PT analysis and design 6

I have used PT anchors in dams before.
For your first question Post tension cables are working all the time. That's why we call them active anchors. Depending on the size of the bar you have x amount of kN compressing your structure constantly. 100% of that load is used to resist to tension. You have no reserve left to resist to shear.

That makes sense @ENGIRL (Structural). I haven't thought of that way before. Can you recommend/share any article I could read more on on this behavior.

You need to do some more reading, and not just simplified design methodology but basic PT logic.

The balancing effect is made up of

1. uplift forces (and downward normally at supports) from the tendons due top change in tendon slope.

2. hyperstatic reactions caused by continuity in the member and the forces from 1.

In determinate members and indeterminate members with Concordant tendon profiles, the hyperstatic reactions are 0, while the uplift forces can be significant.

In Limit State design, the hyperstatic reactions in 2 are considered in the design load combinations. They are considered to be an applied load.

The balancing forces in 1 are internal actions. They are not part of the applied load, they are part of the internal resistance providing capacity.

They are not considered a load in ultimate strength calculations, they are the part of the capacity, as a resisting moment P * e.

The effects are included in the shear calculations, sometimes as an axial effect in empirical formuilas.

In crack control design, when considering sections as uncracked they are included but actually ass part of the capacity again. Some designers will use a form of the calculation as reduced load and that is where you are getting mixed up. If you consider the design as a cracked section, it is the same as for ultimate strength and P.e is part of the resistance.

This is the same as for uncracked section design if you use the complete equation

stress = P/A + Ms / Z + P.e / z + Msec / z

Rather than including P.e as part of the applied moment, the last 2 terms are the resistance effects.

Thanks for the detailed explanation, RAPT. One part I have always missed was that the hyperstatic forces add up zero. Everything makes sense now! Super helpful! I greatly appreciate your help!

RAPT, kindly take a look and let me know if what I have sketched is correct or wrong. All PT annotations are in blue with D and L in red. The only thing I didn't draw was the M_sec due to PT. I can better visualize it now. Thanks for your help.

BulbTheBuilder_EI said:

How come the PT doesn't "help" with punching shear?

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Because most of the tendons are located far enough from the columns that the vertical load that they deposit at the support still needs to make its way over to the column via the punching shear mechanism. Consider the sketch below which depicts a tendon band running over a column and its assumed punching shear perimeter (dashed). The green tendons would reduce punching shear but the blue tendons would not.

BulbtheBuilder_EI said:

2. Do you consider live load reduction in a two-way flat plate and what are your views on them?

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If I would deem live load reduction appropriate for a mildly reinforced slab in a given situation, then I would also apply it for a PT slab in the same situation. So, yes, I often do use live load reduction for PT slabs.

BulbtheBuilder_EI said:

I am thinking about considering live load reduction for flexural and unreduced live loads for my shear check just to be on the safer side.

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If anything, I feel that punching shear is more amenable to live load reduction because it's application does not involve assumptions about load sharing participation etc.

BulbTheBuilder_EI said:

Can you recommend/share any article I could read more on on this behavior.

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As far as I know, TY Lin was the progenitor of the notion that prestressed reinforcement is "active". I've included his discussion of that below. I used to find this confusing, before reading the blurb below, because many of my mentors would use that description without describing its meaning well. I imagined it to be something fancier than it is. It's really just the concept that, with prestressed reinforcement, a designer can induce beneficial stresses in the system from the get go rather than just passively developing them when the loads come along.

Thanks a lot, KootK! I think I have to get a copy of T. Y Lin. I like the angle you're coming from with the PT and punching shear. I still have quite a lot of reading to do! I really appreciate your comment!

I read TY Lin's book about 50 years back... it was a classic and one of the best prestressed concrete books available. Another classic of that time was 'Beton Precontraint' by Guyon (French)

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

You've got me on Google and Amazon looking them up. I still have a long way to go

Guyon was like Lin's book, a classic, but I don't know if there was an English translation. At the time, you had to have a second language to enroll at university; this was dropped the year after I enrolled.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

Je ne parle pas bonne Francais. I don't know if that's correct but I used to fluent at French. The conjugation, feminine and masculine articles...I never took it seriously

If the goal is to quickly get production ready for US PT, I'd start with Aalami or Bondy.

My Guyons are in English.

I have ordered Aalami, post-tensioning concepts; design; construction book. I don't know of Bondy, but I will check it out. Books are so expensive!!

Excellent. Aalami will be plenty to get you started. You can get TY Lins on eBay for the cost of a pizza. Get a hardcover though. The international soft covers are printed on translucent toilet paper. I have both as a result.

"My favourite is 'Voici le duck...'

It's a canard..."

canard being the English version of canard... play on words... just for humour. Canard definition: "a false report or piece of information that is intended to deceive people"

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

I guess only one person on ebay knows the actual cost of pizza because those prices were high . I am probably going to talk to the older folks I know who are about to retire. I got me a whole box of books for just 6 beers the last time, best trade ever!

You are just a great guy. A man who knows his engineering, literature, and languages. Tu es incroyable

BlubTheBuilder said:

I guess only one person on ebay knows the actual cost of pizza because those prices were high

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Is this not pretty close to pizza money? The shipping is only high because it would be to me, in Canada.

Surely this is cheap enough that, if you actually care to have this fine, historically significant tome, that is within your reach.