Post Tensioned Concrete Slab Maximum Punching Shear Stress - ACI 1

[RFreund]

I'm curious to know what you use for the maximum allowable concrete shear stress v_c and total maximum allowable concrete shear stress v_n.
ACI limits these to 3*sqrt(f'c) and 8*sqrt(f'c). There is a very good PT book (Post-Tensioned Concrete Principles and Practice by Bondy and Allred) that suggests the use of 1.5*sqrt(f'c) and 6.5*sqrt(f'c). They say that these values are based on the values used in the ICC-ESR reports by stud rail manufacturers. I looked for some ICC-ESR Reports and it appears they now only recommend limiting the shear stress to 1.5*sqrt(f'c) if the member is part of the seismic force resisting system. My guess is that the reports used to recommend 1.5 and 6.5, but maybe they have since revised this.

EIT

http://www.HowToEngineer.com

 

[HTURKAK]

I will suggest you to look ;

i) ACI 421.3R-10 Guide to Seismic design of punching shear reinforcement in flat plates (ACI Committee 421 )

ii) ACI 421.3R-15_ Guide to Design of Reinforced Two-Way Slab Systems-American Concrete Institute.

There are worked examples at ACI 421.3R-10 app. D .

 

[RFreund]

Thanks. Looks like those documents use 3 and 8. I'll have to see if I can find an ESR that uses 1.5 and 6.5. Maybe I can contact them to understand why.

EIT

http://www.HowToEngineer.com

 

[NJ92]

I'll preface this by saying I have not done any PT design myself so I can't weigh-in on your question directly. That said, I wanted to point out that Dirk Bondy (co-author of the book you reference) has a Youtube channel under the same name. He's uploaded videos of his full post-tension design course (as well as his seismic design course). I've watched the seismic course videos whilst studying for the SE and, imho, I think he does a really good job of explaining the topics both from a school perspective but also from a practical/practicing sense since he practices engineering as well. I skimmed though his PT videos & it seems Lecture 21 is where he deals with PT flat plate design (around the 33 min mark, he mentions the discrepancy that you brought up between ACI & the ESRs for punching shear).

Also, he gives out his email in his videos and from my limited experience, he's seems very willing to help so if the video doesn't answer your question fully, you could try emailing him directly.

Lecture 21: Link