Are steel roof bracings part of braced frame? 2

[NewbieInSE]

Hello Dear Engineers,

Are the steel roof bracings part of OCBF or other braced frame systems? Do we need to make them seismically compact following AISC Seismic 8.2, Table I-8-1 of AISC 341?
I was thinking that, they are just part of the roof diaphragm and not part of seismic braced frame, and hence do not require to be seismically compact.

Thanks all.

 

[JoshPlumSE]

I consider roof bracing to be somewhat akin to a diaphragm.... It's not an OCBF, but merely a part of the horizontal system that transfers forces into the OCBF.

 

[NewbieInSE]

Thank you JoshPlumSE.

 

[Deker]

AISC 341-16 added provisions for horizontal truss systems acing as diaphragms (Section B5.2). You have two options:

1. Design the truss framing and connections for amplified seismic loads, in which case there are no compactness or detailing requirements.
2. Design the braces for the basic seismic load combinations and provide compactness and detailing similar to SCBF.

 

[NewbieInSE]

Deker, thanks for the information.
Ours is a gable shed. Mainly wind forces govern, I think amplified special loads (considering Overstrength factor) won't cause any change in the shed roof bracings. However, it should be checked.
Is there any relaxation in seismic requirements for buildings where wind forces govern rather than seismic?

 

[NewbieInSE]

One more thing, shed roof bracings which are at an angle, say 7 degrees, can be considered horizontal roof truss system, right?

 

[NewbieInSE]

Another question, the AISC seismic chapters seem to talk about V, Inverted V, K bracing. But probably nothing (i didn't find)about X bracing. Why so? Are X bracings not allowed?

 

[Deker]

Is there any relaxation in seismic requirements for buildings where wind forces govern rather than seismic?

The seismic requirements are dictated by the seismic design category (SDC) of the structure. If the SDC requires that a designated seismic force resisting system be used, the detailing requirements for that system must be satisfied.

One more thing, shed roof bracings which are at an angle, say 7 degrees, can be considered horizontal roof truss system, right?

Yes. The term "horizontal truss" is meant to include truss elements that replace the function of the diaphragm to deliver loads to the vertical elements of the lateral system.

Another question, the AISC seismic chapters seem to talk about V, Inverted V, K bracing. But probably nothing (i didn't find)about X bracing. Why so? Are X bracings not allowed?

X-bracing is permitted. There are additional requirements / restrictions for V and K bracing due to the unbalanced load that occurs in the boundary members after the compression brace has buckled.

 

[NewbieInSE]

Thank you very much, Deker.

I have some more questions, if you don't mind.
For a tapered I section (in the case of gable shed rafter), how do we classify the section as compact, noncompact? At which point do we check this? If I have a member whose depth tapering from 300mm to 600mm and web thickness is 12mm, how do I classify the stiffened web as compact or non-compact?

 

[NewbieInSE]

Are the seismic connection design required for Moment Connections only (considering probable moment, following AISC 358)?
The sub-beams/secondary beam connections, can their design be as usual considering the LRFD combination force effects? If their design has to be special considering probable moment, then where can I find the reference procedure.