Subject: Re: Code Created Malpractice Opportunity-Rigid v. Flexible Diaphragms
Date: Sun, 30 May 1999 14:28:47 EDT
The basic premises described by Mr. Shipp are accurate, however, some of the
specific issues in my opinion need further discussion.
First: Mr. Shipp wrote:
"Calculations for the stiffness of various wood framed
shear walls (plywood, drywall, stucco) are documented in the literature
and/or not that difficult to develop."
Structural engineering for earthquakes is not (as much as we would like it to
be) exact science. Our best learning tools are usually not the finite element
analyses and lab tests, but the performance during a real earthquake. Case in
point, the performance of moment frame connections during the Northridge
earthquake. By all standards of engineering science and the previous lab
tests, these connections should have performed as assumed in design. The weld
was supposed to emulate the base metal performance and the steel was assumed
to be a homogeneous material, with identical and predictable properties in
Wrong! Weld behavior depended substantially on weld metal and the workmanship
and steel didn't quite behave as a homogeneous material. My point of bringing
this up is that, with wood diaphragm construction the degree of uncertainty
multiplies compared to steel moment connection. There are many elements that
comprise the end product and workmanship plays a big role in how it is all
put together. So, relying on some equations that try to predict all these
features may not be a prudent thing to do.
Second: Mr. Shipp wrote
"A better use of an engineer's time will
be to focus on a design that will perform instead of looking for an escape
clause for flawed assumptions."
The real life experience of these diaphragms though, has not indicated, at
least to my knowledge, that the assumption of flexibility in light framed
wood construction is flawed. As far as we know, this assumption has not
contributed adversely to structural, or nonstructural performance of any
buildings during the recent earthquakes. So, the better use of an engineer's
time may be to spend the time on ensuring that he/she is designing a well
thought out structure as whole, rather than performing diaphragm rigidity
calculations (for these types of building) that may have little to do with
actual behavior of the lateral system.
So the question is: should we follow science for the sake of science or
should we think with the end result in mind, which is to strive for proper
behavior of the structure in real life situations. To put it another way,
should we follow the letter of the code or the intent of the code. I
personally rather do the latter.
Ben Yousefi, S.E.
San Jose, CA