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Re: URM

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Mark,
I haven't done a multi-story URM for a while but I would like to respond. I
am doing so without my UCBC in hand so bare with me here.
Your answer depends upon the capacity you assign to your crosswalls. The
amount of crosswalls is also a function of the DCR chart which places you in
regions based upon the aspect ratio of the diaphragm less any openings. The
DCR analysis will determine how much crosswall you need to provide to bring
your model in compliance with the region that it exists.
The actual capacity of the crosswall will therefore, depend upon how much
available wall you have and the maximum capacity of twice the diaphragm in
the line of the wall.

To be exact, I believe it was the original ABK methodology back in the very
early eighties that determined the need for a crosswall to be less stiff
than a "shearwall" but only flexible enough to deflect a maximum of 1" per
story. One inch was the magic number that we designed crosswall frames for.
To stiff and the element became a shear wall. Remember, Special Proceedure
is not a uniform distribution, but uses crosswalls as dampers.
To be more exact, you would analyze your crosswall for deflection (I used
the '91 UBC Standard).

Intermediate floor crosswalls were historically attached to the floor
framing above and were considered adequate crosswalls when considering
plaster finishes on both sides. It was not necessary to further connect or
drag the diaphragm into a plaster crosswall (or insure that the framing was
even blocked). Where framing was running parallel to the wall, the
assumption was that the plaster wall was adequately connected to the
diaphragm above unless you could prove otherwise. I never considered it a
positive connection when I needed more diaphragm capacity into a plywood
crosswall and assured extra connections of the wall to blocking and a
positive connection to the diaphragm above.
UCBC (if I recalled) only allowed a maximum capacity for 3/8" sheathed
plywood per side of walls. If I further remember, the diaphragm capacity at
floor level (500 plf for straigth and 600 for diagonal sheathing) rarely was
less than the sheathed capacity (somehow 320 plf sticks in my mind for 3/8"
on each side of the wall). Since the crosswall picks up the capacity of the
diaphragm from both sides of the wall the diaphragm at 2*Vu would exceed the
crosswall capacity Vc. Therefore, additional drags or collectors would not
be required required.
Crosswalls at roof level were a different story since the twice the
diaphragm capacity of straight sheathing is less than a crosswall sheathed
one side with plywood. The second problem is that upper-story walls need to
be extended to the roof by nature of the use of carpenter trusses, separate
ceiling and roof joists or the use of trusses with attic spaces.

Now to make it more complicated. Assume you design a crosswall frame which
you intend to give the entire diaphragm capacity at both sides (approximatly
2*600plf*D) Depending upon the length of the frame, you would design
appropriate collectors and drags to develop the diaphragm.

Now, I would assume that you could deviate from the code and design a
plywood crosswall to take the entire diaphragm capacity into one wall so
long as you can justify the deflection at 1". This is tricky, but I think it
is in keeping with the intent of the code. In this case you might use
collectors to develope the capacity you need.

Finally, in multi-story apartment buildings, it was never necessary to make
a positive connection of a plaster crosswall to the floor above. We just
added the lengths of qualifying walls at each level and considered that our
actual crosswall capacity (total) to compare in the DCR charts.

Hope I havn't confused anyone since I have been away from multi-story URMs
for over five years (I did a few but must be loosing my memory in my old
age).

Dennis Wish PE
PS. I realize that some of this may no longer be acceptable since
Northridge. You need to check current code to see if this is still an
acceptable judgment.


-----Original Message-----
From: Mark D. Baker <shake4bake(--nospam--at)earthlink.net>
To: seaoc <seaoc(--nospam--at)seaoc.org>
Date: Tuesday, January 06, 1998 3:12 PM
Subject: URM


>To all the URM people out there:
>
>UCBC, retrofit of unreinforced masonry buildings, special procedure:
>
>Since a new crosswall is not considered a shear wall and its intent is
>to decrease the displacement of the diaphragm at the center of the
>diaphragm relative to the end shear walls, is it necessary to provide a
>collector element dragging load into the crosswall?
>
>Regards,
>
>Mark D. Baker
>Baker Engineering