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Re: CBC 1921.4 Frame Members Design

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Sorry, our concrete guru retired a few months ago, but here are my

I think I see the conflict in the requirements.  Although, I don't see a
problem with it;  that situation will just will not happen.

I don't have the CBC in front of me, but I looked at the ACI 318 -02.
Section 21.4 in the ACI I think is the equivalent to the CBC section
1921.4.    This section is for members with high axial loads.  When the
axial load is small, less than Ag*fc/10, ACI section 21.3 applies.  ACI
Section 21.3 also has the requirement that Vc=0 when  the axial force is
less than Ag*fc/20.   Here the requirement could apply.  I think the
section on transverse reinforcement was just copied from 21.3 to the 21.4
section for consistency.   Again, I don't see a problem, the situation will
just will never occur under 21.4.

Ed Haninger
Aliso Viejo, CA

                      nitin Christopher                                                                                                
                      <nitinchristopher@y      To: seaint(--nospam--at)                                                                   
                      11/03/03 11:32 AM        cc:                                                                                     
                      Please respond to                                                                                                
                      seaint                   Subject:  CBC 1921.4 Frame Members Design                                               

Hi Concrete Gurus,

Per CBC section 1921.4 - an earthquake load resisting frame member
subjected to bending and axial loads should have an axial load exceeding
"Ag f'c / 10". This is usually a frame column.
 The shear strength of such a member should meet the shear demand per
section 1921.4.5. In section 1921.4.5.2 it is suggested to ignore the the
shear capacity due to concrete (Vc=0), if the conditions per items 1 & 2
are meet.

My question is - How can condition # 2 ever be meet? i.e.

Item #2 states : "The factored axial compression force including earthquake
effects is less than Ag f'c / 20"

Section 1921.4 is addressed only when Pu is greater than Ag f'c / 10, Then
how could Pu be less than Ag f'c / 20 in order to use Vc=0?

Please comment.


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