one-way PT slab or beam can be designed over 12vfcÆ?

[Once20036]

I know two-way PT slab need to be designed <7.5√fc’ per ACI 318 18.3.3. But no specific requirement limit you design even larger than 12√fc’ as long as you consider some requirement in other parts of the chapter 18: like using cracked section for deflection calculation per table R18.3.3.
Is my understanding right: if by engineer judgement and accept cracked structure and consider all other requirement, you can design PT structures up to or even larger than 12√fc’ theratically?

 

[Michel60]

Good morning Structural20036,

I think the short answer is yes, it would be permissible. But in practice it would become more difficult to meet other requirement when you're getting out in that range. "Ie" would be dropping like a stone and it would become difficult to meet extreme fiber compression limits and/or find places to anchor all the tendons. It's probably some place you don't want to go unless you've been in the neighborhood before and had success.

regards,
Michel

 

[Michel60]

..Sorry I seem to have gotten it all backwards, actually the limitations would be that you would be moving more to a predominantly mild reinforced section with less benefit of pre-stress. So it would become a question of is there enough benefit coming from the pre-stress to justify it's use.

regards,
Michel

 

[Once20036]

Thank you Michel60, Good explanation for normal limit we are using everyday. I am checking other people's work and they designed to high limit,say 11√fc’, they know it is a mistake, but as long as code allows, we can't say anything.

 

[Michel60]

You are welcome. It has been a couple of years since my last PT parking garage so hopefully others will have better handle on the resulting suitability of the design. Also review requirements for crack control noted in Table R18.3.3.

regards,
mk

 

[ron9876]

Check the last sentence in 18.3.3 ( in ACI 318-08). Two way slabs are limited to 6.0 sq. rt. f'c.

 

[Once20036]

Ron,
You are right. Even I knew it, I still typed wrong.

 

[rapt]

Structural20036,

Using the ACI 2way slab design approach, it is not permissable under any circumstances to allow a stress greater than the 6√fc’ limit, no matter how much extra untensioned reinforcement is added. This is an absolute limit. Other clauses in ACI that allow partial prestressed design cannot be used to override this limit.

The reason for this is that the stresses being used in the calculation are average stresses over the full width of the slab panel. They do not allow for stress concentrations around columns and at midspan so do not represent the true stress condition in the slab.

Design codes/methods used in other parts of the world do not have this limit, but they do not use the ACI 2way slab design approach. They use a column/middle strip approach similar to that used for RC slabs in ACI. Because they are allowing for the actual stresses rather than the average values, the limitations of the ACI design approach do not apply and partial prestressed design is possible and is allowed.