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Buckling of a beam with tension side braced

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jimmytwotimes

jimmytwotimes

Structural
Feb 25, 2013
19
Does anyone have any recommendations/guidance for the buckling of a beam if the tension side of it is fully braced against buckling?
 
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The compression side is the one that counts, can you use any details to extend the tension side bracing to the compression side?
 
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bracing (to reduce the column length and increase buckling load) isn't effective on the tension side.

Can you add a localised fttg to support the other (compression) side of the beam ? yes, this'll put bending into the brace (something more to analyze) ?

Can you relocate the brace to the compression side ??

Is the load reversible ?

another day in paradise, or is paradise one day closer ?
 
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@rowingengineer

Consider a floor joist that is on the top side of the floor. Except instead of a 2x6 piece of wood the joist is a 2x6 steel tube.

It's more of a thought exercise to be honest, so I'm more interested in determining if/when a beam would buckle in this scenario, not so much how to prevent it from buckling
 
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As far as I am aware, tension buckling isn't something to be considered or analyzed. If your bracing is only provided on the tension side of the member, then by extrapolation, there is no bracing on the compression side. Therefore, you'd size as an unbraced member, and determine capacities that way.

Tension bracing does not provide any restraint for compression buckling. As an aside, this is a question that has been plaguing lateral torsional buckling analysis i-joists for decades, no one has been able to determine an adequate way to model how inflection points affect torsional stiffness and restraint for composite members.
 
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Intuitively to me a tube spanning 20ft would buckle at a lower UDL than the same tube with a continuous form of bracing on its tension face...but I don't know if my tuition is wrong or how to quantify it even if it was right
 
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I would think that to be true as well in the case of an HSS tube with an adequate connection to the bracing member to prevent rotation. I would think the torsional stability of the tube would provide a form of bracing to the compression flange that would be a function of the tubes geometry.

In this case, i.e. a 2"x6" tube, I would expect almost adequate bracing provided to the compression flange via the webs bending resistance, assuming the connections are adequate between the bracing members and the beam. However, say a 2"x14" HSS stair stringer, I'd be less confident that it would provide any benefit.
 
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Are talking about a wide flange beam with HSS6x2 joists welded to the top? Or just a general case where you have uplift on a roof beam with no bottom flange bracing (fly bracing in some parts of the world)?

Unbraced length (L[sub]b[/sub]) is defined by AISC as the "length between points that are either braced against lateral displacement of the compression flange or braced against twist of the cross section." So if you were to weld a tube to the top flange and have an uplift load causing the top flange to be tension, it would brace the beam against buckling if and only if it provided sufficient torsional restraint. So 1) the HSS joist would have to be stiff enough, 2) the weld and HSS would have to be strong enough, and 3) you'd have to have full height web stiffeners in the beam to prevent warping and complete the torsional restraint.
 
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OP said:
Intuitively to me a tube spanning 20ft would buckle at a lower UDL than the same tube with a continuous form of bracing on its tension face...but I don't know if my tuition is wrong or how to quantify it even if it was right
Click to expand...

Your intuition is correct. When you restrain any aspect of a member's natural buckling mode shape, you increase the load at which buckling occurs. With tension side bracing, you effectively remove the "lateral" from the lateral torsional buckling mode. What your left with is often referred to as constrained axis buckling. Buckling is constrained to occur about the bottom flange. It's a commonly considered failure mode in diaphragm chord members which is why you see Sabelli's name on the article below.

c01_qch8sz.png
 
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Something like a pony-truss where the compression chord is braced and the vertical members provide the lateral support for the compression top chord.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik
 
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KootK said:
What your left with is often referred to as constrained axis buckling
Click to expand...
Ahh, good point. I do very little steel design, so wasn't aware of this kind of literature. now that you mention it, this does sound vaguely familiar. Will have to look at the reference document.
 
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If you traverse the world of steel design you will also find that we've resolved the inflection point bracing thing as well: there's no such thing. A flange that's only in compression over part of its length creates a reduced LTB demand but no LTB restraint whatsoever.
 
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Tension side bracing often helps, as it tends to restrain lateral movement, but you typically ignore it in day to day steel design. A lateral restraint on a tension flange is typically considered unrestrained.
 
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Here's a chart that gives an indication of how the level of the brace on the cross-section affects buckling capacity. These are published elastic buckling results, so you still need to feed these through your design code procedure to get a design capacity accounting for imperfections. Hollow sections would be K=0.1 (or less) and bottom flange bracing is at the left of the graphs.

BraceHeight_cwmhrn.gif
 
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Don't forget about the good 'ol web sidesway buckling. Which I suppose is a flavor of constrained axis buckling. But it applies only(?) to concentrated forces, not UDLs. Not sure how this plays with closed shapes (HSS) though.

web_sidesway_buckling_vlbgcn.jpg
 
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