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RE: Hold downs of stacked wood shear walls

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Steve,
I have always designed each wall independent. The tension and compression
from the upper level is placed on the first story wall and the statics are
done from the foundation. In other words, the uplift from the second story
wall is cumulative and added to the uplift at the first story wall.
I consider the second floor diaphragm to be a hinge rather than considering
the walls to be rigidly fixed at the second floor and free of the diaphragm
(in other words a wall with one load from the roof and a total height of 2H
(simplified explanation).

In either case, the tension and compression at the bottom of the wall at the
second level must be considered in the first story wall. If the walls are
not coplanar then the uplift (and compression side) of the upper level wall
needs to be resolved and held in place by what ever framing is available.

Simply stated, the load path must resolve down to the foundation.

CAUTION: A great concern has to do with shrinkage of the wood at the second
floor diaphragm which can cause excessive deflection or play in the hold
downs. I tend to use proprietary wood products to minimize dimensional
changes - this is a great sales point for TJI or equivalent floor framing
systems. Timberstrand or Parallams rim joist, TJI joists and similar
experience very little shrinkage and the play in the holddown is
dramatically reduced.

One more caution. Since the compression side is cumulative you may need to
take a second look at plate crushing at the foundation.

Hope this helps.
Dennis S. Wish, PE


-----Original Message-----
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 design and/or
plan review of single-family homes of wood frame construction (custom or
production):

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 foundation hold
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 moment arm
to the 2nd-floor only (essentially neglecting the uplift from the 2nd-story
shear wall above).  This is reasonable for the situation of "in-plane
offset" shear walls ... but uplift from the 2nd-story shear wall would still
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 (feel free
to email me directly if you prefer).

Thank you for your time,
Steve Hiner, SE
Folsom, CA