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

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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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