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Re: 10/lw factor

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Dennis and Stephen:

Thanks for bringing this topic up. It encouraged me to reflect once again on
how little I understand the seismic section of this new '97 UBC.

I am a self-employed civil engineer; practically my sole source of income is
in designing residential construction for my clients, who are other
engineers, owner-builders, and contractors. Therefore, it is of great
importance to me to get this straight in my mind: the intent behind the
reliability/ redundancy factor (rho), the 10/lw factor for shear walls (ri)
(BTW, just what is defined as a shear wall?? Does this apply regardless of
construction material?), the lower and upper bound on rho, and that strange
omega factor.

Once I understand the intent behind all this, then I can talk about this
intelligently as an engineer. As an engineer, I sense that I would be able
to decipher the code's logic if I understood its intent. Presently, I am
faced with compliance with a code that I do not understand, plan-checkers
who don't understand it either, and no sufficient commentary available (as
far as I know) to explain it. I have expended an exorbitant amount of time
trying to decipher the code's intent - particularly with this new seismic
section, so much so that I cannot justify passing on the expense of this
time spent onto my clients - I am sure that I am not alone on that score.

I think this forum is an excellent avenue by which we - as engineers - can
help each other be engineers, rather than just rats in a cage, blindly
following the prescribed code (in saying this, I am readily implying that I
am blindly following the code when I don't fully grasp its intent).

Stephen, if you are recommending that I comply with the Blue Book, then
please try to explain the intent behind such limitations.

- Richard Flower, P. E.
Los Angeles, CA

-----Original Message-----
From: Dennis S. Wish <dennis.wish(--nospam--at)>
To: seaint(--nospam--at) <seaint(--nospam--at)>
Date: Friday, May 05, 2000 10:07 PM
Subject: RE: 10/lw factor

>Stephen (spelled the same as my middle name:o)
>I think you are on safe ground if you limit 10/lw to 1.0 maximum for wood
>structures with plywood diaphragms. This is almost a non-issue based on all
>of the published comments regarding the intention of the policy makers -
>especially those of Dick Phillips who came up with the "Arbitrary" 10/lw
>However, Dr. Goshen (spelling?) is highly respected in the profession and
>his latest comments have all but guaranteed that there will be no limit for
>masonry and concrete buildings where the diaphragm is known to act rigid.
>problem with Ron Hamburgers summary of Dr. Goshen's comments was that the
>professional community expects to use the unrestricted limits to prevent
>abuse within the community. This is the same as limiting an engineers
>professional judgment when we, as a professional community, have been
>extolling the need for competent engineers to rely heavily upon their
>understanding and rationalization for mechanics and the subsequent
>performance of structures.
>As an example, the code restricts the aspect ratio of a shearwall to 2:1.
>Does this mean that building official is obligated not to accept any
>where the wall ratio exceeds this. Or, does it simply require the engineer
>to prove by additional analysis that a narrower wall can be designed (as
>would a proprietary shear wall) with appropriate stiffness despite the lack
>of compliance to code?
>Most of the engineers I spoke to about this will still design walls that
>greater than 2:1, however, they will study the deflection AND look closer
>the wall stiffness relationship to adjacent walls.
>Where are we going with this code? It's one thing to educate engineers to
>aware of the issues regarding performance, stiffness, deflection etc.. It's
>quite another to establish new restrictions that will prevent an engineer
>from using his experience and knowledge!
>Finally, to complicate matters; the issues of liability are completely
>unrelated to interpretation of the code in this case. Inasmuch as there is
>no expectation of tort reform and as long as engineers still debate
>ambiguities written in the code (leaving issues to be interpretive) there
>will always be a market for lawyers who know they can profit from the
>insurance industries unprotected pot of gold as it is more expensive to
>litigate than settle out of court. The state of law provides no protections
>against frivolous lawsuits and our policy makers (SEAOC included) have done
>nothing, in my opinion, to take the issues of liability serious enough to
>work to protect their professional brethren.
>Dennis S. Wish, PE
>-----Original Message-----
>From: Stephen Fisher's 400MHz [mailto:slf(--nospam--at)]
>Sent: Friday, May 05, 2000 10:28 AM
>To: seaint(--nospam--at)
>Subject: 10/lw factor
>The 97 UBC does not limit the 10/lw factor to a maximum value of 1.0 but
>1999 SEAOC blue book on page 8 does limit it to a maximum value of 1.0.  I
>have read on this list that the current version of the IBC will limit 10/lw
>for wood shear walls to a maximum of 1.0 but will not say anything
>concrete or masonry shearwalls.
>It seems to me that the Blue Book should be enough to base our structural
>designs on and therefore limit the 10/lw to 1.0 even though the 97 UBC does
>not make the same statement.
> Am I going out on a legal limb?   What do you think?
> Stephen Fisher,SE