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Re: UCBC97

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I believe that Section A114.2 of the UCBC is applicable to all stone walls
of an unreinforced stone masonry building.  I don't read from the Code the
criteria that you stated that walls that meet the height-to-thickness ratios
of Table A-1-B do not need bond beams.

The H/t requirements of Table A-1-G are quite restrictive, but most stone
walls I've seen satisfy those requirements.  Do you have walls that exceed
those values?  If the building is a registered historic site, and in
California, the California State Historical building Part 8, title 24 of the
California Code of Regulations may be applicable, and useful in developing
alternative means of complying with the intent of the Code.

I believe that the reason for the restrictive H/t requirements applied to
stone buildings is that many stone walls were built without bond stones
[which are the equivalent of header courses in brick walls that periodically
interconnect the inner and outer wythes].  Without bond stones, the
out-of-plane response of a stone wall is not predictable, but it is likely
that the full thickness of the wall will not be effective in providing
out-of-plane stability.  If the presence of bond stones can be verified to
be equivalent to the requirement of Section A106,3.2.1 [very difficult to do
except in the case of ashlar lay-up of the stonework in which you can
actually see each end of the same stones each side of the wall], I believe
that the higher H/t values of Table may be justified.  The CA State Historic
Building Code requires that the inner and outer wythes of stone walls be
pinned together [using epoxy to bond the pins to the masonry] as part of a
retrofit strengthening [Section 8-805-2.2].  I believe, though it is not
stated, that the purpose of the pins is to provide through-the-wall bonding
equivalent to that provided by bond stones or header courses.  For
relatively thin stone walls, I would place the pins in a more closely spaced
pattern than that required by the Historical Building Code if I were to
justify using the Table A-1-B values of H/t.

In Section A110.2, the out-of-plane analysis that is not required if the
walls comply with Table A-1-B is an analysis in accordance with Chapter 16
of the UBC [and no one wants to get into that kind of analysis for an
unreinforced masonry wall in seismic zone 4].  If the walls exceed the Table
A-1-B limits, the bracing allowed by the exception is bracing that is used
to reduce the wall spans to achieve H/t values that comply with Table A-1-B.

Bond beams have a function that is different from that of out of plane
bracing.  At the top of the wall, they provide continuity in the zone of a
stone wall that is very fragile: the stones at the top of the wall are not
confined by the weight of material above and are easily displaced by
earthquake shaking.  Another function of bond beams is to provide a strong
material to secure anchor bolts into, so that the diaphragm-to-wall
anchorage can develop the high forces that are concentrated at points of
anchorage, and then distribute those forces somewhat uniformly [and at
relatively low stress] to the stone masonry.

A requirement related to stone masonry walls in seismic zones that is not
stated in the UCBC or the Historic Building Code, but that is very important
to the survival of a stone building in strong shaking is that the wall
anchors and the epoxy pins need to be installed diagonally into the corners
to restrain the corners from separating.  I saw this need in a stone masonry
building that had been retrofit and then had been damaged by the Northridge
earthquake of 1994.  The walls all remained tightly restrained to the
diaphragms because of the effectiveness of the retrofit wall anchors, but
the corners of the building fell away, and that nearly led to complete
unraveling of one of the walls.  With a little more shaking, support of the
framing would have been lost, and the diaphragms would have been on the
ground, still anchored to bands of stones adjacent to their edges.

Let me know if you'd like to discuss this with me further.

Nels Roselund
Structural Engineer
South San Gabriel, CA

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