Lateral torsional buckling 2

[Yazan Atoum]

Hello Engineers,

I have a question. For a composite slab I used to design the main girder's bottom flange to be unbraced for its whole length, but one engineer told me that this is extremely conservative and secondary joists with depths around >= 1/3 depth of Girder would provide bracing for LTB even with a shear conncection, I had another debate with a Doctor in engineering, he mentioned that I have to assume the bottom flange totally unbraced for the whole length as there is no such refernce in the codes that assumes a shear connection can prevent LTB, So I am really confused now, I know this would not affect simply supported beam but it does have a remarkable effect on girder with fixed ends or if it's a part of a moment frame where there is an iflection point between positive/negative moment.
Can anyone guide me to a respected reference that gives a solid answer regarding that or if there is a proportion between Girder/ joist profile depths that assure no LTB would occur.

Thanks.

 

[KootK]

and secondary joists with depths around >= 1/3 depth of Girder would provide bracing for LTB even with a shear conncection

By "joists", you mean secondary beams right? Not open webbed steel joists? The rule of thumb that I was taught was that, with a suitable connection morphology (shear tab, clip angles, etc), the secondary beams could be considered to provide rotational restraint to the entire cross section so long as the depth of the connection was at least 60% of the depth of the girder.

I had another debate with a Doctor in engineering, he mentioned that I have to assume the bottom flange totally unbraced for the whole length as there is no such refernce in the codes that assumes a shear connection can prevent LTB

I don't agree with the good doctor. Firstly, structural engineering is more -- a good deal more -- than what is written in the codes. It's judgment, experience, research, and actual understanding. Unless codes specifically prohibit something, we're at liberty to employ these other tools as required to solve engineering problems. Secondly, there's a section in the AISC steel specification (appendix 6) that deals specifically with the strength and stiffness requirements for bracing. Granted, it doesn't specially state that a shear connection can provide rotational restraint. However, the stiffness requirements make it pretty clear that it doesn't take a whole lot of rotational stiffness to get the job done in most instances.

Here's an excellent reference on beam bracing: Link. It probably doesn't answer your question as simply as you might like however.

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.

 

[Yazan Atoum]

TheRick109, I wouldn't be a structural engineer if I haven't known these basics, please reread my question carefully, I am talking about the ability of secondary beam with a shear connection to provide full LTB resistance to the main girder at the inflection point of the negative moment, where the bottom flange would be in compression.


KootK, thank you so much for your reply, but don't you think that the depth of the section what matters more than the depth of the connection, as it might not be stiff enough to resist the lateral movement of the compression flange.

 

[Trenno]

Not always Rick, cantilevers have the tension flange as the critical flange.

 

[KootK]

KootK, thank you so much for your reply, but don't you think that the depth of the section what matters more than the depth of the connection, as it might not be stiff enough to resist the lateral movement of the compression flange.

You're welcome. Both the depth of the connection and the stiffness of the connected member are important. The latter, at least. Is relatively easy to evaluate numerically. It's also pretty easy to accomplish with most practical member sizes.

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.