I-beam torsional stress calculation
I-beam torsional stress calculation
(OP)
Can anyone tell me how to calulate torsional stress on an I-beam section? I have read texts that state that the shear stress of each individual section is based on the the sum of torsional resistances of each individual section of the beam. Is this the standard way of computing the shear stress (due to torsion) for non-circular members?
What is the torsional constant "J" as listed in the Torsion Properties table of the AISC Manual of Steel Construction. Is this value used in the stress calculation as described above?
Thanks in advance,
What is the torsional constant "J" as listed in the Torsion Properties table of the AISC Manual of Steel Construction. Is this value used in the stress calculation as described above?
Thanks in advance,






RE: I-beam torsional stress calculation
There are 2 torsional constants which in general are both relevant: Pure torsion J which is dominant for closed sections, and warping torsion which governs for I beams.
If you have access to FEA, a simple answer is to model the beam with plate elements. This is ideal for warping torsion.
Twin beam theory is a hand method for warping torsion of I beams, and it works well for simple models. Basically, each flange is considered a beam that is bending about its strong axis but in opposite directions. The resulting stresses are in the longitudinal direction and must be added to the other bending and axial stresses.
You might also see some horrible text book solutions with graphical solutions for torsion - I suggest avoid them unless you are into pain.
The simplest solution however is to avoid torsion if possible. ie change your design to eliminate torsion, or change to hollow sections so that you can analyse it using the J value.
RE: I-beam torsional stress calculation
www.achroneng.com
RE: I-beam torsional stress calculation
It is very comprehensive and has several examples and tables to use in the calculations. Enercalc software has a limited I-bema torsional design and analysis. I say limited because of the boundary conditions of the beam.
Good luck
RE: I-beam torsional stress calculation
RE: I-beam torsional stress calculation
RE: I-beam torsional stress calculation
One simple, but conservative, way of dealing with torsion on an I beam is to replace the torsion with two lateral forces F on the flange, such that the torsion T = FD, where D is the distance between the flange centres. You then analyse the flanges alone under the action of this minor axis loading to obtain the stress. If it is stress that you are interested in, you can use our old friend Von Mises to check the combined effects of the bending and torsion, or if you are simply checking capacity, you can use the standard interaction checks including the additional term My_flange/Mc_flange where My_flange is the minor axis moment due to F and Mc_flange is the moment capacity of the flange alone. You should also check the twist as this is likely to be large for an I section.
Martin Heywood
RE: I-beam torsional stress calculation
If you refer to the text pointed out earlier from lutfi (Author: Omer Blodgett), the section addressing torsion points out (almost in the first paragraph) "Design rule no. 1: use closed sections where possible". Not only is it tough to calculate the reaction of the section under torsion (since the polar moment is almost impossible to calculate accurately and with justification - leading usually to a very conservative value for this), but you'll probably find that an elastic band will offer you better resistance to torsion!!
-- drej --
RE: I-beam torsional stress calculation
One other thing that is easy to forget...The connections at each end of the beam need to resist the regular shear forces but also the torsional reactions at the end of the beam, which could lead to rather beefy looking connections.