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RE: Improving Shear Capacity of Existing Plywood Sheathed Shear Walls

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Nels,
A couple of questions:

1. Will the historic code allow you to design to the existing capacity of
the weakest member as did the URM retrofit code? This should solve at least
your problem at the roof since it is unlikely that the roof sheathing is
plywood and more likely straight sheathing with a lower capacity.

2. The floor capacity is most likely too high compared to the first story
walls, but the idea is to design based upon a demand to capacity ratio
rather than a straight distribution of shear as is typical in new
construction.

3. I believe that there have been tests to increase the capacity of walls by
adding a second layer of sheathing (assumed to be staggered from the lower
layer so that edge nailing occurs where the field nailing exists in the
lower panel. I remember a thread a few months ago from an engineer who had
done this, but I don't know if this occurred on the SEAINT List or on the
AEC-Residential List. Possibly John Rose can jump in on this one.

4. Obviously, increasing nailing (decreasing spacing) can help boost shear,
but you will still need to tear into the wall to retrofit in the sill
connection for shear as well as any uplift that may need to be tied. You are
probably dealing with full size studs and this may help when increasing the
number of nails, but then again, you probably are also dealing with
inconsistent stud spacing as I find typical in these old building.

5. I don't see how you can avoid getting messy on the first floor, unless
you can redirect all shear to the exterior of the building. If you have a
raised first floor then you still have to tie the shearwalls down and
possibly tear through the floor to get a grade beam in place. It might not
be any more messy than removing the existing sheathing and shearing two
sides.

I guess the big question is what does the historic code require? As I recall
from URM work, it required a good deal less than even the minimum RGA or
UCBC requirements. Is there a tradeoff for wood frame historic retrofit as
well?

Nel's don't forget the rigid analysis (just joking)!!

Regards,
Dennis


-----Original Message-----
From: Nels Roselund [mailto:n.roselund(--nospam--at)worldnet.att.net]
Sent: Friday, February 02, 2001 3:48 PM
To: seaint(--nospam--at)seaint.org
Subject: Improving Shear Capacity of Existing Plywood Sheathed Shear
Walls


A  historic 100 + year-old two story wood-framed hotel building is being
rehabilitated for adaptive reuse.  A previous strengthening project of about
25 years ago included installation of 3/8 plywood shear walls.  Drawings for
the old strengthening project are available, and appear to reliably reflect
what was constructed.  The shear walls do not have capacity to satisfy
current Building Code requirements.

At locations where adding plywood to the opposite side of the wall is an
undesirable or unavailable option, is there a reasonable way to increase the
capacity of the shear wall?  For example, have there been any tests of
increasing capacity by installing a second layer of plywood over the
existing, perhaps with vertical edges at studs staggered from the existing
vertical edges, and with blocking nailed to the sole plate and top plates to
provide additional backing for edge nailing at those horizontal edges?

Nels Roselund
Structural Engineer