SOURCE
https://www.eng-tips.com/viewthread.cfm?qid=388484
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QUESTION
I am a recent graduate and trying to understand the beam bracing analysis/design.
I read in many texts that the beam flange needs to be braced otherwise it would buckle ( like a wave shape as we see in a column under axial load.)
Now my questions are:
If I have a beam supporting floor slab, (top flange is fully braced- great, no worries) Now, bottom flange needs to braced so it does not rotate (torsional buckling?)
how do I know if I really need to brace or not ? OK, AISC manual gives a Cb equation I can calculate my un-braced length
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How can I correlate Cb equation with the moment applied to the beam, or stability of the beam ? Cb should tell me how much more moment capacity I am gaining for providing that brace at that distance (correct?)… what does it have to do with stability ?
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What If I have moment connection (double curvature moment diagram) is it different than simply supported ? ( Do I always need to brace both flanges Tension and compression ?
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How much load will be brace see? how much should be designed for, is this load coming from the moment or out of plane load ( wind, seismic )?
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When do I need to brace/add stiffener to the web of the beam ?
My first thoughts are: If the beam works for giving load for moment, shear, and deflect - perfect…But know I have stability issues I need to understand.
I am just trying to make sense of equations, rules, and develop a sense of what is right and what is wrong (how can I prove by calculation).
REPLIES
KootK
To begin with, it’s important to recognize that lateral torsional buckling (LTB) has nothing to do with flange buckling. Rather, it is the tendency of the section as a whole to rotate about a point in space that is on the same vertical axis as your beam shear center. Therefore, it’s most useful to think of bracing the entire section against rotation rather that bracing individual flanges against buckling.
If I have a beam supporting floor slab, (top flange is fully braced- great, no worries) Now, bottom flange needs to braced so it does not rotate (torsional buckling?)
how do I know if I really need to brace or not ? OK, AISC manual gives a Cb equation I can calculate my un-braced length
For a gravity loaded, simple span beam, the point of LTB rotation is at a distance below the bottom flange of the beam. Even with the bottom flange unbraced, top flange bracing alone is enough to prevent LTB rotation about this point. With the top flange braced, it’s actually still possible for the beam to LTB rotate about a point in space at the elevation of the deck restraint. This LTB buckling mode is analogous to tension chord buckling in trusses (Link). The good news is that this second mode of LTB buckling requires a good deal more energy to initiate and is therefore fairly easy to prevent. It normal circumstances, it is prevented by the bottom flange acting as a horizontally spanning girt between supports. This is part of the reason that our codes insist on torsional restraint at the ends of simple span beams that engages a majority of the beam cross section.
- How can I correlate Cb equation with the moment applied to the beam, or stability of the beam ? Cb should tell me how much more moment capacity I am gaining for providing that brace at that distance (correct?)… what does it have to do with stability ?
The LTB equations are derived assuming uniform moment along the length of the beam which is the worst case from a stability standpoint. Most beams do not have uniform moment diagrams which is an improvement. Cb is simply way to approximate that improvement. Note that Cb has nothing to do with unbraced length and will not alter it in any way.
- What If I have moment connection (double curvature moment diagram) is it different than simply supported ? ( Do I always need to brace both flanges Tension and compression ?
With negative moments in play, there will likely be unbraced lengths of the beam over which bottom flange (compression flange) bracing will be required. The main exception is cantilever beams which are most effectively braced at the tension flange. For the most part, double curvature within a single unbraced length affects Cb and is accounted for there.
- How much load will be brace see? how much should be designed for, is this load coming from the moment or out of plane load ( wind, seismic )?
The load is coming from the moment. The AISC manual has an entire section dedicated to require brace strength and stiffness which addresses this issue quite thoroughly. In the past bracing for 2% of the compression force in the flange was common. I think that, still, AISC’s seismic manual require bracing for 5% is some situations where the flange is expected to make excursions into the plastic range.
- When do I need to brace/add stiffener to the web of the beam ?
There are several reasons to do this including web shear buckling, web crippling, web yielding, and overall section rotational restraint at supports. All are addressed in the AISC manual and only the last really impacts LTB.