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Good response.  For the below situation, for example, if you had say a metal deck horizontal diaphragm at the top of the shear wall that had only a capacity of 900 lb then the most "the system" can deliver to the shear wall is 900 lb which is less than your design load of 1000 lbs.

Thomas Hunt, S.E.

"Terangue Gillham" <tiger(--nospam--at)>
05/16/2006 08:22 PM
Please respond to seaint

I think that the statement you are referring to simply allows you to account for mechanism formation, resulting in some form of yielding, that would limit the amount of shear that could manifest in your shearwall IF said shear were LESS than that required by equation 1612.4.  In other words, no need to design for 1612.4 if mechanism formation will limit the design forces to something less than what 1612.4 requires.  That said, the calculation of the maximum shear due to mechanism formation would need to include material overstrength and other factors that will almost certainly push UP the ultimate capacity of the yielding member, and hence increase the maximum possible shear.
In this case, the 1k of shear calculated based on applicable load combinations has little meaning - the actual overstrength capacity of the members transferring the shear into the shear wall, as they will be built in the field, is the controlling factor since that determines your maximum shear during the design earthquake (or competing mechanisms such as flexural yielding of the wall itself).
As far as discussions on this, I seem to recall that the Blue Book does discuss these issues in detail, but I don't have a copy with me so I can't say for certain.
Hope this helps.

Terangue *Tiger* Gillham, PE
GK2, Inc.
PO Box 8061, Koror, Palau 96940
Phone: (680) 488-7282 office
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tiger(--nospam--at) <

-----Original Message-----
S. Gordin [mailto:mailbox(--nospam--at)]
Wednesday, May 17, 2006 10:19 AM

UBC/CBC Section 1630.8.2, IBC Section 1617.1.1.2 similar.
"Em need not exceed the maximum force that can be transferred to the element by the lateral-force-resisting system."
Suppose I have a shear wall that collects 1000 lbs of shear, ASD, as calcualted based upon the applicable (not special, UBC 1612.4) load combinations.  What - if any - limitation it imposes on the above Em calculation?
Where can I find a sensible discussion on the subject (Seismic Design Manual is as vague as the codes, and the Blue Book does not even mention the issue).
Steve Gordin SE
Irvine CA


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