huangyhg

allowable rotation for steel beam in torsion
i am a recent grad working for a firm and am working with steel beams in torsion for the first time. i have performed analysis and the beam checks out for combined loading; however, i cannot find anything in the steel code limiting the angle of rotation. i have been all throughout the code but can't find anything specifically addressing rotational servicability.
thanks
there is nothing in the code limiting the angle of rotation. the rotation of a girder with a beam framing into it at right angles will often be self limiting. the beam end rotation will be a function of its loading and stiffness therefore the girder cannot roll over more than the beam will allow it to.
one limit you could use is keeping the total deflection (twist and vertical) to below the limit you normally use for vertical only (l/250, l/360 etc).
as apsix said, it is often the beam feflection that limits the total.
also keep in mind that this rotation will be zero at the ends and maximum in the middle possibly giving a differential deflection along the span of the beam.
can you give us some more specific details of this project so we can give you more specific advice. what is the limiting factor for deflection? what causes the torsion in the beam? what is the configuration of the beams?
i agree with the above posts regarding just checking the total defelction. this is of most concern on spandrel beams with a slab overhang or a lintel with a brick relief angle because the rotation causes rigid body movement of the assembly which, in the situations listed (and others), cause a relatively high amount of vertical displacement with a relatively small amount of twist.
usually, the problem is not with the beam itself, but with non-structural items which attach to the beam. for example, if you have steel studs resting on top of the beam, the twist at midspan will cause a rotation at the base of the stud wall, which will cause deflection at midheight of the wall. this type of thing needs to be checked.
daveatkins

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