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Re: Weakened Shear Plane In Brick (URM) Wall, Zone 4

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Jon,

Have you determined the probable cause of the crack?  What had to move to
form the crack, and in what directions?  How long has the crack been
there -- does anyone remember a time when the crack was not there?  Is the
steel bar related to lintels over the openings?  Are there anchor plates on
the exterior of the walls aligned with the bar? Is the crack visible from
both sides of the wall, or only one side?  Was the building built before the
1906  Earthquake?  Some interesting repairs were done in buildings that
survived that quake.  A low-strength shear plane related to the
mortar-to-steel contact would not be surprising; perhaps the strength in the
adjacent wythes provides adequate compensation, considering that it is high
in the wall.  I would be reluctant to "repair" the crack without figuring
out why it is there and whether the "repair" is really needed.

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




----- Original Message -----
From: "Jon Brody" <jon(--nospam--at)jonbrody.com>
To: <seaint(--nospam--at)seaint.org>
Sent: Friday, August 20, 2004 12:57 PM
Subject: Weakened Shear Plane In Brick (URM) Wall, Zone 4


| Hi All,
|
| I'd be interested in hearing other professional opinions regarding a
building I was
| recently called out to look at.  I'll describe it in detail, because
ultimately it's a
| judgment question.
|
| The subject is an existing two-story unreinforced brick building in
seismic zone 4.
| Slab on grade at ground floor, 2nd and roof framed with lumber and 1x
sheathing.
| Seismic retrofit work apparently has been performed on two occasions;
once, to
| brace the parapets and tie the walls, then, after changing the ground
floor use, new
| steel braced frames (with collectors) and plywood shearwalls were added up
to the
| second floor.
|
| The building is located on a corner.  The two street walls have many
openings on
| both levels (with narrow "piers" of masonry between the openings); these
are the
| two walls that now have frames from the ground floor to the second floor.
The other
| two walls are less punctured with openings, and new plywood shear walls
were
| provided next to them at the lower level for redundant (or
post-brick-cracking) shear
| resistance (and, presumably, to have a load path that met code).
|
| I was called in because a long horizontal crack was noticed at a grout
line just below
| the ceiling framing of the upper floor (near the roof).  The crack extends
perhaps
| three-quarters of the length of one of the solid walls, and continues
across one of
| the street walls (above the window openings).  Upon investigation, it was
revealed
| that there is a flat steel plate running continuously horizontally at the
grout line that is
| cracked, embedded in the bed joint.  The plate appears to be 3" or 4" wide
and
| about 1/4" thick.  It's not clear what role this plate plays in the
construction; it may be
| related to the original connection of the ceiling framing to the wall.
The crack seems
| to have been grouted over in the past; it does not seem to be a recent
development,
| and there is no sign that it has widened recently.
|
| I recommended that in situ shear tests be performed at the crack line, and
that
| others be performed a couple of feet above or below for comparison.  The
resulting
| average shear strength was 26 psi at the crack, and 192 psi away from the
crack.
| My concern is that the weakened shear plane at the crack renders the two
upper
| level brick walls nearly useless in resisting in plane seismic loads.
There are a
| number of partition walls in both directions at the upper level only,
which, at this
| point, represent the majority of seismic resistance at that level.
However, I believe
| that the structure is much more at risk of partial collapse in a major
earthquake than
| the average structure of that construction type, because the masonry walls
have
| such minimal shear resistance in this condition, and because the roof and
floor
| framing appear to rely on the brick walls for vertical support.  If the
building relies on
| the partition walls for seismic resistance, it might move too much and
result in P-
| Delta issues with the walls and wall piers.  My recommendation is that, at
a
| minimum, the crack be repaired with epoxy grout injection from BOTH the
inside
| and outside (the plate is in the middle of the wall thickness).
Alternatively, it could
| be strengthened with steel plates that bridge the crack on the outside
face of wall
| and are epoxied in above and below the crack line.   I'm also
recommending, as a
| highly desirable, but more optional, additional improvement, that they
continue the
| braced frames up to the roof and add plywood over the 1x (straight) roof
sheathing.
|
| My question is, mainly, would you recommend that the building be vacated
until the
| strengthening takes place (the upper floor is studio apartments)?  The
fact is, even if
| the crack weren't there, the building would have a questionable seismic
load path
| from roof to second floor, so mightn't that be over-reacting?  Let me
point out that
| there is no code reason (that I know of) that the building be strengthened
above the
| second floor, except if a dangerous condition (such as the crack) exists.
It boils
| down to the fact that there is a substantial defect in a load path that
isn't "all that" to
| begin with.   Which brings up the question, Is fixing the crack enough?
Or should
| the building be left with an explicit seismic load path, even though there
isn't a code
| requirement to do so? Of course, I will explain that to my client, and
will strongly
| advise them to retrofit the upper story (not just the wall); but, when
they find out how
| much it will cost to retrofit, they may not be able to proceed.  They will
want to know
| HOW dangerous is leaving it the way it is (crack issue addressed, upper
story load
| path still dubious).  The fact is, then it's as dangerous as other
buildings of the same
| type (except that the lower level braced frames probably stiffen the lower
level and
| therefor increase the load at the upper level).
|
| All opinions welcome, and thanks in advance,
|
| Sincerely,
|
| Jon Brody, SE
| San Francisco
|
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