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RE: Redundancy Factor

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At the seminar last week in Alhambra, CA, the author of this particular
piece of code, Dick Phillips, clarified this issue. He said the intent of
the 10/lw was to "help" long shear walls but not to penalize short ones. So,
if you had 3-8' shear walls N=3. If you had one 30' shear wall, N=3. Sooo,
how to apply this "10/lw" in accordance with the author's intent is still
unclear but not nearly as conservative has you have interpreted it.

I believe a clarification is due to come out either as a result of the
Seismology committee meeting (going on as we speak) or as a result of a
request by Dick Phillips upon John Shipp to produce a document clarifying
this issue as well as the application of a low R factor on the whole
structure just because you had some cantilevered columns at the garage

Stay tuned.

Bill Allen, S.E.
Laguna Niguel, CA

> -----Original Message-----
> From: chuckuc [mailto:chuckuc(--nospam--at)]
> Sent: Saturday, August 14, 1999 10:51 AM
> To: seaint(--nospam--at)
> Subject: Re: Redundancy Factor
> Ron-
>     I've been looking at the ICBO vol. 1 Seismic Design
> Manual regarding the
> redundancy issue for shearwalls.  As I read the UBC, it says
> to determine what
> percentage of  the story shear is carried by each wall and
> then apply the
> 10'/wall length adjustment factor.  The example on page 50
> does just that (but
> since their example walls are all 10' or more there was no penalty).
>     However, on page 54 there is a different calculation
> which looks completely
> wrong.  On page 54, rho supposedly is related to the number
> of 10' walls, which
> is senseless.  In my design work, a 9.5' long wall is a
> handsome shearwall but
> using the "method" on page 54, N would be zero and rho
> becomes 2. That says add
> 6" to my walls and halve the design load--dumb idea.
>     It looks to me like the "wordsmithing" in the UBC got it
> right, although
> the standard wall should probably be 8' not 10' since 99% of
> our test data is
> on 8' walls.
>     Also, the load distribution examples show the "flexible" diaphragm
> distribution.  For the diaphragms dimensions shown, rigid
> distribution is more
> likely.  Using narrow shearwalls and flexible diaphragms I
> expect rho would get
> pretty high.
> Chuck Utzman
> "Ron O. Hamburger" wrote:
> > A recent thread of discussion on the list server has called
> my attention to
> > what I believe is an uintentional and also unfortunate
> problem with this
> > factor in the 97 UBC.
> >
> > When the committee first developed this factor, the intent
> was that the
> > rmax represent the % of the story shear carried by the most
> heavily loaded
> > element.  We then proceeded to define what an "element" is.
>  For example,
> > each brace is an element, etc.  When we got to shear wall
> structures, the
> > intent was that each individual wall pier across a
> horizontal plane cut
> > through the building would be an "element".  Then, someone
> on the committee
> > noted that if you had a 100' x 100' tltup type structure,
> with a number of
> > 20' wide panels, this would be considered to have high
> redudance (because
> > each 20' panel would be an element) however, if you had the
> same structure
> > with cast-in-place walls, then it would be non-redundant,
> as the whole side
> > of the strucure would be only one element.  In order to
> solve this problem,
> > for shear walls, we introduced the rule that when a shear
> wall exceeded 10'
> > in length, each 10; segement (or part thereof) could be
> considered an
> > element.  The intent was as follows  - If you have a wall
> line with 10 - 4'
> > piers between windows, each pier would be an element.  If
> you had a wall
> > line with a 40' wall, you would have 4 elements.
> >
> > Somehow, in the word smithing that went into the actual
> code language, this
> > logic got badly messed up.  Now each wall segement is
> multiplied by 10/lw.
> > This has the desired effect for long walls, but has a
> penalty effect for
> > short wall segments.  This was, in my opinion, never intended.
> > This has some serious negative impacts on wood frame construction.
> >
> > Please consider this matter, at your next Seismology
> Committee meeting.
> >