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Rho, Rho, Rho your code:o)

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Is that an attention getter or not?

A couple of days ago, Martin Johnson was kind enough to respond to my
comments about Gary Searer's paper on the problems with the Redundancy and
Reliability Factor Rho. Statistical studies were presented that indicated
that Rho should not necessarily be written out of the code and I responded
defensively, siting that statistical studies can often be biased as my be
considered the case of the studies prepared for the NAHB-RC and HUD on the
performance of wood framed homes subsequent to the Northridge earthquakes.
In the studies prepared by an independent firm on randomly choose homes,
indicated that the homes in the San Fernando Valley actually performed very
well. Please understand that I am doing this study an injustice by trying to
summarize the work in one or two sentences.

I read the statistical studies and with my limited knowledge, was able to
identify some areas that concerned me as to how the homes in the study were
chosen and the fact that the study did not specifically seek out and
evaluate damaged homes. I presented some of my questions to Jay Crandell, PE
of the NAHB-RC who spent a great deal of time by e-mail to provide me with
other information to justify the accuracy of such studies.

I would consider Jay to be an expert and very well qualified in the field of
statistical analysis. However, the bottom line was that I did not feel
comfortable accepting the study as an appropriate method to draw such a
broad conclusion. The end results was that home performed as expected based
upon the life safety provisions in the code. The $20-billion in damages was,
in Jay's opinion, due to over-inflated claims, inflated-cost of repair
during a particular supply and demand market - more so the abuses within the
system than an actual representation of the normal cost of repair.
Furthermore, inasmuch as lives were protected as the code requires, Jay's
opinion is that the expectation of the homeowner as to the performance of
the houses is raised to too high a level.

Now, before I get reprimanded from Jay by taking him out of context, my
point is that there is a diverse difference of interpretation to statistical
studies and what you attempt to seek out that plays on the results. I have
taken Jay out of context and while I don't want to be unfair, I am relaying
our discussion from memory. What I do remember most is leaving the
discussion thinking that Jay may have had a better handle on reality than
most engineers. However, a few days later, the continuous thought of $20
billion dollars in damages, the knowledge of what I experienced dealing with
these homeowners, the abuses from both the insurance industry as well as
homeowners was enough to make me revert back to my original thought that the
dollar value was probably closer to a realistic figure offset by abuses on
both side (insurance not willing to pay full claims, and paying unnecessary
claims for less than reasonable damage) and that this figure was too much
for the Insurance industry and homeowners to expect.

Now we have a statistical study that suggest that we should not throw away
the Rho factor and in other than residential structures, I might agree. I
originally heard the argument as presented by Prof. S.K. Ghosh who, as I
interpreted the e-mail on this list, believed that Rho was a factor to be
used to insure that engineers are designing properly. In other words, as a
forced factor of safety. I apologize if this was not the intention, it is
merely my interpretation of what I read - most of which was second hand.

I don't think that a professional community needs the code to include a
"fudge" factor to protect us from our own error or incompetence. I do,
however, believe that this is a consideration best left to the judgment of
the engineer of record based upon his experience. We certainly don't need
Rho to increase the base shear as the code only provides us with a minimum
value based on the definitions within the code.

I know of few residences other than possibly lofts, which are designed with
few or no interior partitions that may be lacking in redundant systems. The
majority of residential homes, and multi-residential structures have
sufficient interior partitions to more than meet the requirements as a
redundant system - the intent of the code.

While I have the attention of at least Martin, I'm sure that many engineers
feel the same - they disagree with the conclusion that was suggested from
the statistical study mentioned and would rather see Rho removed from the
code in application to residential structures of wood framing with wood

Dennis S. Wish, PE

Dennis S. Wish, PE
The Structuralist Administrator for:
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