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RE: Rho, Rho, Rho your boat (foundation)

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Perhaps you could make the argument that rho should not apply to a mat foundation - but what about a steel braced frame system with isolated footings?  An increase in brace forces would increase loads on footings.
Perhaps the code could be refined to address this - but a more refined code is also a more complex code!  Do we want to keep adding complexity to refine each individual design?
Bill Sherman

From: Garner, Robert [mailto:rgarner(--nospam--at)]
Sent: Tuesday, August 19, 2008 11:31 AM
To: seaint(--nospam--at)
Subject: RE: Rho, Rho, Rho your boat (foundation)

Thank you all for your replies.  As with many aspects of the Code, this looks like one where it's best to "Just do it", and I will henceforth.  "I'm trying to keep my mind right, boss".


Although I'm still looking for the logic - if you have a lack of redundancy in your framing system that could lead to a possible failure of the framing system, how would that lead to a possible failure of the foundation, thus requiring an increase factor for the foundation?  It seems you are making the foundation stronger to make up for a deficiency in a separate structural component.  I can see a need for foundation redundancy but only if the foundation itself has some deficiency that makes you need an additional reliability factor.  I see rho as an assurance factor for the framing itself; a penalty factor on the framing some have called it.  It is related to the framing, some configurations of which have proven unreliable.  But foundations have not been proven to need some means of redundancy, have they?


Not meaning to argue, just looking for engineering logic.


Bob G.


From: Garner, Robert [mailto:rgarner(--nospam--at)]
Sent: Tuesday, August 19, 2008 9:22 AM
To: seaint(--nospam--at)
Subject: Rho, Rho, Rho your boat (foundation)


I'm designing a steel framed structure in Seismic Category D.  For the steel framing, Rho will be 1.3.  For the foundation (a concrete mat foundation), we are using a Rho of 1.0 - our reasoning being that the purpose of Rho is to assure that no single seismic force resisting element above the foundation (the building frames) will likely fail in an earthquake.  Nowhere in the Codes can I find a statement that Rho is to be applied to the foundation.  Can anyone challenge me on this with a Code quote or logical argument?


Thanks for any help you can offer.



Bob Garner, S.E.


R. Garner

Moffatt & Nichol

Tel.:  (619) 220-6050

Fax.: (619) 220-6055

e-mail: rgarner(--nospam--at)