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RE: Screen Anchors in Unreinforced Masonry

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Regarding the use of "Screen Anchors," there are a couple of different
applications which need to be addressed.  It sound's like Fred's
application is for temporary anchorage, not for permanent seismic
upgrades, but let me address the upgrades first.

1.  Hilti (and probably other manufacturers) maintain a seperate report
for URM upgrades.  Our ICBO ER # is 4815.  We also maintain a City of
Los Angeles Research Report # 24564.  In order to get the COLA report,
we are required to find buildings for our test program with 30 psi
mortar strength (which by the way is a real feat.)  A push test is done
on the building, then anchors are tested in the specific configurations
that are used in the code for seismic upgrades (22.5 degree anchors for
example.)  Regardless of how high your test results are, you are still
limited to the code values of 1200 lbs. tension.  In our tests, the
anchors were installed at the "T" joints (Intersection of the head and
bed joints.)   We were still able to far exceed the 1200 lbs. value,
even though the highest mortar strength I saw in the test report was 34
psi. (Not that UMB's in Los Angeles are not considered safe if the
mortar strength is les than 30 psi.)  Although the rules on mortar
strength are not as strict for the ICBO ER, most manufacturers probably
use the same basic testing in low psi mortar to submit to ICBO as well.
ICBO also limits the tension load to 1200 psi.
 2.  Our other ICBO ER # 5193 is probably more applicable to the Fred's
question.  It has an allowable load table for "screen anchors" installed
in clay-brick masonry.  The loads are based on the test values (divided
by a safety factor), not on code value.  Note 3 states that "The anchors
may be installed in the vertical or horizontal mortar joints, or in the
brick itself."  The published loads envelope these conditions.  The
testing was performed in an old building with around 45 psi mortar
strength (still very low.)

So what are the answers to Fred's questions?

1.  Structurally I don't think you can say one is better than the other.
Failure modes even for the same size anchor in the same wall tend to
vary.  The loads that are published are conservatively low enough to
cover all conditions during the design, and are generally verified with
on-site testing.
2.  The only place that you can install an anchor and not hurt the brick
is in a "T" joint between the bed and head joints.  Mortar joints are
generally between 3/8" and 1/2", and a 3/8" diameter "screen anchor"
requires a 1/2" hole.  So, if you drill into the "T" you generally won't
hit the brick.  The mortar is more easily patched (and is perhaps being
re-pointed at the same time anyway.)  For a removable anchor, you can go
to an "internally threaded insert" installed into a screen, but to my
knowledge they all require a hole >1/2" in diameter.

I hope that I have provided some insight.

Chris Gill
gillchr(--nospam--at)us.hilti.com



> ----------
> From: 	FredT5(--nospam--at)aol.com[SMTP:FredT5(--nospam--at)aol.com]
> Sent: 	Monday, June 15, 1998 4:13 PM
> To: 	seaoc(--nospam--at)seaoc.org
> Subject: 	Screen Anchors in Unreinforced Masonry
> 
> Can you help me please with two questions about using screen anchors
> in
> unreinforced masonry like the Hilti Hit Adhesive Anchor System?
> 
> 1. Is it structurally preferable to drill a hole right through the
> face of
> clay brick masonry for anchors with shear loads or should the holes be
> drilled
> in the mortar joints? Does the preference depend on the relative
> strengths of
> the brick and the mortar? In our case, we have very weak mortar and
> fairly
> decent brick, but only two wythes. I'm concerned that drilling in weak
> mortar
> might propogate more cracks than just drilling right into the brick. 
>  
> 2. For historical buildings, which type/location of holes is more
> "reversible"
> i.e. to patch at a later date? The holes are intended for temporary
> security
> measures. The preservation architect fears more irreversible cosmetic
> damage
> to the building if the holes are drilled right in the brick. Which
> hole
> location can later be more easily patched to match the original
> undrilled
> color/texture? 
> 
> Thanks for your thoughts,
> 
> Fred Turner
> Staff Structural Engineer
> Ca. Seismic Safety Commission
> 1900 K St. #100 Sacramento, CA 95814
> 916-327-1606 916-322-9476 Fax
> FredT5(--nospam--at)aol.com
> 
> 
> 
>