When to consider I-beam torsion
When to consider I-beam torsion
(OP)
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.






RE: When to consider I-beam torsion
RE: When to consider I-beam torsion
RE: When to consider I-beam torsion
I'm having a hard time following your description.... at least in any way that makes sense to me.
RE: When to consider I-beam torsion
What you have is two cantilevered beams with a simple span framing between the two ends. Torsion could be calculated based on the simple beam slope, but it will be negligible.
BA
RE: When to consider I-beam torsion
-consider torsion when it is due directly to the action of external loads (a so called primary effect)
-do not consider torsion when it is due to the internal constraint of the structure, in other words it is caused by the deformation of the structure under the external loads (a so called secondary effect)
prex
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RE: When to consider I-beam torsion
RE: When to consider I-beam torsion
You should always 'consider' torsion when it exists. You may not need to put specific numbers to it but you should always take it into account even if it is just allowing 10% overcapacity to allow for its effects.
In the case that you describe the amount of torsion depends on the flexibility of the end beam, the more flexible that end beam, the more torsion you will get.
what I have done with very similar situations is to do hand calculations based on the corners being pinned (i.e. two cantilvers with an end load and one simply supported beam) and then use the resultant forces in an alnalysis to calculate final stresses and deflections. The analysis is also important to calculate weld stresses.
If you are not confident to make these sort of engineering judgements then you should calculate for all these things until such a stage that you are confident enough to discount them.
RE: When to consider I-beam torsion
Now, maybe in an case where the loaded bm is very flexible and of a very long length and moment connected to the two cantilevered bms, then maybe I would look at it..even in this case it is self-limiting.
RE: When to consider I-beam torsion
It is my opinion that proper mentoring of young engineers will take them thru the trail of the various types of structures with detailed calculations for the elements.
RE: When to consider I-beam torsion
if the loaded member is fixed, the fixed end moment creates torsion on the supporting member.
RE: When to consider I-beam torsion
RE: When to consider I-beam torsion
what will the out-of-plane load do to the supporting members ?
is the out-of-plane load applied thru the shear center of the uspporting members ? no, but the torque created is probably roughly 1/2 of squat.
if you Really wanted to, i'd consider the torque as a couple acting along the supporting I-beam caps, so it would augment the bending streses.
RE: When to consider I-beam torsion
RE: When to consider I-beam torsion
RE: When to consider I-beam torsion
The loaded beam carries a concentrated load W at midspan. The simple span bending moment, M is WL/4 where L is the span. The end rotation is M*L/4EI = W*L^2/16EI which, in the worst case, is also the rotation of the end of each cantilever beam.
The rotation per unit of length of each cantilever beam is W*L^2/16EIC where C is the length of cantilever. Torsional stresses can be determined from that using torsional constants for the beams. They should be very small.
BA
RE: When to consider I-beam torsion
RE: When to consider I-beam torsion
That is a different scenario. In this case, all beams are in the horizontal plane.
BA
RE: When to consider I-beam torsion
RE: When to consider I-beam torsion
RE: When to consider I-beam torsion
-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
http://www.xcalcs.com : Online engineering calculations
http://www.megamag.it : Magnetic brakes and launchers for fun rides
http://www.levitans.com : Air bearing pads
RE: When to consider I-beam torsion
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 ...
RE: When to consider I-beam torsion
RE: When to consider I-beam torsion
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.