orsion in concrete beams

huangyhg

torsion in concrete beams.
when do you design for torsion in concrete beams?  i used to automatically check the torsional stress across the section when a beam carried an out of plane load.  more recently i feel that is a beam is monolithically connected to a slab, on either or both sides, it cannot twist on account of the restraint offered by the slab.
is this the correct approach or is torsional deformation to be considered to limit this approach?  is there any time when a beam and slab would need to be checked for torsion?
many people automatically the torsion induced by computer programs but fail to consider the resistance offered by the associated diaphragm.
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i've seen many application of torsion to concrete beams; my experience is that if you have it, you'll know it.  
many of the applications i've seen are rectangular beams with edge blockouts (or seats) for supporting some other structural member.  the concrete beam is usually supported at the centroid of the orginal rectangle.  in many cases the support for the beam varies from large diameter concrete piles to smaller h-piles.  with the latter application, the rotation just screams to be addressed with the concrete piles you really have to wonder.  i guess i recomend that you assess the rotation with the slab or whatever edge stiffening   
well, to get right to the point, i check torsion whenever such as situation arises.  i always start with the code and work my way through it.  but when faced with your situation, i always check the rotation or twist with the slab (diaphragm) just as the slab can be flexible given the right conditions, thickness, supports, span lengths etc.
two other common incidences of torsion occur in soffit beams and in beams where you have differential live loads, such as a parking garage, even if you consider the slabs.  longer spans also induce torsion, so as qshake says, just keep on checkin'.
out of interest have any papers been written concerning the torsional restraint offered by slabs?
check the aci structural journal and the asce structural journal.  very likely you'll find something on the subject.
for many years i designed concrete structures in texas (as they are quite common there) and we usually didn't focus a lot of attention on torsion.  typically, we used either beam and slab or beam, joist and slab systems.  with these, we would typically specify closed stirrups for perimeter beams where the structural slab was on one side only.
aci dictates that if you design the adjoining slab (whether it be a slab, joist, or other system) based on a simple end condition (i.e. the joists or slab don't depend upon the torsional rigidity or strength of the perimeter edge beam) then you can reduce torsional effects to a small amount (see aci 11.6.2.2)  this we did, and calculated the "typical" torsion on various size   
if you are considering that the slab/diaphragm offers resistance to torsion in beams, then you should check that wether that slab can resist that much amount of additional moment (that is being transferred to it because of torsion). the safest way is to design the beam for the torsion, rather than trying to assume that slab can take tat additional loads.
to answer the other part, encountering torsion, since almost all the structures here in india are deigned in rcc, all elements more or less monolithic, we come across torsion very frequently, like projections on lintels, projections not lying as the same level as the slab, free cantlever steps from a beam, brackets in beams supportin cross beams/slabs etc.   
the torsion should always be checked.
you mention calculating the twist/deflection.  these parameters are typically calculated for "service" conditions, and should not be confused with "limit states".  at "limit states" or ultimate strength upon which most concrete design is based the cracking, twists, deflection etc. are significant and redistribution of loads will form, possibly negating your redundancy.
always provide redundancy in your designs.  since torsional failure is sudden and non-ductile, you should always check it and confirm adequate strength in the primary   
it seems that there are many ideas offered and most imply that it is best to check the torsion - even if the resulting reinforcement detail becomes somewhat excessive.
the failure mode in torsion of a beam and slab system is very interesting.  i would have expected the beam to rotate under torsion and the slab to act as a simple strut or tie to stop the rotation (simple pencils and ruler theory)!  the effect would be to impose an axial force in the slab rather than a moment.  eg a beam spinning about a longitudinal axis could be restrained by a stap perpendicular to the axis.
at the same time a monolithic connection could introduce a moment into the slab..................
i have designed many beams for torsion but it is good to question the rationale behind the codes from time to time.
thanks  nb
can someone explain to me the mechanism of torsion in the simplest manner? the code requires additional longitutinal reinforcements at the mid section of the beam for torsion resistance. if the addtiional reinforcecement is located at the top or the bottom of the beam, what would happen? is it necessary to place the additional bars at the mid section even if the beam isn't "deep" say, 12" overall depth beam?