Concrete edge beams torsion design 1

[Gus14]

As per ACI for ribbed slabs where torsional resistance is not required for equilibrium the primary beams shall be designed for compatibility torsion only ( To prevent excessive cracking). For the 8.6 meter by 8.6 meter ribbed slab in the attached file ( where ribs are not continuous " only one span ")

1) If I design the ribs as pin supported ( torsion is not needed for equilibrium ) can I get away with specifying narrow beams along the perimeter and design for compatibility torsion only ?

2) If I specify top reinforcement in the rib would that cause torsion to the primary beam even if I design the ribs as simply supported ?

Side note, My gut tells me to design the perimeter beams for applied torsion equal to (10) percent the rib positive moment but I would prefer a reliable reference.

 

[HTURKAK]

I think you have confusion for equilibrium torsion and compatibility torsion. In this case, the spandrel beams 200 X700 mm has limited torsional resistance. That is, with a small rotation , negligible equilibrium torsion will develop..

The following picture, i found just by searching compatibility torsion..

Yes.. but negligible.. in order to avoid crack you should provide top reinf. for the ribs ..

What is your modelling for waffle sab and spandrel beams?

If you post you model, you may get better responds..

P.S. I would choose the width of beams and columns at least 300 mm ..

 

[Gus14]

Thank you HTURKAK for replying.
My theory behind the post is that since there is only one rib span and not a continuous span, torsion is not a compatibility torsion but it's an equilibrium (because the rib would be statically determinate). However, after reading your reply I think that I was overthinking it and that both methods are correct and safe. I just don't like the looks of compatibility torsional cracks, and since the structure will be exposed. I will design for some torsional resistance ( the 10 percent of positive moment mentioned earlier ).