LTB Calculation of Stacked Beams
LTB Calculation of Stacked Beams
(OP)
My situation is an S6x12.5 monorail that spans too far without some sort of reinforcing (24'-5") simple span without any lateral bracing between supports. My supervisor (not a licensed engineer) wants me to weld a W8x18 or similar larger beam to the top of the S6x12.5.
My question is how should I calculate the LTB capacity of the combined section? If I used the old ASD method of Fb = 12000 * Cb / Lb * (d/Af), it seems like I am limiting myself due to the increased depth of the section without accounting for any resistance to rotation from the top flange of the S6 and bottom Flange of the W8 (located near the neutral axis of the combined section). It's seams like the newer method in 13th or 14th edition may take into account weak axis stiffness and torsional/rotational stiffness and may yield a higher capacity.
A W14x22 (Sx = 29, Ix=199) by itself just barely works for this loading, and my combined section has just slightly lower section properties (Sx = 26.8, Ix = 192). But my gut feeling is that the combined section would be able to resist more torsional rotation than a W14x22 and if so, you could increase Lp and therefore increase LTB capacity.
Side notes - I may lobby to use a WT in lieu of a W-Shape to reinforce the monorail (thoughts)? A larger S-Shape for the monorail is not an option.
Any advice would be appreciated.
Thanks,
-T
My question is how should I calculate the LTB capacity of the combined section? If I used the old ASD method of Fb = 12000 * Cb / Lb * (d/Af), it seems like I am limiting myself due to the increased depth of the section without accounting for any resistance to rotation from the top flange of the S6 and bottom Flange of the W8 (located near the neutral axis of the combined section). It's seams like the newer method in 13th or 14th edition may take into account weak axis stiffness and torsional/rotational stiffness and may yield a higher capacity.
A W14x22 (Sx = 29, Ix=199) by itself just barely works for this loading, and my combined section has just slightly lower section properties (Sx = 26.8, Ix = 192). But my gut feeling is that the combined section would be able to resist more torsional rotation than a W14x22 and if so, you could increase Lp and therefore increase LTB capacity.
Side notes - I may lobby to use a WT in lieu of a W-Shape to reinforce the monorail (thoughts)? A larger S-Shape for the monorail is not an option.
Any advice would be appreciated.
Thanks,
-T






RE: LTB Calculation of Stacked Beams
However, you should use an HSS to eliminate LTB as a failure mode. That's the only way you can be certain that you've done it correctly, without understanding what you're doing.
RE: LTB Calculation of Stacked Beams
Usually a flat channel cap can be used on top. This significantly increases the shape's lateral stiffness (Iy goes up for LRFD) and increases the old ASD rT of the shape and helps LTB considerably.
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RE: LTB Calculation of Stacked Beams
Typically, the choice of retrofit section is dictated by fabrication and serviceability concerns. I've personally seen the channel cap used or HSS sections on each top flange tip. I've heard of adding a WT or another beam entirely (as JAE mentions, this can be a mixed bag depending on the section).
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RE: LTB Calculation of Stacked Beams
RE: LTB Calculation of Stacked Beams
Also, to clarify, this is not a retrofit and the monorail will be hung from the top WT or W-shape Stiffener.
RE: LTB Calculation of Stacked Beams
RE: LTB Calculation of Stacked Beams
RE: LTB Calculation of Stacked Beams
I've seen LTB values figured for built-up shapes before (as a single piece) and it's not that much extra capacity most of the time. (At least enough that justifies a lot of time & expense to pour into it.)
RE: LTB Calculation of Stacked Beams
jayrod12 - I have to use an S6x12.5 per the client specifications for compatibility with their existing trolleys. They have service rails all over this plant and all are S6x12.5.
RE: LTB Calculation of Stacked Beams
Loading the bottom flange does not create a restorative moment because the top flange can displace laterally and twist torsionally. Lateral-torsional buckling is not a flexural tension failure.
RE: LTB Calculation of Stacked Beams
The only caution with adding a W14 to the S6 is that lateral loading from the monorail creates additional torsion on the W14 as the lever arm is bigger. Rationalize it as the torsion is resisted by the S6, or check the W14 for the load... any way statics is satisfied and you have capacity in the system.
RE: LTB Calculation of Stacked Beams
There is no stabilizing P-delta in simply supported monorails. The tension flange doesn't buckle laterally or torsionally. The compression element of a W14 will not buckle because you added an S6 below it. This isn't torsion. It's buckling resisted by lateral-torsional strength of the compression element.