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Re: Screen Anchors in Unreinforced Masonry

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In a message dated 6/15/98 2:14:30 PM, you wrote:

<<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

Thanks for your thoughts,

Fred Turner


1.  Regarding shear capacity in a weak mortar/good brick application, the
anchor will have a slightly higher capacity in the brick as opposed to being
installed in the mortar. There will be more of a difference in capacity if the
shear loading is in line with the mortar joint. ICBO ES and LA City give
maximum capacities regardless of jobsite conditions and mortar strengths,
however I believe San Francisco allows higher capacities if job site testing
supports higher loads.  Drilling into weak mortar doesn't usually cause
problems because the drill bit goes through it so easily, where the brick
might require much longer drilling time and cause more vibration, even in
rotary only mode.
2.  The mortar location should be easier to return to existing finish
condition.  The anchors cannot be removed once they are installed, but can be
cut off and ground down.  A lime or other mortar patch can be used over the
anchor.  I'm sure it would be easier to match the mortar color than the brick.

Hope this helps.

Howard Silverman, PE
Covert Operations, Inc.
Anchoring and Crack Injection Systems