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Re: SEAOC seismology opinion regarding 10/Lw factor for calculat

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Martin W. Johnson wrote:
> During the Sept 29 seismology meeting, the committee voted to submit
> a code change proposal for the 2001 IBC Supplement No. 1, with the
> following language:
> "In light-frame construction, the value of the ratio of 10/Lw need
> not be taken greater than 1.0."

Sadly this recommended code change by the SEAOC Seismology Committee 
is fundamentally flawed.

The thought behind the 10/Lw term was to allow long walls to count as 
multiple elements (each 10 ft wall segment = one element), not to 
count short walls as less than one element.  The appropriate 
expression of this counting function has nothing to do with the 
material used in the construction of the shear wall.

Adding text such that 10/Lw need not be taken greater than 1.0 
satisfies the original intent.  However the committee chose to 
qualify an otherwise appropriate revision with "in light-frame 
construction".  Hopefully someone on the ICC hearings (a SEAOC 
representative would be most appropriate) will repair this error.

As further evidence on this issue, I append a contribution to this 
listservice made by the current president of SEAOC.

-Mike Valley

------- Forwarded Message Follows -------
From:          "Ron O. Hamburger" <ROH(--nospam--at)>
To:            seismo-all(--nospam--at)
Cc:            seaint(--nospam--at)
Date:          Fri, 30 Jul 1999 14:45:57 -0700
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

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

Please consider this matter, at your next Seismology Committee 

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