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RE: SEAOC Seismic Design Manual Seminar - Vol. 1

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Bill,

I am glad you were good, we had over 200 at this seminar, and many of us
were up and made the drive to put on the seminar.

Williston "Bill" L. Warren, IV - S.E.
Newport Beach, CA


From: "Bill Allen" <Bill(--nospam--at)AllenDesigns.com>
To: <seaint(--nospam--at)seaint.org>
Subject: RE: SEAOC Seismic Design Manual Seminar - Vol. 1

Amazingly, I DID decide to get up on Saturday morning, make that 1 hour
drive into LA (1-1/2 hrs home) to attend the seminar. All in all, it was
worth the time, money and lost sleep. The three presenters, John Shipp,
Henry Huang and Richard Phillips did an excellent job on presenting a
difficult topic to a very large (much larger than 150) audience. The
audience was fairly well behaved (including myself amazingly enough) with
not a single tomato thrown at the podium. There are a few areas,
particularly relevant to my practice that I would like to point out here.

1. Designing a masonry or tilt wall in accordance with section 1632.2
I had previously posted some questions on this list and a thread ensued
regarding the design of mass walls to comply with this code section. Some
engineers, including myself, interpreted the section to require a varying
distribution of load based on the minimum (0.7*Ca*Ip*Wp) to a height of
0.367*hr then increasing to ap*Ca*Ip*(4)*Wp/Rp at the roof level. Others
suggested that the wall force be a uniform force based on the average of
the
two values. Based on the seminar and design examples, the latter method is
the one that was used. For the life of me, I cannot find where it says in
the code explicitly that the designer is to average the forces between
levels. I specifically asked John Shipp if this method was in the Code and
he said that it was. If someone can point this out to me, I would be most
appreciative. Otherwise, I have to assume that this is merely an engineer's
interpretation of the code which is much more different than an explicit
directive.

2. Rho factor for shear wall structures.
Well folks, I got it from the horse's mouth. Dick Phillips, who wrote this
section of the Code, indicated both from the podium and to me personally at
the break, that the intent of the 10/lw term was to "help" long shear walls
but not to penalize short shear walls. I pointed out to him (which he
already knew) that, currently, the r value is merely the shear stress in
the
wall multiplied by 10/lw. This of course is NO indication of redundancy and
will no doubt require a rho value greater than 1 on some structures where
this was clearly not the intent of the author.

3. R penalty on the whole structure
In the classic case of having a pair of cantilevered steel columns at a
garage opening (or elsewhere in a wood structure otherwise laterally
supported by wood shear walls), currently the code reads that the R value
of
2.2 be used for the design of ALL elements at that level in that direction
including diaphragms, shear walls, etc. Of course, the consensus of the
engineers I have heard believe this is an unrealistic provision and that
this issue should be addressed.

During the seminar, Dick Phillips asked John Shipp to look into items (2)
and (3) to see if SEAOC could develop some sort of document that
would/could
clarify these issues. As most of you are aware, there will be no 1998 or
1999 addenda to the 1997 UBC (unlike in the past), so there are no
opportunities to amend the building code. It would be unfair to be (too)
critical of the various code committees since the effort consists of
entirely voluntary effort by engineers doing this work on their own time.
However, there should be avenues to "make things right" and practical when
deficiencies become apparent as the code is put into use. If SEAOC would
present a position paper or a white paper and the design engineer would
confer with a building official in the jurisdiction responsible for their
project, a determination could be made whether the conclusions presented in
the position paper could be applied on a CASE BY CASE BASIS. This position
paper could also be used as a defense document in a court of law.

4. Rigid Diaphragm Analysis of Wood Framed Buildings (particularly
residential structures).
Sorry, folks no such clarification at this seminar. But there needs to be
something done here. Do we really need an elephant gun to go squirrel
hunting?

I have a particular opinion on the lateral design of wood framed
structures.
I believe that if one would:

a.) select a lateral force coefficient somewhere between 0.15g and 0.20g
(visualize my hand reaching up into the air as if to grab a pesky knat).
Then,

b.) distribute the load vertically is some rational manner (triangular,
rectangular, it doesn't really matter). Then,

c.) distribute the forces horizontally in some rational manner (flexible,
rigid, it really doesn't matter). Then,

d.) provide a positively connected load path from roof to foundation. Then,

e.) go to the jobsite (design engineer, not his proxy) to observe that the
project is built according to the intent of the structural design. Then,

I believe you will have a well performing structures IN MOST CASES.

Hopefully, someone acting as Frank Lew's proxy at the August 13th
Seismology
Committee meeting will stand up and shout "Where are the bodies"? We really
need some common sense applied to the horizontal distribution of forces for
wood framed structures.

5. Limitation of Na=1.1 in accordance with 1629.4.2.5 will never happen for
anything but the simplest of wood framed structures, right?

Regards,

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