Torsion on moment frame

[xlren]

I have a concrete space moment frame structural with PT slab. For vertical load cases, there are large torsions at perimeter members and some of the interior members when running ETABS model. I think this torsional problem should be seen often for most of the concrete structures with perimeter beams as a result of unbalanced tributory area. Is there a general solution for this kind of issue?

 

[csd72]

Check the code, I believe that you are allowed to reduce the torsional stiffness of the section which would then reduce this.

In reality, for a concrete beam to fail in torsion it has to elongate, and the slab will help prevent it from doing so.

csd

 

[slickdeals]

For spandrel beams, you typically have torsion (due to unbalanced moments). This occurs because of the rotation of the beams framing into the spandrel girder. I am assuming that you have a concrete system.

The code allows for redistribution of moments and it is therefore not necessary to design a beam for all the torsion it sees as long as a path for redistribution is present. Typically what is done is that the beams that frame into the spandrel are designed for wl^2/8, assuming a pinned end. The spandrel beams are designed for compatibility torsion (mainly for torsional cracks). Once the beam can't take the torsion, it will kick it back into the joists (which have been designed to carry all moment, assuming no fixity at the ends).

Hope this answers your question.

 

[drile007]

I always reduce torsional stiffness in thise beams. With this provision I achieve redistribution torsional moments from beams to moments in slab and consecutive more reinforcement in slab which is OK.

Drile007

 

[sdz]

AS 3600 8.3.2
Where torsional strength is not required for the equilibrium of the structure and the torsion is induced soley by angular rotation of adjoining members it shall be permissible to disregard the torsional stiffness in the analysis and torsion in the member, if the torsion reinforcement requirements of Clause 8.3.7 (Minimum reinf) and the detailing requirements of Clause 8.3.8 are satisfied.