Web stiffeners and Warping 1

[Abuh001]

Hmmm....Just been thinking, what kind of resistance would web stiffeners provide against warping. Say you could provide a post to restrain the beam from torsion, would adding web stiffeners allow the connection to be fully restrained against both torsion and warping. Thanks

 

[KootK]

You're covered for St. Venant torsion. For warping, normal stiffeners perpendicular to the web would be ineffective. You'd need something more like side plates weleded between the flange tips and running parallel with the beam.

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.

 

[WARose]

It's like the stiffeners are not even there when it comes to torsion. I got into this when I had a beam with stiffeners subjected to torsion and I was worried about the weld (to the flange). Checking it in a FEA program, the required weld was virtually nothing (for the torsional forces)......and the stiffeners changed nothing with regards to torsion.

 

[Abuh001]

Thanks for the responses, how effective do you think a steel post is as lateral torsional buckling restraint. Is it only effective if it can restrain the top compression flange somehow or would simply welding it to the bottom of a beam provide some degree of restraint. Thanks

 

[Lomarandil]

Yes, for a post below a column, providing web stiffeners at the post is a traditional method of providing LTB restraint to the top flange. I think AISC even calls for it, although I'd have to look to remember where.

In some cases where you aren't obliged to follow AISC specifically, I've accounted for less ideal restraint using the web stiffness. One old way to do it from the BS standards was to add 2x the beam depth to the physical LTB braced length for your calculations. I would probably do this only for fairly stocky beams (with good web proportions) -- not some highly slender floor joist W section.

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The name is a long story -- just call me Lo.

 

[dik]

It has a more important reason. In the event the structure goes into 'plastic' mode, the stiffeners maintain the cross section at the location of what is usually the first 'hinge'. This even works for sections that are not Class 1.

Dik

 

[KootK]

Is it only effective if it can restrain the top compression flange somehow or would simply welding it to the bottom of a beam provide some degree of restraint.

While Lo has answered this rather adroitly, I think it worthwhile to beat on a couple of dead horses in the interest of clarity/safety:

1) The system that you've described with full -- or nearly full -- height stiffeners is ubiquitously used for LTB prevention.

2) The system that you've described without stiffeners is potentially catastrophic and should only be used if you've rigorously evaluate the level of restraint provided by the column using methods similar to what Lo has described.

3) As with most things, judgement needs to be exercised, even with the stiffeners. Would I use this system with a W36 beam running over top of a 3x3 HSS column? No way.

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.