Joist bracing beam for LTB

[BAGW]

Hi,

I have a question regarding the joist bracing the beam for Lateral-Torsional Buckling.

Can the joist spanning into beam top flange like shown in the image below brace the beam for LTB AISC 360 section F2.2?

 

[SWComposites]

with the tiny weld shown? probably not.

 

[271828]

The weld just looks tiny on the figure. It's not large, but not tiny either. Also, these joists are usually not very far apart.

Typically these would count as lateral-torsional buckling braces. The weld / joist / diaphragm "panel bracing system" would need to be evaluated per the AISC Specification Appendix 6, Section 6.3. It would need to have adequate strength and stiffness.

 

[KootK]

One would normally count this as LTB bracing for situations where the top flange would move the most laterally during LTB but not for situations where it would be the bottom flange that would move the most. So one needs to consider the nuances associated with uplift, cantilevers, Gerber system, continuous beams etc.

We usually treat this as lateral flange restraint rather than whole cross section torsional restraint.

 

[BAGW]

I am more looking for a LTB restraint for gravity loading when the top flange is in compression. I believe in such situations the joist seat welded to the top flange of the beam will provide the LTB restraint.

 

[jayrod12]

I would agree with that assessment

 

[lexpatrie]

As a point of order, can we start encouraging folks to say "Open web steel joist Bracing Steel beam...." versus "joist bracing beam..."

For most rolled structural steel roof beams, the difference between bracing every 5' or 6' and "continuous" bracing of the top flange (by the steel deck), is relatively minor, in terms of strength loss with considering discrete (5' or 6') bracing, for uplift it's more complicated, but there is Cb to be applied there (See Steel Design after college).

Fisher -

https://steeljoist.org/wp-content/uploads/2021/04/EJ-2006-JMFisher-Bracing-using-OWSJ-V43_01_25.pdf

AISCs slides for Steel Design After College -

https://www.aisc.org/globalassets/c...ndouts/steel-design-after-college-handout.pdf

Does anyone have the Zoruba article? I feel certain I have this in paper format but I can't find it.

https://www.aisc.org/A-Historical-a...the-Cb-Coefficient-in-the-AISC-Specifications

Same question regarding the Hemstad paper:

https://www.aisc.org/Cantilever-Beam-Framing-Systems

Regards,
Brian

 

[BulbTheBuilder]

@Lexpatrie: See attached for the document