Seismic Design of OCBF

[Toby43]

Hi all,

I'm curious about the AISC341-16 provisions concerning Ordinary Concentric Braced frames "OCB" (I'm in Australia so don't know the standard that well). It states that the slenderness of the braces in a "V" type configuration must satisfy
Lc/r<4sqrt(E/Fy)
I understand this seems have the intent of minimising un-favourable degradation of overly slender braces (happy to be corrected). But on appraisal in a recent project, members sized using this are so "stocky" that they would not buckle nor yield in tension under "elastic" seismic action (i.e Response Modification Coefficient R = 1.0)
This leads me to the conclusion that the connection to and overturning on the footing would also need to be sized for R=1.
Example
Consider single storey of height "h" with "V-type" OCBF
R=3.0
Seismic Shear V3=100kN
Seismic OT Moment M3 = 100h
Brace compression C3 = (0.5)1.41V3 = 70kN (45 degree angle and 0.5 to tension diagonal)
Select brace due to slenderness limit results in buckling capacity of 400kN
thus at
R=1.0
V1=300kN and Brace force is (0.5)1.41V1= 211kN<400kN

Sizing all the connections and beams for the expected capacity and/or over-strength seismic demand is one thing, but would it not make sense to size the footings for overturning from the "elastic response", otherwise the footing will do so regardless prior to any significant energy dissipation in the frame.
The potential savings in footing size may be fools gold...
Would like to hear others thoughts or experiences.
Cheers
Toby

 

[Shu Jiang PEng SE]

The detailing requirements have to meet “AISC 341 Seismic Provisions for Structural Steel Building” if the structure seismic design category, SDC, (which is determined by the site soil classification seismic data) is “B” or above. Per AISC 341, for OCBF seismic resistance system (SDC B or above), the brace connection strength shall meet:

” The required strength of diagonal brace connections is the load effect based upon the amplified seismic load.
Exception: The required strength of the brace connection need not exceed the following:
(1) In tension, the expected yield strength of the brace multiplied by 1.0 (LRFD) or divided by 1.5 (ASD), as appropriate. The expected yield strength shall be determined as RyFyAg.
(2) In compression, the expected brace strength in compression multiplied by 1.0 (LRFD) or divided by 1.5 (ASD), as appropriate. The expected brace strength in compression is permitted to be taken as the lesser of RyFyAg and 1.14FcreAg where Fcre is determined from Specification Chapter E using the equations for Fcr except that the expected yield stress RyFy is used in lieu of Fy. The brace length used for the determination of Fcre shall not exceed the distance from brace end to brace end.”- Excerpt from AISC 341.

Please note that the over strength factor of 2 could be applied to the calculated brace internal force, but the internal force of brace used for design shall not be larger than the requirements mentioned above.

The maximum brace connection strength may be used for brace design.

 

[Toby43]

Thanks for that extra information, yet I'm still not clear on whether a reduction in footing size would be valid if the structure would respond elastically with R=1.0