Just thought some of you on this list serve might benefit from reading the
following post from Dennis Wish. Thanks, Dennis, for your help!
-Richard Flower, P. E.
From: Dennis S. Wish <dennis.wish(--nospam--at)gte.net>
To: seaint(--nospam--at)seaint.org <seaint(--nospam--at)seaint.org>
Date: Sunday, May 14, 2000 1:16 PM
Subject: RE: 10/lw factor
>Richard, I'll try to keep this simple as I, like you, are not theoretical
>nature and have tried to understand the intention of these formula's as
>as their function to performance.
>10/lw was an arbitrary ratio which came from the mind of Dick Phillips - a
>member of the Seismology Committee. As I am able to ascertain, the ratio
>considered for masonry and concrete buildings in an effort prevent tall
>narrow panels that will become too flexible. If this happens, then the
>are penalized to increase the load applied to the wall so that it can be
>designed for greater stiffness (less deflection) or a compromise can be
>to substitute a longer wall.
>As Dick Phillips pointed out at the last ASD/LFRD seminar in City of
>Industry, the ratio was chosen arbitrarily and there was no discussion as
>its application to wood structures with wood diaphragms. The adoption of
>code last July simply assumed that ALL MATERIALS WILL COMPLY WITH THE SAME
>METHODOLOGY without consideration to practicality.
>Consternation within the profession forced SEAOC to evaluate the decision
>use 10/lw in wood framed residences. Only after the code became enforceable
>was it discovered that most residential homes do not have an abundance of
>10'-0" long shearwalls. Therefore, not only were wood framed homes to
>with much more restrictive code methods and higher forces, but they were to
>be penalized for using shearwalls that meet the revised 2:1 aspect ratio
>were less than 10' in length.
>SEAOC decided to create an exception to the rule. In 1997, the exception
>simply stated that for wood framed structures with wood diaphragms 10/lw
>not expected to exceed unity or 1.0. Walls shorter than 10' would default
>a 10/lw of 1.0.
>When discussions started about other types of structures (masonry and
>concrete) Professor Goshin from Illinois (and leading ICBO lecturer) was
>adamantly against changing the definition for 10/lw for any structure other
>than wood (I do not know is position on wood). Therefore, the next code
>cycle will only issue an exception for residential or wood framed
>with wood diaphragm.
>The frustration with the current code stems from prematurely codifying a
>theory without first testing its application and its practicality to
>types of buildings that are economically sensitive to change. As the
>compliance requirements for non-conventional structures (those with
>irregularities) becomes more restrictive the gap between conventional
>construction (prescriptive methods) grows and becomes an economically
>significant issue to developers and builders of spec homes. This creates
>incentives for independent developers (those who build ten or less homes
>year) to eliminate minor irregularities so as to comply with prescriptive
>methods. Furthermore, the code grants more latitude to those who wish to
>prescriptive methods but have existing irregularities in the design. The
>code allows only the non-conforming irregularity to be addressed by an
>engineer while the rest of the structure complies with prescriptive
>Therefore a code that was intended to improve the performance of
>structures creates an incentive to produce more homes by prescriptive
>methods which have historically been designed by engineers. This is
>occurring in my city as more than 2000 homes have been built - the majority
>by prescriptive methods with the few exceptions for higher end homes.
>You are not alone in the quest for answers. So far, we have been promised
>Martin Johnson and Ron Hamburger that given the time for a special
>sub-committee over viewing light framed structures, issues that were left
>unresolved would be addressed. As we close the first year since the
>of the code we still have no established standard for the following issues:
>1. What constitutes a rigid diaphragm and how do we treat non-rectangular
>structures where proportionally large diaphragms are connected by
>2. The current ICBO Design Manual Volume II uses an Envelope solution but
>clear to point out that this is the authors choice and is not a
>recommendation. Why not? How do we compete with engineers that interpret
>code as indicating a non-flexible diaphragm is rigid and therefore only a
>rigid analysis is required?
>3. How do we address skewed shear walls (there is no standard of agreement
>within the professional community and the two commercially available
>software's ignore skewed walls).
>Rather than go on I suggest you check the archives for other unanswered
>questions that are left to resolve. Just remember, you are not alone.
>Dennis S. Wish PE