in the midwest. As such, I'd recommend this: Bijan Aalami. If you need free:

1) Google "VSL + Post-Tensioned Concrete". They've got all kinds of great docs that summarize the technology succinctly.
2) Google "PTI + Post-Tensioned Concrete". They have a bunch of tech docs and power point presentations that train basic PT nicely.

There's some great stuff out of Europe and Australia as well but that tends to be bonded PT heavy. Consider that a next step once you get the basics down pat for your market.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Beginner PT Design Info

(OP)
Thanks Koot! Out of curiosity, why is unbonded the preferred method in the midwest?

RE: Beginner PT Design Info

Quote (BadgerPE)

Out of curiosity, why is unbonded the preferred method in the midwest?

Ughh. You're going to get me in trouble with the bonded loving Aussie/Euro folk. Here's an excellent document that summarizes many of the differences between the systems: PTI Bonded/Unbonded

My personal take on it is this:

1) bonded systems perform better and are better from a purely technical perspective.

2) several decades of North American experience in thousands of buildings has proven that banded/distributed unbonded systems perform satisfactorily.

3) several decades of North American experience in thousands of buildings has proven that banded/distributed unbonded systems are the least expensive to construct in North America.

In my estimation, 2 + 3 = unbonded is the superior system in an engineering sense where an optimal balance between performance and economy is sought in a North American market.

How's that for politically correct?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Beginner PT Design Info

(OP)
Handled like a seasoned politician. KootK for President....of eng-tips. Don't think you want to be in on that other "mess"!

Thanks for the info!

RE: Beginner PT Design Info

Another way to look at it is to consider that the primary motivation for using post-tensioned floor slabs is improved deflection performance and the associated reductions in slab thickness that come along with that ($$$). The improved deflection performance for both systems comes primarily from the balanced load effect where curvature in the tendons tends to oppose the effects of gravity. Whether or not the tendons are bonded doesn't change this aspect of performance much. So, if unbonded is less costly locally and the main benefit of PT (deflection) is the comparable, you can see why unbonded technology tends to come out on top.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Beginner PT Design Info

(OP)
I thought that economics may have been the main driver. Is unbonded typically used on AASHTO projects or say parking structures? I seem to remember a while back a thread where an unbonded system was used in a parking structure (?) and was experiencing degradation of the tendons due to water infiltration.

RE: Beginner PT Design Info

Quote (BadgerPE)

Don't think you want to be in on that other "mess"!

I actually miss American politics quite badly now that I'm back in Canada. I hate to say it but Canadian politics is pretty dull by comparison. One of my favorite things about living in WI was how politically engaged people there tend to be. I got to shake Obama's hand in Madison during his initial campaign run. Followed by a kick-as Springsteen street concert...

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Beginner PT Design Info

(OP)
Did you live/work in Madtown?

RE: Beginner PT Design Info

Quote (BadgerPE)

Is unbonded typically used on AASHTO projects or say parking structures?

You do tend to see bonded PT in a lot of transportation/parking structures where durability concerns come more heavily into play.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Beginner PT Design Info

Quote (BadgerPE)

Did you live/work in Madtown?

2000 - 2007. I've been trying to get back ever since but it's tough logistically with a career oriented wife and school age kids. Also gotta wait for the healthcare situation to settle down a bit. I think that the Median Salary:Awesomeness ratio in WI is about as good as it gets.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Beginner PT Design Info

(OP)
Awesome! I live about 20 minutes west of Madison and work west of town too. If you want to connect on LinkedIn, send me an email at engtipswontallowemails@gmail.com and I will get in touch with you.

RE: Beginner PT Design Info



Banded distributed systems can be made to work as long as your tendons provide a correct load balance path to the supports and limit stresses especially over the supports. I once say a design with a tendon layout with the tendons in the banded direction spread over the full width, so basically distributed/distributed. This is NOT a load balance layout. Just a few other things to think about

If you have drop caps or drop panels, you cannot use the ACI/PTI average moment/full panel width logic and if you have drop caps they cannot be used for flexure calculations at all. So disregard anything in PTI literature or Aalami's book or Ken Bondy's musings that say otherwise.

If you have beams in your slab, you cannot use the full panel width as the flange and you cannot include the tendons parallel to the beam in the slab as beam tendons again contrary Aalami's book or Ken Bondy's musings. Use the ACI effective flange width for beams.

If you allow the tension stress to reach the ACI limit in ACI/PTI average moment/full panel design, the slab will be cracked around the column and this will affect the deflections, despite what ACI says. The minimum reinforcement requirements will probably be sufficient to control the crack widths if provided properly, but that does not stop the deflection increase.

Also, you cannot justify partial prestress design with use the ACI/PTI average moment/full panel width logic. The stress limit in the code is an absolute limit. It should be much lower if you want to assume uncracked sections for deflection calculations.

You cannot use deflection multipliers to predict long term deflection as suggested in many of Aalami's musings. Even ACI code says you cannot.

And finally, bonded PT is actually cheaper in most countries because the PT companies provide full service PT, supply, install, supervise, stress and grout. It is not simply a material supply operation as it is in the USA in many cases, so outside North America the PT company is pricing in the full cost of the PT.
My 3 problems with unbonded PT are that the tendon is relying on a friction grip with its wedges for its whole life, even when it shakes a lot in an earthquake, the steel is not being used to its full capacity as it is not bonded and 3rd despite what the industry says, there are major problems in USA with long term performance of the tendons, especially at the anchorages and also of slabs designed using banded/distributed tendon systems with unbonded tendons.

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