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Re: seaint Digest for 14 May 2000

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This is in response to a posting by Dennis S. Wish
RE: 10/lw factor

Dennis said:

>10/lw was an arbitrary ratio which came from the mind of Dick Phillips - a
>member of the Seismology Committee. As I am able to ascertain, the ratio was
>considered for masonry and concrete buildings in an effort prevent tall
>narrow panels that will become too flexible. If this happens, then the walls
>are penalized to increase the load applied to the wall so that it can be
>designed for greater stiffness (less deflection) or a compromise can be made
>to substitute a longer wall.

None of the above is correct.  First, it was not invented by Dick Phillips, but
rather by a subcommitee of the Seismology Committe that Dick chaired.  Other
members were Chia Ming Uang of U.C.S.D., Nabih Youssef, Bob Thacker, Ted Zsutty,
Ajit Virdee, Gary Hart and myself.  The 10/lw factor was "invented" so that
designs with a single long wall were not penalized in the calculation of
building redundancy, the theory being that since shear travels in approximately
45 degree planes, a long wall should be considered to be equally redundant to a
series of short walls, each with an asepct ratio (height to length ratio) of
1.0.  Since building have story heights of approximately 10 feet, the magic
10/lw was invented.  It had nothing whatsoever to do with penalizing short
walls, though Dr. Gosh and several later members of the Seismology committee
decided to use it for that purpose.

>As Dick Phillips pointed out at the last ASD/LFRD seminar in City of
>Industry, the ratio was chosen arbitrarily and there was no discussion as to
>its application to wood structures with wood diaphragms.

This is correct - the committee was thinking of masonry and concrete buildings,
not wood buildings. However, it didn't really matter.  The intent was never to
have 10/lw when calculated as less than 1, act as a penalty.  Unfortunately, in
a recent attempt to change the code to say that, Dr. Gosh, and others who did
not understand what the 10/lw was really about, got in the way of this and the
change was made only for wood structures.  Too bad!!

>When discussions started about other types of structures (masonry and
>concrete) Professor Goshin from Illinois (and leading ICBO lecturer) was
>adamantly against changing the definition for 10/lw for any structure other
>than wood (I do not know is position on wood). Therefore, the next code
>cycle will only issue an exception for residential or wood framed structures
>with wood diaphragm.

Dennis if you are going to use the gentleman as an authority, please use his
name correctly.  It is S. K. Gosh.

>You are not alone in the quest for answers. So far, we have been promised by
>Martin Johnson and Ron Hamburger that given the time for a special
>sub-committee over viewing light framed structures, issues that were left
>unresolved would be addressed. As we close the first year since the adoption
>of the code we still have no established standard for the following issues:

>1. What constitutes a rigid diaphragm and how do we treat non-rectangular
>structures where proportionally large diaphragms are connected by relatively
>short connections?

I can't speak to the non-rectilinear layout, but in February, Martin Johnson
made a post to the listserver, that indicated the Seismology Committee's revised
position on this was available to review on the Seismology Committee page of the
SEAOC web site.  The Seismic Design manual Vol. II was revised to reflect this
new position - the major reason why publication was delayed as long as it was.

I can't respond with regard to your concerns on skewed walls.  Perhaps we need
some more research on that one.