# Column for OCBF



## Adeeel (Jun 3, 2011)

Do we have to use the over strength factor "omega node" to design the columns in OCBF if Pu/Phi-c Ptn &gt; 0.4? or this condition is not applicable to OCBF?

Is there any part in AISC 341 mentioned anything about this?


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## blybrook PE (Jun 4, 2011)

Per my interpretation of the following note, I would say that no, you do not need to utilize the Omega (overstrength) factor in the design of OCBF members. The amplified seismic load is that determined by the load factors utilizing the Omega factor.

From AISC 341-05, Section 14.1 (full specification available free from AISC)

"User Note: Previous versions of these Provisions have required that the members of OCBF be designed for the amplified seismic load, effectively reducing the effective R factor by half. To make the design of OCBF consistent with other systems, this requirement has been eliminated from these Provisions, consistent with a corresponding reduction in the R factor for these systems in SEI/ASCE 7-05 Supplement Number 1. The required strength of the members of OCBF will now be determined using the loading combinations stipulated by the applicable building code (and the reduced R factors prescribed in SEI/ASCE 7-05 Supplement Number 1), without the application of the amplified seismic load."


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## Adeeel (Jun 4, 2011)

blybrook PE said:


> Per my interpretation of the following note, I would say that no, you do not need to utilize the Omega (overstrength) factor in the design of OCBF members. The amplified seismic load is that determined by the load factors utilizing the Omega factor.
> From AISC 341-05, Section 14.1 (full specification available free from AISC)
> 
> "User Note: Previous versions of these Provisions have required that the members of OCBF be designed for the amplified seismic load, effectively reducing the effective R factor by half. To make the design of OCBF consistent with other systems, this requirement has been eliminated from these Provisions, consistent with a corresponding reduction in the R factor for these systems in SEI/ASCE 7-05 Supplement Number 1. The required strength of the members of OCBF will now be determined using the loading combinations stipulated by the applicable building code (and the reduced R factors prescribed in SEI/ASCE 7-05 Supplement Number 1), without the application of the amplified seismic load."



Thanks blybrook PE for your clarification. This is my understanding too.


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