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RE: News from the IBC front

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Bill:

I think you did have it figured out.  It appears that the code
bureaucracy has screwed up the redundancy factor beyond belief.  In
addition to your conversation with Dick Phillips, consider the email
message from Ron Hamburger to this list.  (I've inserted it after my
comments.)

Although Ron notes that the 10/lw factor "has some serious negative
impacts on wood frame construction", that observation follows a long
discussion of the fact that the original code writers NEVER intended
that 10/lw be taken greater than 1 FOR ANY TYPE OF CONSTRUCTION.  The
SEAOC proposal to change the IBC erroneously applied only to
"light-frame construction".  SEAW attempted to fix the SEAOC proposal
on the floor at the code hearings.  The proposed modification was not
accepted; as a result, the 10/lw problem was only fixed for
light-frame construction.  Evidently SEAOC can't (collectively) figure
out and agree upon what 10/lw means.  Because of that, it hasn't
really been fixed.

A related problem was also NOT fixed at the IBC hearings.  There was a
need to clarify that the redundancy factor (which is related to the
performance of the entire system) should not apply to prescriptive
forces that are not related to the performance of the entire system.
The most obvious example is the prescriptive diaphragm forces.  As
discussed in the "Blue Book", the redundancy factor should only apply
to the diaphragm forces that are related to transfers of forces at
offsets in the vertical elements of the lateral-force-resisting
system.  In other words, diaphragms should only be penalized for lack
of system redundancy when they are part of the global load path (and
not when they are simply delivering local loads that are unrelated to
system-wide redundancy).  The SEAOC proposal to fix this problem was
rejected.

In my opinion, both of these issues reflect a growing problem.  Many
of those involved in today's code writing and adoption process have
either forgotten or never known the reasons behind the code
provisions.  Rather than read code commentaries (the Blue Book, NEHRP
Commentary, and others) or ask those who were involved, they close
their eyes and dream up (flawed) rationale for the current code
provisions.  More often than not, that rationale reflects their
personal/professional bias rather than the original intent or real
data and the code becomes a confusing political (not engineering)
document.

-Mike

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Michael Valley, P.E., S.E.                   E-mail: mtv(--nospam--at)skilling.com
Skilling Ward Magnusson Barkshire Inc.              Tel:(206)292-1200
1301 Fifth Ave, #3200,  Seattle  WA 98101-2699      Fax:        -1201


-----Old Message on this topic-----
From: Ron O. Hamburger [mailto:ROH(--nospam--at)eqe.com]
Sent: Friday, July 30, 1999 2:46 PM
To: seismo-all(--nospam--at)seaint.org
Cc: seaint(--nospam--at)seaint.org
Subject: Redundancy Factor

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.

> -----Original Message-----
> From: Bill Allen [mailto:Bill(--nospam--at)AllenDesigns.com]
> Sent: Friday, April 21, 2000 6:51 AM
> To: seaint(--nospam--at)seaint.org
> Subject: RE: News from the IBC front
>
>
> Whoa! Forget IBC 2003 for a minute and let's talk about
> something a little
> closer to my reach in my bookshelf, the 1997 UBC.
>
> Am I missing something here? According to a conversation I
> had with Dick
> Phillips at one of the SEAOSC seminars in the City of
> Commerce, it was NEVER
> his intention for the 10/lw provision to be a penalty on
> short shear walls
> but to be an aid on long shear walls.
>
> So, with regards to the 1997 UBC, is it the intent of the
> code to use a
> value of 10/lw greater than 1? If so, you are going to end
> up with a rho
> greater than one on wood framed structures with multiple
> shear walls (albeit
> short ones) which is rediculous and is contrary to the intent as I
> understand it of the redundancy issue. This is because
> r-sub-max will be
> based on a maximum value of f-sub-v and NOT the shear load
> in an individual
> shear wall divided by the shear force at a particular level.
>
> According to Dick Phillips, the purpose of the 10/lw feature was to
> eliminate the penalty of long shear walls. Visualize a
> shopping center with
> the rear wall being long and solid. The intent of this
> feature was to
> consider this shear wall as individual walls 10 feet long.
>
> Wasn't SEAOC going to write a white paper clarifying this issue?
>
> Are building departments enforcing this issue? I haven't
> had any problem
> with this issue in the projects I've submitted, but that
> really doesn't mean
> anything.
>
> Sheesh! I thought I had this damn thing figured out.
>
> Regards,
>
> Bill Allen, S.E. (CA #2607)
> ALLEN DESIGNS
> Laguna Niguel, CA
> http://www.AllenDesigns.com