St. Venant vs Warping torsion for I-beams
St. Venant vs Warping torsion for I-beams
(OP)
I am designing an I-beam for torsion.
The FE-program gives the St. Venant torsion. Is it allowed to design the I-beam using this St. Venant torsion only (and neglecting the warping torsion)? Is it a conservative assumption?
Eurocode states that I-beams shall be designed to warping torsion. But on the other hand the plasticity theory states that every equilibrium system that does not violate the yield strength is a safe approximation of the carrying capacity of the structure.
The FE-program gives the St. Venant torsion. Is it allowed to design the I-beam using this St. Venant torsion only (and neglecting the warping torsion)? Is it a conservative assumption?
Eurocode states that I-beams shall be designed to warping torsion. But on the other hand the plasticity theory states that every equilibrium system that does not violate the yield strength is a safe approximation of the carrying capacity of the structure.






RE: St. Venant vs Warping torsion for I-beams
In some cases (support condition and loading), you could argue that warping isn't going to be very significant in the final design. But, it is best to do this from a position of knowledge.... after first estimating warping stresses.
RE: St. Venant vs Warping torsion for I-beams
flexibilityrestraint you will alwaysunderestimateoverestimate twisting deformation. In the case of an I section, yourunderestimateoverestimate is likely to be a grossunderestimateoverestimate as warpingflexibilityrestraint in such a section is usually a major contributor to rotationalflexibilitystiffness.2) Section warping will introduce axial stresses in the flanges which will interact with the axial stresses imposed by the flexural and axial loads applied to the member causing first yields to occur sooner and asymmetrically.
3) If your software doesn't model warping
flexibilitystiffness, then it will beoverestimatingunderestimating the torsional stiffness of the members and the amount of torsional force drawn to them. You'll want to keep an eye on that.I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: St. Venant vs Warping torsion for I-beams
This approach is conservative for calculating the normal stresses because:
- you are ignoring the torsion taken by pure torsion action
- you are ignoring any contribution from the web
RE: St. Venant vs Warping torsion for I-beams
Just a quick correction on item #1. The warping effect adds stiffness to the section. Therefore, when you ignore the warping effect your twist is much, much higher for I beams.
That in turn makes item #3 incorrect as well. If your software ignores warping in the stiffness calculation, then the analysis will underestimate the amount of torsional force drawn to the member.
RE: St. Venant vs Warping torsion for I-beams
RE: St. Venant vs Warping torsion for I-beams
A pretty fundamental correction I'd say. Having given it some careful consideration, I agree on all counts. Thanks for taking the time to set me straight.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: St. Venant vs Warping torsion for I-beams
I have another question:
I have an I-beam welded to a RHS-columns (no stiffener plates in the joint). The width of the beam flanges and width of the RHS-column is the same. Is this joint restraint for warping?
RE: St. Venant vs Warping torsion for I-beams
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: St. Venant vs Warping torsion for I-beams
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: St. Venant vs Warping torsion for I-beams