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Re: Redundancy/Reliablity Factor (ro) in Wood Frame BLDG
- To: seaint(--nospam--at)seaint.org
- Subject: Re: Redundancy/Reliablity Factor (ro) in Wood Frame BLDG
- From: bcainse(--nospam--at)aol.com
- Date: Thu, 21 Sep 2006 12:44:39 -0400
My experience with the rho factor has been that if the structure is laid out properly with adequate shear walls placed where they should be the rho factor is 1.0. If the layout is skimpy and lacks redundancy, torsion and unbalanced deformations will result in poor performance. Rho is in the code for a valid reason, and though it is not a rationally derived nor perfect measure of reliability/redundancy issues, it does encourage good design if one takes the time to understand it (I.e., don't blindly accept the penalty provided for lack of redundancy but understand where your deficiencies are in your structure). The most troubling feature of rho has been the 10/lw which was intended as an amplification for high aspect ratio walls but rather acts as a penalty on long walls.
If the Plan Examiner asks for a rho calculation, I give it to him. It is in the California Building Code as a requirement to be met and he is entitled to ask for it. I have sometimes included comments trying to educate the plan checker on when the rho factor is important and when it is going to be 1.0. These comments have had a positive effect on future checking of my projects in reducing the number of requests for the rho calculation. It is important to note that while SEAOC has published a Position Statement (See Ben Yousefi's previous email for a link) recognizing the deficiencies in the the rho factor, such position DOES NOT ELIMINATE the CBC requirements. Those are State law until changed. It does, however, provide future code writers with substantial food for thought on probably more direct and workable ways to achieve the same result: to ensure proper performance of the structure during a major earthquake.
As I see the issue, we should stop trying to be "code jockeys" cutting everything to the minimum limits in the code. We should practice good judgement in developing well-engineered structures which will withstand the "laws of nature" which "laws" do supersede code requirements on a practical level. This is verified every time we have a major earthquake. And codes then get adjusted to prevent failures to observe those "laws of nature."
Bill Cain, S.E.
Berkeley CA 94705
Sent: Thu, 21 Sep 2006 5:59 AM
Subject: Redundancy/Reliablity Factor (ro) in Wood Frame BLDG
Dear List members:
There are some "Plan Examiners" that have been requiring computation of Redundancy/Reliability Factor (ro) in Wood Frame BLDG.
My understanding is that the SEOASC has issued a white paper in this regard and does not require the need for this in light wood frame structures.
1- What has been your experience with Redundancy/Reliability Factor when designing a wood frame building?
(would you still provide the "Plan Examiner" with "ro" computation?)
2- And how does this relates with IBC 2003 requirement?
Casey (Khashayar) Hemmatyar, PE