Lateral Stability Ratio 1

[jheydon2]

Hello All,

I am currently working on a design of a simple monorail system. Some of my old mentors spreadsheets has a "Lateral Stability Ratio" which can not exceed 40. The equation is the length of the beam divided by the width of the beam. Does this sound familiar to anyone? If so, could you please point me in the right direction to find where something like that is stated?

Regards,
jheydon2

 

[WARose]

Doesn't ring any bells with me (doesn't sound like a proper slenderness ratio check either). Of course, with hung monorail systems, the challenge is deciding on unbraced length. If you do a search, you will see a lot of threads on that topic.

 

[SlideRuleEra]

jheydon2 - Your mentors must have been around a long time. The "Lateral Stability Ratio" dates back to the 1930's, and before. That is how what is now called "lateral-torsional buckling" was addressed... before it was well understood. Using that the beam length to flange width ratio is obsolete (and not always accurate, anyway).

Go to this page of my website and download the Carnegie Pocket Companion - 1934, located near the top of the page. See the portion of the book "Safe Loads For Sections Used as Beams" beginning on page 182. The 40:1 ratio is addressed on page 184.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[KootK]

If I had to guess:

1) A poor man's check on lateral torsional buckling (LTB) or;

2) A poor man's check on lateral stiffness although I wouldn't technically call that stability.

Is there a separate LTB check built into the spreadsheet?

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.

 

[KootK]

Damn it! Typed too slow... Nicely done SRE.

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.

 

[Hokie93]

The unbraced length-to-width ratio of the compression flange in hot-rolled beams, plate girders, and built-up girders was limited to 40 in the first through fourth editions of the AISC Specification. The units for both "l" and "b" were inches. As best I can tell, the provision was not included in the 5th edition AISC Steel Construction Manual. As Kootk said, I would term this a poor man's version of a check for lateral-torsional buckling. Out of curiosity, how old are your mentors??

 

[SlideRuleEra]

Thank you, KootK. Sometimes I feel like a 1930's engineer who came though a time warp.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[jheydon2]

Thanks for the responses. My mentor is around 72 now, so not quite sure where he got it. Could anyone point me in the direction to check the lateral torsional buckling properly?

 

[Lomarandil]

You can find one treatment of LTB in chapter F of the AISC Specification here:

https://www.aisc.org/WorkArea/showcontent.aspx?id=39246

As WARrose alluded though, if this is a hung monorail, the trick will be defining the unbraced length between points of torsional support. That's a problem that hasn't really had a codified solution (as far as I'm aware).

I'd recommend reading this thread for sure:

http://www.eng-tips.com/viewthread.cfm?qid=387114

There are also a handful of threads about suspended beams here on eng-tips if you want to get into a lot of depth.