heoretical torsional analysis question

huangyhg

theoretical torsional analysis question
here is the situation.  i have a wf in an existing building that is being modified.  the modification is going to introduce significant torsion on wf.  i am going to weld the web of a channel to the flanges to create a closed section.  in trying to dumb down the torsional analysis, i am only checking the closed section for torsion (assumed hss with wall thickness = the smallest of the channel web, wf web, or wf flange) and neglecting the outstanding parts of the composite section (i.e. channel flanges, and wf flanges not connected to the channel web).
the member is fine for torsion (as would be expected).
it is my understanding that because it is a closed section the warping stresses are negligible such that there will be no addition to the bending stresses from torsion.  is that a fairly accurate statement?
if it is, would it be reasonable for me to use the wf with just the web of the channel (such that the section looks like this  ___  )
           _|_|
for flexure and do two seperate checks?
i would do one check for torsion using the conservatively assumed hss section only and do a second check using the shape above for flexure.  again, my reasoning for assuming the two seperate analyses are valide is that the closed section reduced the warping normal stresses (what would add to the flexural normal stresses) to a negligible amount.  i would also design the weld of the channel to the wf using blodgett - he has a good example of the shear flow in a built up section subject to torsion.
i would really appreciate some opinions.
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make sure there is some loadpath out of the closing web; in your biz, probably a weld.
if there are discrete points of torsion input into the composite beam, why not close off the other side of the i-section locally ?
i am assuming you are talking about the weld (per blodgett) that i mentioned.
additionally, it is loaded eccentrically from glass as well as acting as a wind girt.  it is also loaded eccentrically from the cladding when the wind blows.
it is not discrete locations, but a uniform load (from both wind and gravity - which happen to be additive in this case) at a given eccentricity.
the only section that has no axial stress induced by torsion is a pipe. all others have axial stresses induced by warping, for closed sections this is much less than for open sections.
csd,
i know what you are saying, but my steel textbook (salmon and johnson) says that for closed sections the normal stresses due to warping are negligible.  is this an inaccurate statement?  
"is this an inaccurate statement?"
yes.
yet again, s&j is the place to look... lol.
yes, the effects of torsional warping can be neglected for closed sections.
yes, though you need to take them into account to design the welds.
structuraleit:
i think you're fine with neglecting the warping torsional stresses.  the saint venant shear stresses will be more than adequate for sizing the weld, in addition to the bending shear stresses.  if you have access to a simple fea program, a quick shell model of the beam will confirm your assumptions and maybe give you some additional confidence.
castigliano
i've used blodgett's example (problem 5) on page 2.10-9 before and was planning on using it again.  i don't believe that it accounts for warping stresses.  it adds the torsional shear stress to the bending shear stress (horizontal shear ->  vq/i) to design the weld.
thats fine to design a perpendicular weld, but what about longitudinal shear?