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Residential Design Discussions

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There are issues that have not been resolved in residential design. I've
been off this list since the end of June and am returning in hopes that
I can resolve some of these conflicts.

1. The 97 UBC provisions allow us to design a light framed wood
structure as flexible using the Simplified Static Design provisions.
However, the code still requires that the designer calculate the
deflection of the wood diaphragm (blocked) and convert the deflection to
an unblocked diaphragm by multiplying the results by 2.5 (as determined
by the APA).  However, unless the diaphragm has a cut-out or comes close
to a 4 to 1 aspect ratio, the diaphragm will calculate as much too stiff
to prove flexible. In this case, the provisions of the Simplified Static
Design can not be used.

2. I've read the alternative methods from Los Angeles City and the
Tri-County version written by Ben Youseffi a couple of years ago. As I
recall, the provisions result in a design that is not less than the
minimum code requirements. If I recall correctly, the designer still
must show that the diaphragm calculates as flexible and again, you don't
generally have that condition without a long narrow diaphragm or one
with cutouts.

3. The issue of Rho may be covered in the opinions stated by the SEAOC
Seismology Committee but unless the code is revised (which it will not
be) the base shear will need to be increased by a Rho of 1.5. Most of
the professionals I have spoken to disregard the Rho factor because of
the work done by Gary Searer that discredits the interpretation of Rho.
Since it is codified, the opinions of those who created the code are of
no value in a court of law. The engineer of record is still responsible.

4. We have started a discussion forum on the Residential Listservice of
the Structuralist.Net at
The purpose of this is to focus on what other engineers who design
low-rise light-framed wood structures have accepted as practical methods
to resolve these issues. I invite anyone to subscribe to this list who
is interested in residential construction. This is a low volume
list(although I often get wordy) but I think it is important as I have
been doing some contract plan checking and find that almost none of the
plans I have received are design in full-compliance to the current code.

Specifically, I would like to know what other professionals in different
states or countries are doing in the design of residential structures. I
believe that the restrictions created by the current code has been
counterproductive to the intent of the code to improve construction
quality. The cost of compliant design has created an incentive for
smaller developers and designers to avoid engineering by reducing the
irregularities in the structure and having it conform to prescriptive
Conventional Construction provisions. 

Again, I invite you to join us in the residential list to evaluate how
others are designing homes.

Dennis S. Wish, PE

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