Web Stiffeners On Long Cantilever

[BSVBD]

I have a W16x57, with a 7'-6" cantilever, supporting common roof trusses at 24" o.c.

In such a case, I always provide (1) set of web stiffeners, same or greater thickness as beam web, each side of web, directly above the cantilever supporting column.

Although I've never done so in the past (I frequently design "long" cantilevers), I'm considering providing web stiffeners 12" to 24" o.c. from supporting column, both sides of column for one or more space(s). But... I don't want unnecessary labor = $$$.

In this particular case, my shear is only 21% stressed.

What are your standard practices?

I often see details of either (2) sets of stiffeners - (1) each side of column; or (3) sets - (1) each side and (1) directly above column.

Although it HAS not happened in the past, i consider that that long of an unbraced cantilever may tend to torque or buckle.

Any thoughts? Am i overthinking?

Thank you!

 

[BAretired]

No, you are not overthinking. Thinking is a good thing and should not be discouraged.

Over the column, you need to supply enough stiffeners to satisfy bearing stress and stability considerations. The location of stiffeners will depend on the type and orientation of column.

Beyond the column, there appears to be no need for stiffeners unless needed for bearing, which seems unlikely given your description of the problem.

BA

 

[JAE]

Just use stiffeners at the column. Others on either side of the column don't do anything useful unless you have a very thin web (which you don't with a W16x57).

The vertical stiffeners at the column provide lateral resistance to lateral instability at the column-beam interface.
Vertical stiffeners elsewhere don't provide that same lateral stability against beam twisting unless they are tied into something external to the beam.

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[BSVBD]

Thank you both!

 

[MotorCity]

Also note stiffeners are required at unframed ends of beams that are not otherwise restrained against rotation about their longitudinal axis (page 16.1-138 AISC). So that would apply to cantilevers, and perhaps your case if you do not consider the trusses preventing rotation of the W16.

 

[BSVBD]

MotorCity... is your reference from AISC 14th?

AISC 13th (16.1-361) J10.7, Unframed Ends of Beams and Girders says, "Full-depth stiffeners are required at unframed ends of beams and girders not otherwise restrained to avoid twisting about their longitudinal axes."

JAE, BA... I understand that web crippling or buckling can only occur where there is a resistant element to cause such action. At a cantilever end there is no resistance / support.

Assuming the wood trusses cannot provide adequate resistance, please explain why you think the unrestrained cantilever cannot rotate and/or why stiffeners would not assist in resistance to rotate?

Thank you!

 

[MotorCity]

Yes, 14th edition. I assumed the ends of your cantilever beams are "unframed" or otherwise unrestrained at the end. An unframed end CAN rotate if it is not restrained. Stiffeners do not prevent rotation, they simply reduce the effects of local buckling in the compression flange

 

[KootK]

My interpretation of the unframed ends businesss that it's intended for beam ends that sit on something in bearing. At a cantilever end, stiffeners will do next to nothing to prevent rotation.

The unrestrained cantilever can rotate beyond the support point but, with lateral restraint provided by the trusses, rotation is forced to occur about the top of the beam rather than a point below the beam. That requires a lot more energy and results in an effectively braced beam for all intents and purposes.

Stiffeners not attached to anything external to the beam do not help with rotation because they do not appreciably help with torsional stiffness. All stiffeners do in this regard is prevent relative lateral flange displacement at discrete points which yields little improvement.

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.

 

[TehMightyEngineer]

There's also issues with elastic lateral-torsional buckling of cantilevers: [link msc.aisc.org/globalassets/modern-steel/steel-interchange/2005/2005v03_si_web.pdf]msc.aisc.org/globalassets/modern-steel/steel-interchange/2005/2005v03_si_web.pdf[/url]

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com

 

[JAE]

BSVBD - KootK summarized my response as well with regard to vertical web stiffeners on WF shaped beams.

It seems many people assume that a vertical web stiffener somehow deals with the shape's overall twisting (lateral torsional buckling or lateral beam instability)...they don't.

The stiffener can deal with web crippling or buckling where there are concentrated loads creating compression in the web.

But your issue, as I gathered from your original post, was about a cantilever beam and overall lateral stability of the cantilever.

At the ends of the cantilever, I'd try to use bridging or some kind of continuous blocking at the end to prevent end-of-beam rotation.

At the bearing support of the cantilever, I'd use the stiffeners to prevent web crippling/buckling as well as create resistance to the beam from rotation...assuming there is adequate connection between the beam and the support below....the stiffeners serve as a sort of support extension up through the beam to prevent this rotation.



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[BAretired]

I agree that stiffeners at the end of a cantilever serve no useful purpose.

BA

 

[KootK]

The unrestrained cantilever can rotate beyond the support point but, with lateral restraint provided by the trusses, rotation is forced to occur about the top of the beam rather than a point below the beam. That requires a lot more energy and results in an effectively braced beam for all intents and purposes.

While I still contend that this is true, it's difficult to apply in practice. In light of the article that TME posted, I recommend the Nethercot procedure discussed there as the best way to evaluate cantilevers in a design office setting.

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.