Subtlety re Torsional Response of Concrete Edge Beam

[pa4912]

Sorry to revisit an old topic, but i really need clarification on a fine point with regards to torsion in concrete edge beams.

The real essence of the problem is this:

1. Can we just neglect the torsion by pinning in our model (therefore zero torsion from model, so zero torsion entry in our design spreadsheet),

OR

2. Do we have to re-run our model with fixed ends of secondary beams(even though we will eventually detail secondary beam as simply supported)in order to see whether the torsion in edge beam exceeds the 'threshold minimum' in ACI318-08, in which case we can design for the less of this design torsion or the 'maximum Tu in event of redistribution' - clause 11.5.2.2 in ACI?

I.e. do we still need to put in extra stirrups for the (minimised, redistributed) torsion force even if our pinned model shows zero torsion force in the beam?

Thanks!

 

[rowingengineer]

Too quote the commentry C8.6:
"Two conditions determine whether it is necessary to consider torsional stiffness in the analysis of a given structure:
(1) the relative magnitude of the torsional and flexural stiffnesses, and (2) whether torsion is required for equilibrium of the structure (equilibrium torsion) or is due to members twisting to maintain deformation compatibility (compatibility torsion). In the case of compatibility torsion, the torsional stiffness may be neglected. For cases involving equilibrium torsion, torsional stiffness should be considered."

Thus I assume your edge beam is in compatibility torsion, thus you can neglect you torion stiffness, but there will be detailing requirements, you will need to have fully enlosed ties i would think. you also need to provided shear ligs as per Cl 11.5.5.3, (I don't know your code as well as i should and am unsure about tension steel effects, if any)If you were in Australia, there are some tension requirements but I am yet to see some inculde the tension reo, unless it is a signification compatibility situation.


When in doubt, just take the next small step.

 

[csd72]

There is another posting on the same topic currently running on this forum.

 

[JAE]

pa4912, take a look at this thread (if you haven't already):

thread507-235070