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Slender CMU Wall Design

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Rich Lewis wrote:

"I am doing a preliminary design on a slender CMU wall.  I have several
questions.

1. The wall is for a pre-engineered metal building.  The wall (eave
height)is 30 feet tall.  The spacing of the rigid frames are 26.5 feet.
The architect is bumping out the face of the wall at the frame
columns...."

Rich,

I consider this a very marginal situation for a masonry wall.  I
designed a building very similar to what you are describing about 6
years ago, and would not want to do it again.  (My client was a block
manufacturer and this was his new factory.  He figured on using all his
"leftover" (i.e. reject) block, which he considered free.  It ended up
costing him an arm and a leg.  For advertising purposes he wanted to use
a different type of block in each bay.  Geez.)

If memory serves, I ended up specifying a deflection limit for the PEMB
of Ht/200 under the DESIGN wind, at which point the rep I was working
with just went nuts.  They're used to Ht/100 or worse under a 10-year
wind.  It probably still wasn't stiff enough, but I don't know, as I've
never been back to the jobsite.  My client couldn't find a contractor to
bid the job (that, of course, was my fault too) and when he finally did
the guy knew he could do whatever he wanted, and did.  My client did me
the huge favor of deselecting me for construction inspection (he allowed
as that he knew enough to do it himself) and I quietly thanked God and
left.

Anyhow, I would try hard to use the PEMB standard girts to hold up your
wall, just as though it was metal siding.  Spec them to be as stiff as
feasible.  In theory you might consider some kind of expansion joint
material under the face shells near the bottom, to make something of a
hinge, but of course that interferes with the idea of it being a shear
wall.  To be honest, I would strongly consider letting the PEMB
manufacturer resist the lateral load; or, if not, then you'd better put
a big black note on your drawings telling him that his building's
stability is his problem until the masonry sub (not under his control,
usually) finishes the walls.  You'll also need to contractually dictate
who is responsible for every last screw, bolt, and shear tab at the
interface between the PEMB and anything else not supplied by them,
including tolerances.

In general, every time I've tried to do something with a PEMB that was
not right out of their catalog, I've regretted it or cost my client
money (usually both).  As a result, I've come to have something of an
attitude toward them.  With how well I hide it, you may not have
noticed.  As Paul Ransom said yesterday (Geez, Paul, take a day off once
in a while) it's a very competitive environment for these guys, and it
shows.  You can get a very good deal on a building if you want exactly
what they offer, but to do something custom jacks the price and often
doesn't work very well.  When you add the very basic deflection
incompatibility between your two materials, problems will occur.

Mike Hemstad, P.E.
TKDA
St. Paul, Minnesota

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