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FW: Discontinuous shear wall load path

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Sharad:

I don't wood in practice, but I can offer some general seismic comments
regarding your post:

The critical aspect of a discontinuous wall system, as I see it, is
providing a complete load path that will allow the transfer of loads to the
foundation DURING THE ACTUAL EARTHQUAKE, as opposed to under reduced design
forces.

Understanding that the actual eartquake will impose forces exceeding those
used for design, one must use magnified forces OR forces that are a function
of the capacity of the elements that are going to "yield", which you are
already doing.

So, wrt the "elements" that must be deigned for the larger forces, I would
say that ANY elements that form the path for the forces to travel from the
discontinuous wall to the foundation should be considered as elements to be
designed under the larger loads.  That is, of course, unless you want one of
those elements to yield instead of the second level wall, in which case that
becomes the yielding element and the other elements are treated accordingly.

The load path would include a horizontal path (diaphragm) as well as
vertical path.

As a side note, from you description of the situation, it sounds like the
vertical element supporting the upper level wall is offset and a beam is
transferring the vertical wall reaction over.  If this is the case, I would
check the stiffness of the system, since it sounds like it may be quite
limber (depending on the stiffness of the transfer beam).  This would become
an issue, in my mind, if there were competing lateral resistance systems
(intended or not) that may take more load than the disc wall system due to a
higher relative stiffness.


Hope this helps.

T. Eric Gillham PE


-----Original Message-----
From: Sharad T. Patel [mailto:stpatel(--nospam--at)vcaengineers.net]
Sent: Friday, May 25, 2001 7:02 AM
To: seaint(--nospam--at)seaint.org
Subject: Discontinuous shear wall load path


In accordance with UBC 1630.8.2, "elements" supporting discontinuous systems
shall have the design strength to resist the combination loads resulting
from the special seismic load combinations of Section 1612.4, with the
exception that Em need not exceed the maximum force that can be transferred
to the element by the lateral force resisting system.

Now, assuming that the "maximum force that can be transferred to the system"
is adequately determined, my question is:  What constitutes "elements"?
Specifically:

Assume that in a wood framed structure, there is an interior second floor
shear wall which occurs over floor joists that are perpendicular to the
wall.  There is no stacking wall below and the nearest shear wall is several
feet away (load is transferred via the diaphragm to adjacent walls).  Also
assume that there are overturning loads and resisting devices at each end of
the wall.  So...which element(s) need to be designed for the special load
combinations?

Here is a list of the elements:  Hold down device and post, beam supporting
hold down device, beam connection to adjacent wall, post and footing
supporting this beam, floor diaphragm (magnify shear load to check strength
level of diaphragm?), and finally...the adjacent shear walls (since,
technically, they are the "supports" for the lateral load from the above,
offset, shear wall?


Sharad T. Patel, S.E.
VCA Engineers
295 N. Rampart Street, Suite A
Orange, CA 92868
Phone:  (714) 978 - 9780 ext. 120
Fax:  (714) 978 - 9926


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