When to consider I-beam torsion

[McT178]

I am working with an I-beam grid that can be approximated as a "U" that is fixed at the two ends of the U. A load is applied to the middle of the beam that would act as the bottom of the U. Is there any guidelines as to when torsion should be considered? For instance, is there a ratio of length to web height when exceeded torsion needs to be considered? I understand that torsion will exist to some degree where the beam at the bottom of the U meets the side beams, but this would be limited to amount that the bottom beam will deflect. I appreciate any advise.

 

[prex]

Saying the same as others above with different words:
-if you need to calculate the supported beam as being fixed (unlikely, but you could have narrow space constraints), then the torsion in the cantilevers is primary and you must account for it (but of course I profiles wouldn't be OK for this)
-if you can calculate the supported beam as being pinned, then you can forget about torsion, as the stresses are self limiting (a basic characteristic of secondary stresses); and if you wanted to determine those stresses (just to know, as they don't need to be accounted for), BAretired has the procedure: the torsional imposed rotation is equal to the flexural end rotation of the supported beam.

prex

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[rb1957]

actaully isn't this a case where you calc the partial fixity of the loaded beam ? the torsion stiffness of the supporting beams is the end fixity of the loaded beam.

the end moment of the loaded beam creates torsion in the supporting beams. Over the length of the supporting beam you can calc the torsional rotation which is then the slope of the loaded beam ?

next we'll figure out how many angels can danced on the tip of a needle ...

 

[flash3780]

Ahh, I see it now. I was definitely understanding the problem incorrectly. Yeah, the torque in that situation is essentially nil in most cases... but could be important for a significant span. I think you can get there by solving the beam equation with a torsional stiffness at the boundary... although a closed form solution to the differential equation eludes me at the moment.

 

[dhengr]

rb1957:

I doubt that any angels would attend this kind of party, they have work to do, although we certainly do provide the tip fo the needle to dance on. They would be all torqued off my now.

I can’t see the original attachment, but I think I get the picture. Maybe another way of saying it is that if the loaded beam were sufficiently loaded and very long, it would have significant deflection and end slopes to induce a meaningful torsional loading (rotation) in the ends of the supporting cantilevers. And, as usual BA has boiled it down to the basic equations from our first Strength of Materials class.