LTB restraint by grating

[Eagleee]

Hi all,

I have a simply supported 3 meter I beam (180mm deep) which supports grating panels on both sides. The beam is overutilized in bending to around 130% if it is considered susceptible to LTB. The grating panels are clamped as shown in the picture per 800mm. You can also see in the picture the direction of load bearing bars of the grating. The load comes from the gratings (i.e. no grating no load). Would you consider the detail shown to restrain the top flange, or would you increase the size of the beam (implying extra weight, extra work, etc.)?

 

[BridgeSmith]

Restraint for LTB requires very little strength, as long as there is very little movement before the restraint is engaged. In other words, as long as the beam remains straight, it takes very little to keep it straight, but if it is allowed to deflect laterally and develop eccentricity, then the forces can become large.

The setup you show should work if the bolts remain tight, but I would be hesitant to assume that.

If it was me, I would consider a tab welded to the grating bars that extends down beside the flange to lock it in. Alternately, adding a tab to the top flange of the beam between the grates would also work. That's assuming the gratings themselves are tightly restrained, of course.

 

[JoshPlumSE]

I don't generally consider metal grating to restrain the top flange of a beam. AISC appendix 6 has some criteria for stiffness and strength required to consider a member braced. You might take a look at that. Run the numbers to see. Though, I have a hard time believing that grating alone would be sufficient.

 

[WARose]

I never, ever consider grating as any kind of restraint. In a industrial environment, the clips get knocked/torn off regularly.

 

[JAE]

We don't consider grating as a bracing element either. The grating generally has a small in-plane shear stiffness so there is very little diaphragm action present.
Plus the grating sometimes can be partially removed.

The numbers might work out per AISC but these would be based on guesses as to the rigidity of the grating itself.

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[Eagleee]

thank you all for your input

 

[SteelPE]

I am also of the opinion that grating does nothing to resist LTB.

 

[12345abc6ttyui67]

Another vote for grating not resisting LTB.

 

[CARunderscore]

Seems like this topic is basically wrapped up, but something else to consider: Your typical LTB check assumes loading at the center of gravity. For loads that are just bearing on the top flange without providing any sort of restraint, such equations are actually unconservative.

I don't know how unconservative offhand (sorry), but I assume it isn't much in this case. I guess my point is that there are little wrinkles that get ignored all the time, and it's not a good idea to scrape the bottom of the barrel for extra capacity while still ignoring unfavorable factors.

Those clips will totally get busted off, though. Don't rely on them.

 

[Settingsun]

Probably wrapped up, so I guess I'm free to follow it along the tangent. Is it indeed the case that the American code doesn't consider the position (height) of the applied load?

 

[Eagleee]

as i ve said above, the input of all you guys is much appreciated. i would normally not consider the grating restraint for deeper beams and more significant loading, but wanted to get a feel on the general opinion for this matter. which was accomplished. so feel free to take this thread wherever .

@carunderscore and steveh49: i had included the negative influence of loading on the top flange. i deal with eurocodes, with info on much info on ltb being present only in background docs / litterature etc and not in the codes themselves. while american standards could be similar in this respect, surely the position of the load has to be accounted for somewhere in the calculations as its influence as mentioned by others can be signifcant