From: "Dennis S. Wish" <dennis.wish(--nospam--at)gte.net>
Date: Sun, 14 May 2000 12:54:08 -0700
Richard, I'll try to keep this simple as I, like you, are not theoretical by
nature and have tried to understand the intention of these formula's as well
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 was
considered for masonry and concrete buildings in an effort prevent tall
narrow panels that will become too flexible. If this happens, then the walls
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 made
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 to
its application to wood structures with wood diaphragms. The adoption of the
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 to
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 comply
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 but
were less than 10' in length.
SEAOC decided to create an exception to the rule. In 1997, the exception was
simply stated that for wood framed structures with wood diaphragms 10/lw was
not expected to exceed unity or 1.0. Walls shorter than 10' would default to
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 structures
with wood diaphragm.
The frustration with the current code stems from prematurely codifying a
theory without first testing its application and its practicality to certain
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 per
year) to eliminate minor irregularities so as to comply with prescriptive
methods. Furthermore, the code grants more latitude to those who wish to use
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 residential
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 by
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 adoption
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 relatively
2. The current ICBO Design Manual Volume II uses an Envelope solution but is
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 the
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