I knew an engineer(SExxx)who did it that way back in the early 80's, but
then again, he would not even put in holdowns at all if the wall was not too
narrow and less than 300#/'. Then one day a plan checker called him on it.
You will not find anyone that will rationalize that ideology today. One way
around the large uplift values is to design a beam under and parallel to the
shearwall to couple out the holdown forces and lower the lowest level uplift
forces, may be more work than its worth. It's pretty hard to get stacked
wood framed three story shearwall holdowns to work unless you have double
shear concentric holdowns.
|~|From: Steve Hiner [mailto:shiner(--nospam--at)folsom.ca.us]
|~|Sent: Thursday, January 20, 2000 10:30 AM
|~|To: SEAINT (E-mail)
|~|Subject: Hold downs of stacked wood shear walls
|~|Here's one particularly for those involved in the structural
|~|plan review of single-family homes of wood frame construction (custom or
|~|The subject has to do with sizing of the hold downs on a two-story house
|~|where a shear wall in the 2nd-story is stacked directly above a 1st-story
|~|shear wall below (say both are 10 feet long). The standard for
|~|this type of
|~|construction locally is to provide plywood (or OSB) only at the specified
|~|shear wall locations ... not at the entire exterior of the house.
|~|Statics would dictate that the hold downs at the foundation be sized
|~|considering the overturning moment due to the force at the roof times the
|~|moment arm to the roof "plus" the force at the second floor
|~|times the moment
|~|arm to the 2nd-floor (less any dead load resisting moment).
|~|Many designs do not approach it this way. Often times the
|~|downs are sized considering the overturning moment due to the
|~|"total" of the
|~|force at the roof "plus" the force at the second floor times the
|~|to the 2nd-floor only (essentially neglecting the uplift from
|~|shear wall above). This is reasonable for the situation of "in-plane
|~|offset" shear walls ... but uplift from the 2nd-story shear wall
|~|need to be addressed as far as load path through the 1st-story goes.
|~|The reasoning often used to justify the later approach often varies from
|~|"that's the way we have been doing it for 30 years", and/or "it's just a
|~|house". I'm not really comfortable with either of these explanations.
|~|Just curious what feedback any one else may have on this subject
|~|to email me directly if you prefer).
|~|Thank you for your time,
|~|Steve Hiner, SE