Lateral torsional Buckling

[civileng01]

for a chs beam do you work out both bending resistance: Mc,y,rd=Wply,Fy/Ymo and lateral torsional buckling = Mb,rd= XltWy(Fy/Ym1). is there a difference or similarity between ltb and bending resistance? or do you use two or one of them in calculation?

 

[Agent666]

A CHS tube is not subject to LTB, because about any axis the section properties are the same. You can achieve the full plastic section capacity if subject to only flexure.

 

[civileng01]

Okay, also is it the same beam classification for I beam that I can use for CHS like d/t= cw/tw and other equations for I beam for CHS. Because in the steel data book it mentions I beam is there another databook just for CHS?

 

[JoshPlumSE]

By CHS you mean circular Hollow Sections, right? If so, circular sections do NOT experience LTB.

 

[civileng01]

yes i mean circular hollow section i now know it doesn't experience ltb but im designing a chs beam and want to determine section size to use and bending and shear resistance, I was wondering can i still use the same I beam classification for CHS d/t= cw/t and other equations. the steel data book i have only mentions I beam so i was wondering if theres another steel datebook for chs to find sections and other equations. Or i follow the I beam steel book but im not doing LTB.

 

[KootK]

Most steel design manuals should list maximum diameter:thickness ratios to ensure that the walls don't locally buckle under flexural compression. After that, flexural design is pleasantly straight forward. Shear determination will also be different from the I-beam calculation and your local steel design standard likely speaks to that.

 

[JoshPlumSE]

Section F8: Round HSS covers the moment capacity of Circular Hollow sections. Though CHS compactness is determined from B4.1b, Item #20..... Note that I'm looking at AISC 360-2016.

Section G5: coves the shear strength of Round HSS.

 

[civileng01]

Thanks very much one final question when finding 𝜎𝑚𝑎𝑥=MZ/I which M, moment do you use is it Med(wl/8)or bending resistance(Mc,y,rd)?

 

[JoshPlumSE]

Well, I'm not sure what you're using that equation for.

The normal max stress (Sigma = M*c/I, which at first yield is My/S = Fy) assumes an elastic stress distribution. Therefore, it is only valid up to My (the moment where the extreme fiber reaches the yield stress Fy).

The nominal capacity of a beam that is not subject to LTB or local buckling will generally be the plastic moment, Mp (the moment at which the entire cross section reaches yield Mp = Z*Fy).