CMAA 74 and MH27.1 Allowable Compressive Stress

[BMart006]

I am personally not a fan of codes without commentaries, and I'm currently working with two of them (CMAA70 and MH27.1). Both of them feature an equation for the maximum compression stress (from flexure) for a single web monorail crane as Stress Allowed (ksi) = 12000 / ((L_bd)/A_f) < 0.6F_y. L_b is the unbraced length of the top flange and A_f is the compression flange area. FYI monorails are usually not doubly symmetric or of the same material as the top flange and web. That being said, this looks like a buckling formula of sorts, but there is no commentary in either code to provide the origin. Anyone have any thoughts on where this comes from or if it is really sufficient for an unsymmetrical section with two different F_ys?

 

[JStephen]

That's Eq. F1-8 in the AISC ASD 9th Edition. It's in the 1947 version of the standard also. Edited: There's about a page of commentary in the ASD 9th Edition relating to this formula.

 

[FLCraneBuilder]

CMAA member responding here; (but I do not speak on behalf of CMAA - probably a legal issue there)
1) CMAA is adding commentary as we can or encounter a need. Most new or revised sections will have commentary. Paragraphs that have been around for a while and have not been revised may be left without commentary for an extended period of time
2) I do not understand your statement that monorails are usually not doubly symmetric nor the comment that the top flange & web are different materials. Most monorails are common WF beams. Are you by chance referring to Patented track monorails? (which are rather uncommon)

 

[BMart006]

Thanks JStephen. It seems MH27.1 has used F1-8 but did not include the other two stress limitations (F1-6, -7) for some reason.

FLCraneBuilder,
Yes, the different materials refers to patented track, that in my limited experience, has been the preference in new installations. The non-doubly symmetric would apply to patented track and many of the designs I've seen in older facilities. These include W- or S- shapes with channel caps or those with a large W-shape as the main "backbone" with a smaller T-beam welded below for the trolley to run on.