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RE: Screen Anchors in Unreinforced Masonry

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Fred,
I've read two other comments and would like to add mine to pot:
1)The shear capacity of the wall is governed by the capacity of mortar joint
as indicated by the Push-test. Therefore, I would believe that the mode of
failure would occur in the weakest link - the mortar and that it would not
matter one way or the other were the anchor is placed as long as it is
outside the joist pocket.

2) Are you referring to repairing damaged brick that might crack due to the
installation of the anchors through it? Repairing mortar joints is much
easier than replacing Red brick. The labor to repair damaged brick would
require removing both inside and outer wythes since the anchor has to
penetrate to one inch of the exterior face of wall, and therefore connects
to both wythes. The repair methods we used in the late 80's required the
removal of brick from each side of the crack. This approach proved much more
costly than to pressure grout the wall.
I read the comments from Covert. I was under the impression that the anchor
capacity needed to be reduced to compensate for a 2-wythe wall since the
UCBC Appendix 1 Chapter 1 capacity of shear anchors assumes ICBO and COLA
values for anchors in a 13" minimum wall. This should be true, unless you
test the anchors and the local jurisdiction accepts testing in lieu of ICBO
reports. I am aware that Covert has tested anchors in 8" walls as well as 6"
clay tile (which they did for me about six years ago). In the structural
clay tile wall, we needed to reduce the shear anchors to somewhere around
600 pounds from code allowable 1000 lbs.
On the jobs I am referring to, MEC Labs in Glendale California performed the
tests using the Covert system. As I also recall, Hilti had not (at the time)
tested their anchors in other than 13" 3-wythe walls. You might want to
research this to be safe.
Personally, I believe that cracked brick will be the least of your concerns
in the event of a sizable quake. You might try to convince the architect to
consider it as "Character" lines rather than damage and do a simple
"facelift".
One final comments. If the mortar is weak as you suggest, I believe that
there is less likelihood that the brick will crack since the first movement
will occur in the bed and head joints. If anything, this should help
preserve the brick, or at least localize the crack and prevent them from
radiating through courses of bricks. More likely, the anchored brick might
crack (which is probably concealed to begin with) and the mortar joints will
crack (stair-step cracks) subsequent to the initial shock - preventing a
radial crack through many courses of brick. This can be remedied by pointing
and a small amount of pressure grouting at the damaged brick. This should
also help preserve the aesthetics of the building. Remember that the contact
area of the anchor increases as the epoxy "Spaghettis" through the screen
and bonds with the brick in the collar joint. If there is over 50% collar
joint coverage, the capacity of the anchor may test out higher than
allowable since the area of contact will have more resistance in the collar
joint area.

Dennis Wish PE



-----Original Message-----
From: FredT5(--nospam--at)aol.com [mailto:FredT5(--nospam--at)aol.com]
Sent: Monday, June 15, 1998 2:13 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 propagate 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