analysis and design the compatibility torsion on the concrete beams

[h-badawy]

Dear Experts

in case of design concrete slab-beam system , shall we ignore the compatibility torsion on the beams by put the torsional stiffness equal to zero (torsional modifier factor=.0001 in etab software ) and design only on the equilibrium torsion ?.


Thank you

 

[rapt]

If you set the torsion stiffness to zero, will you get any equilibrium torsion?

 

[h-badawy]

Yes the equilibrium is not effeted by the torsional stiffness only the compatibility torsion will be ignored in that case ,

Thank you

 

[rapt]

Are you sure about that? Are you talking about torsion or Mxy moments?

 

[h-badawy]

Yes am sure equilibrium torsion will transfer to the been otherwise the system will be unstable , i am speaking about compatibility torsion which appears in the external beam from the end moment of the slab/beam support by this beam or internal beam from the different moment of two adjacent slab with different in span/load.
Also the other question is in case of external beam which is supported slab/beam ,if i will put the torsional modification factor = zero in the sap model and in the site during construction the top reinforcement of the slab/beam which is supported be the external beam take the sufficient development length to transfer the end moment to this external beam , the external beam in that case will be fail or the redistribution will be happened before failure.

Thank you

 

[rapt]

If the beam has zero torsional stiffness, how will SAP differentiate between compatibility torsion and equilibrium torsion and decide to have equilibrium torsion but no compatibility torsion in it even though its torsional stiffness is zero and it therefore cannot attract any torsion?

Even if you have no top reinforcement at the end, the beam will still attract torsion until the concrete cracks in torsion thus relieving the stresses. Just because you have told the beam it has no torsional stiffness does not mean that it has none in practice. That is why design codes require minimum torsion reinforcement, especially in edge beams, even in cases where you have ignored compatibility torsion. So that the beam will not fail in torsion and is able to redistribute its effects.

 

[h-badawy]

Thanks rapt

This is not direct torsional stiffness, but this the torsional modification factor , anyhow i make a lot of experimental models and i sure for the direct torsion (like cantilever slab from the beam without back span ) it has been taken into consideration.
The thing is that the failure in torsion is brittle failure ( happen suddenly), and I don't know if the minimum torsion reinforcement is sufficient to over come on the comparability torsion till the redistribution happened or not .


Thank you

 

[rapt]

Isn't the torsional stiffness multiplied by the torsional modification factor. so 0 * I = 0! My point is that SAP cannot differentiate between equilibrium and compatibility torsion. To it, they are the same thing. I have trouble differentiating also. But, There is a difference between torsion and moment!

Design codes suggest that it is sufficient.