Shear in PT beams when Design column strip for Column + Middle strip resultants is checked

RE: Shear in PT beams when Design column strip for Column + Middle strip resultants is checked

Seth Guthrie — Wed, 16 Mar 2022 22:18:44 GMT

Yes, that's when I think it is appropriate.

RE: Shear in PT beams when Design column strip for Column + Middle strip resultants is checked

Fabian Gonzales — Wed, 16 Mar 2022 21:27:29 GMT

Yes Seth, I'm running the distributed tendons in the N-S direction to achieve the minimum effective prestress (125 psi) required by ACI. The beams were added to improve deflections.

So you would recommend to check "Design column strip for Column + Middle strip resultants" only when we have one-way behavior?

RE: Shear in PT beams when Design column strip for Column + Middle strip resultants is checked

Seth Guthrie — Wed, 16 Mar 2022 21:06:55 GMT

You are running these distributed tendons in the slab to improve the deflection and reduce the moment in the slab right? You could have put all the tendons on the beam and achieved more drape, but you didn't because you also need the slab to "work" for service and strength design. Hence, if the slab should work as a post-tensioned slab I would tend to leave the middle strip alone and check it like any other 2-way PT slab.

RE: Shear in PT beams when Design column strip for Column + Middle strip resultants is checked

Fabian Gonzales — Wed, 16 Mar 2022 20:18:28 GMT

Hi Seth, thank you for the quick reply.

What do you mean when you're saying "making the slab work for N-S span"? I ran the distributed tendons in the N-S direction because the spans are shorter. The slab was originally fully flat but we had to add beams in that area because the columns were failing in punching shear and the deflections were too high.

You're right. This is a 2-way slab.

RE: Shear in PT beams when Design column strip for Column + Middle strip resultants is checked

Seth Guthrie — Wed, 16 Mar 2022 20:11:12 GMT

I assume this is a 2-way slab. If you have distributed tendons in this direction that are there to make the slab work for N-S span, then I don't see why you would want the narrow column/beam strip to be designed for the total resultant. That approach is generally used where the slab is not being considered in the direction parallel to the beam, e.g. a one way slab.