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Re: URM Building per UCBC

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Larry,

I would interpret the veneer anchorage requirement to be applicable only to
an un-anchored or poorly anchored veneer wythe, not to adhered veneer.  The
hazards posed by the two types of veneer are clearly different -- the
un-anchored or poorly anchored veneer wythe being a potential life safety
hazard in case of strong shaking.

I would go with repointing to mitigate low strength mortar indicated by the
tests.  Mortar deterioration generally progresses inward from the wall
surface, changing from serious deterioration at the surface to sound mortar
someplace within the wall.  Repointing should include removal of all unsound
and debonded mortar.  In a wall with low tests, this will probably result in
removing more than 3/4" of old mortar.  The 3/4" minimum depth in UBC
Standard 21-8 is intended to provide, in a 1/2" thick mortar joint, a
depth-to-thickness ratio that will assure that the repointed mortar is not
likely to pop out of the wall.  Removal of all deteriorated mortar will
probably result in a considerably greater depth-to-thickness ratio.

Plywood is not the answer -- it is so much more flexible than 13" of masonry
that it will have no beneficial effect in supplementing the strength of the
URM wall.  Even if you were to install a plywood sheathed wall, just to
cover up the deteriorated URM, the URM should be repointed to mitigate the
hazards posed by the damaged URM wall.

I don't use gunite in URM strengthening unless it solves a unique problem
with no other solution -- almost never.  Gunite adds to the mass of the
building and thus the demand on the lateral load resisting systems.  And,
unless carefully designed, the load path for shear loads from the diaphragm,
through the [presumably low-strength} URM, to the gunite is not effective.
And guniting is an exceedingly messy operation that you want to avoid if the
owner wants to occupy the building during the retrofit construction.

Nels Roselund
Structural Engineer
South San Gabriel, CA
njineer(--nospam--at)att.net




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