Omega and OCBF's

[lana02]

I overheard a conversation reguarding the use of omega in OCBF vs SCBF and hoped someone could confirm/deny/shed greater light on the subject.

For a SCBF the brace is designed for R=6. With omega=2 applied to the connections making the connection design twice as strong as the brace. So a reliability of R=3 appears be the max force necessary to design for.

But an OCBF has a R=3.25* and an omega=2. Applying omega here ostensively enhances the design of the connection to be equivalent to R=1.625 or 2x stronger than the connection of the SCBF!

The conversation I overheard was just that - it is not appropriate to apply omega to an ordinary systems because the reliability is already reduced to minimum levels by the lower R values.

Thanks
AL

*P.S. (Was an R=3.25>3 selected just so the design would be forced to comply with the special seismic provisions?)

 

[WillisV]

Regarding the PS - the R of 3.25 was chosen to discourage users from using OCBFs in seismic regions (as well as to keep it in the realm of the seismic provisions).

 

[JAE]

lana02, the comment you heard that you don't use [Ω] values on OCBF is wrong in my view. The fact that the numbers don't come out to be the same between SCBF and OCBF is what WillisV describes - trying to keep you towards the special frames/braces in high seismic and also a reflection of the higher detailing requirements that ensure better ductility.

 

[ARLORD]

See the latest Seismic Provisions AISC 341 for when to use the overstrebgth factor for braced frames.

I don't have my copy here, but from memory the Expected Yield Force is used for SMF, IMF & OMF connection design

 

[ARLORD]

Per AISC 341 for connection design, for SCBF the Expected Yield Force is used and the Overstrength Factor is used for OCBF. They both use an increased load for the connection design.