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RE: Wood Fr-Upgr : What I'll do and two more questions

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Thanks Bob, but I think there may be a flaw to the "ABK" method here. The
ABK method assumes that you design to the element that will fail first. In
this case, if the roof were plywood unblocked you would be designing for
about 240 psf for the full length of the wall. The stucco finish (and
applicable shearwalls) are the weak link here - yet these are the walls that
will be replaced with the new shear from the addition.

Maybe I'm confused here, but lets assume that the original structure had 10
feet of stucco shear (braced wall panel) even though the roof is
considerably stronger. This translates into about 1800 pounds of shear. If
the new addition contributes 1000 pounds of shear do I design the common
wall for 2800 pounds, or do I design for the diaphram capacity over the full
length of wall (20 some feet total)?

If the assumption is to design for 2800 pounds of shear then this is what I
originally assumed in my question.

Bob, would you mind clearing this up - I think I'm confusing myself:>)


-----Original Message-----
From: Robert J Bossi, PE [mailto:rjbossi(--nospam--at)]
Sent: Thursday, October 29, 1998 9:10 AM
To: seaint(--nospam--at)
Subject: Re: Wood Fr-Upgr : What I'll do and two more questions

Dennis S. Wish PE wrote:

> One last question. Inasmuch as there is insufficient exterior shear in the
> walls, do I shear the common walls of the new/existing construction for
> the difference in shear caused by the new addition, or do I go back and
> to determine the shear from the old section also acting in this wall and
> combine them?


If you are adding load to an existing wall then that wall must meet current
(See UBC 3403.3, second paragraph) other wise you have an unsafe condition.
With regard to combining the existing and new loads, you might want to try
"ABK" like approach for the existing loads such that the demand is not the
calculated tributary load but rather the capacity of the existing diaphragm
deliver that load.  The new load from the addition should be full code and
shear wall(s) should meet current capacity and aspect ratio requirements.

Bob Bossi