Cantilever Beam Torsion ETABS

[N G P]

Dear All,

I have been trying to design some cantilever beams for a 3 storey building using ETABS. As shown in attached figure, the cantilever span is 4.2m and the span of the slab supported on beam in other direction is 12m. I have applied the stiffness modifier for all other beams and slabs as mentioned in ACI.

While analysing the beam can I consider a stiffness modifier for torsion in the cantilever beam. Is it right to reduce the stiffness? If yes, what should be the value I have to consider?

For now I have analysed the structure without any modifier for the cantilever. But the beam is failing in combined shear and torsion with the predominant torsion moments.

Regards,
George

 

[Tommy385]

Hi,
not familiar to ACI, but here I would set torsion stiffness of beams to 0, when you eliminate it, shear shouldn't be a problem.
The stability of structure (in this case) does not depend on torsion stiffness and torsional stiffness of cracked section is kinda tricky thing (personally I avoid it generally)
If the height of your beams is 80cm, dont forget to check deflection of your cantilevers beams.

 

[Agent666]

You don't need to carry the torsion to maintain equilibrium, therefore you can follow the rules in ACI for compatibility torsion.

Usually this involves setting the torsional stiffness to zero and ensuring that reinforcement is anchored effectively and providing a minimum degree of torsional reinforcement to control cracking at the serviceability limit state.

Effectively if you don't require the torsion force for equilibrium the torsion is allowed to be redistributed to moment in the beam between the two cantilevers. For example the two cantilevered beams are allowed to crack and twist to redistribute the torsion into moment in adjacent cross member (i.e. it effectively becomes simply supported instead of carrying some moment at the ends which is equal to the torsion at the ends of the cantilevers).

 

[Agent666]

I'd point out as well that the stiffness reductions in the code are intended for the global lateral analysis. As such when looking more closely at gravity deflections, you should evaluate the actual stiffness reduction due to the service moment and you reinforcement layout to more accurately assess the deflection.

Also don't forget to evaluate the long term deflection effects due to creep and shrinkage. EDIT - note you can offset some of the deflection with precamber to the cantilevers if required.

 

[rapt]

Agent666,

You can offset some of the "visual" deflection using precamber. If there are masonary walls or brittle finishes, you cannot offset the deflection effect on them using precamber as they are affected by the total movement they experience, and precamber does not reduce the actual movement, only the nett deflection from horizontal.

 

[N G P]

Hi all,
Thank you for your replies. My understanding from your your replies is that since the beams are not affecting the equilibrium the the torsional stiffness can be reduced considering the compatibility.
I have applied 0.15 as the torsional stiffness and and the beams are ok.

 

[N G P]

In ACI section 11.5, an equation is provided for the maximum torsion value that can be reduced for the beam design. should we consider those calculation or is it enough to provide some skin reinforcement with area of Steel equivalent to 0.1 percentage of the cross section.?