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RE: '97 UBC Design - Are you too old to change your ways???

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If I had the opportunity to retire (or change professions for that matter),
I would have done it long before the 1997 UBC hit the streets.

Other than that, I vote for something in between. It is obvious to me (based
on how I have read the code and what I know of the background of some of the
code authors) that the seismic provisions of the 1997 UBC were not written
with wood framed, residential construction in mind. Best case, this class of
structure may have been merely accommodated. IMO, between July 1, 1998 and
at least until the 2001 IBC hits the streets, there will be
misinterpretations abound as well as a significant amount of, dare I say,
over-engineering.

It has not been demonstrated to me that the typical custom home located in
even the highest seismic risk area has been subjected to a significant
amount of damage as compared to, say, unreinforced masonry, wall ties of
tilt-wall buildings, soft story designs and open front designs.

Where I am going with this is that I believe there might be a need for an
intermediate code. Something more prescriptive than Conventional Framing
Provisions and less restrictive than the 1997 UBC. Maybe a One- and
Two-Family Dwelling Code. What a concept!

To me, the seismic design of these kinds of structures are relatively
straight forward. First of all, select a base shear coefficient expressed as
a percentage of gravity. Visualize that I am reaching up in the air as if to
grab a knat. Some number between 0.15 and 0.20 g (in zone 4). O.K., now do a
vertical redistribution of forces in some rational manner. Either inverted
triangular distribution or rectangular. Doesn't really matter that much.
O.K., now do a horizontal distribution of forces in some rational manner. It
doesn't really matter if it's flexible, rigid, or rigid-flexible. Just
something rational. Now, stand back and look at the structure. Make sure you
have a load path from the roof to the foundation. Now, when the design goes
into construction, make sure the design engineer (not a proxy) makes enough
jobsite visits to ensure that the design has been accomplished. Outside of
some special conditions (like addressing rotation, etc.), this is the bulk
of what we need as far as a seismic code for residential construction, in my
opinion. If you like this, you should see my solution for federal income tax
:o).

It may be true that the "young pups" might be more "software friendly", but
I doubt this is the case in the big picture. Some of us "old dogs" have
figured out how to turn on a computer. In addition, we have seen a lot of
structures built and I believe we have a better "feel" on what makes sense
rather than just possessing the ability to crunch numbers at blinding speed.

Regardless of the limitations of the "old dogs" and "young pups" it doesn't
make much sense to double the amount of engineering hours it takes to do a
lateral analysis of a house if there is no real benefit in doing so other
than being able to say that you have enveloped every conceivable possible
way a seismic load can pass through this shear wall. This is nuclear power
plant design mentality and is not rational in residential construction. If
the perception now is that rigid diaphragm analysis is the more appropriate
way of analyzing wood framed structures, this method should be adequate. Not
both. However, it would be hard to justify to me how a procedure practiced
in a wide geographic region over multiple decades can be so much at fault.
Look at Kobe. Look at Turkey. Look at Greece. Look at Taiwan. Look at Mexico
City. Look at Iran. O.K., now look at Northridge and Loma Prieta. What is
the difference? Certainly not flexible vs. rigid.

O.K., I'm done.

Bill Allen, S.E.
ALLEN DESIGNS
Laguna Niguel, CA

|| I would be interested in your opinions. Should some of us
|| old timers consider
|| throwing in the towel or should the towel be thrown over the
|| new methodology
|| until it can be justified by "bodies" of evidence supporting
|| damage resulting
|| from inadequate design proceedures?
||
|| Dennis Wish PE