Minimum torsional moment even after assigning low stiffness modifiers to J beam compound

[structural87]

hello,

I would like to ask your opinions about torsional stiffness reduction for beams.
A lot of engineers use a low torsional constant (J = 0.1) in order for the beam torsional moment to be transmitted to the slab, in case of compatible torsional system.
however, despite assigning a low modifier to J, ACI recommends a minimum torsional moment (section 11.5.2.2) to abide by.
As such, torsional moment should be always be considered and can not be neglected in neither case (unless an interior beam with equal span and loading from both sides).
I am seeing a lot of engineers waiving this section and designing only for bending and shear (without any torsional consideration).
your thoughts please ?
thank you

 

[structural87]

Any help ?

 

[Agent666]

Great question (I often see similar things being done), strictly speaking if setting J to an arbitrarily low value, then you should be detailing for the minimum torsional reinforcement required as you note. But only if the original torsion with normal J is higher than the minimum is this required is my understanding of most codes approach in this regard.

For example if you have a torsion lower than the threshold minimum then it can be effectively ignored in most codes (I'm pretty sure ACI has the same requirements). Setting J = 0 in these situations at least means the torsion is accounted for as being redistributed to moment and accounted for if member does crack under torsion.

 

[KootK]

I do it like this:

1) Closed stirrups to meet the minimum on all exterior beams.

2) Open stirrups on most interior beams but I always have generous top steel over the beams which, in my opinion, completes the closed stirrup even if the code doesn't specifically say anything about this.

Evaluating torsion in interior beams is often analytically tedious and of questionable accuracy. And I refuse to live in a world without open stirrups.

HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235

 

[structural87]

Thank you agent666
Kootk so do u waive the code requirement or u stick to it ?
Personally I will always stick to it but I see lot of people which are aware of this section but dont consider it stating that waiving this section will result in some minimal torsional cracks which would be hidden within the architectural finishes.

 

[KootK]

Kootk so do u waive the code requirement or u stick to it ?

As I mentioned above:

1) I stick to it for perimeter beams and;

2) Stick to a modified, over stirrup version of it for interior beams unless I demonstrate that torsion is minimal. Here', I'll specify the quantity of open stirrups such that they would at least match the minimum torsion requirement.

...stating that waiving this section will result in some minimal torsional cracks which would be hidden within the architectural finishes.

I don't know about that. I think that you get the minimal cracks IF AND ONLY IF you provide the minimum torsion reinforcing. That's what it's for after all. Without out it, I think that you risk very large torsional cracks that risk compromising shear capacity in addition to torsion capacity.



HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235