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RE: Screen Anchors in Unreinforced Masonry

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I received an email from Fred explaining the need for anchorage as only to
create a temporary infill installation with CMU in windows and doors. It is
not part of a retrofit program and is considered only temporary until a
decision is made to either tear the building down or renovate it. The bars
need to be removable and the holes filled.
I think that Bill Cain had the best solution to the problem. With that said,
I would like to address Sasha's comments since I had stated very similar
comments and removed them before posting. These comments are intended for
retrofit anchorage schemes rather than what Fred Turner is describing.

I have not measured the mortar joint in a long time, but believe it is less
than 3/4" thick. Therefore, as Sasha suggests, the hole needed for a screen
anchor is generally larger than the bed or head joints. The one thing that
has not been discussed is that size of the plug due to the expansion through
the screen. The epoxy bonds itself with any surface of the brick and mortar.
This is the main reason to specify a "Clean drilled hole" - so that no dust
particles would prevent the epoxy from adhering.
Depending upon the collar joint coverage (the area between the brick wythes)
the adhesion can create much greater shear capacity behind the inner and
outer wythes.
My point is that there is sufficient shear area to transfer load to the
brick rather than to consider the mortar itself - especially if the mortar
is considered soft to begin with. The action of the anchor is virtually the
same as what is performed by the in-plane shear testing. The allowable
capacity of the anchor is determined by the first movement capacity reduced
by a factor of safety. In URM work, once the overburden is removed, 20% of
the lowest values can be ignored and the remaining value used for the
capacity of the mortar.
With this said, the failure is not expected in the brick but in the mortar.
Therefore, as long as there is sufficient contact area with the adhesive
against the surface of the brick both above and below the joints, the
difference between installation within a brick or joint should be
insignificant.
If the difference was greater in brick, I would suspect that the code would
specify installation in solid material over the bed and head joints.
It would seem that the allowable capacity of the anchor has been tested in
various locations to average the results.

One more thing: The capacity of the shear anchor is dependent upon 8"
embedment for full capacity on screen anchors. The testing was originally
done using grouted anchors with 8" embedment into a 13" URM wall. When
proprietary anchors came onto the marked, the same test criteria was
maintained to ensure that epoxy anchors met or exceeded grouted anchor
capacities.
With this said, it would seem reasonable that the shear capacity of the
anchor would need to be reduced (without testing) if the shear area is less
due to a 7" embedment into an 8" wall (1" from exterior face of wall). I
have had anchors tested in structural clay (6") walls and have reduced the
capacity of the anchor to between 400 and 600 lbs. depending on accepting
approximately 25% of the value at failure.
If I recall, Hilti has not (at least since 1993) tested in 8" walls, but
Covert has. At the time, I believe that it was MEC Labs in Glendale
California who did quite a lot of testing for various committees of SEAOC
and was active in testing the Covert anchors in different wythe walls on a
job to job basis.
Anyone interested may contact Covert to verify this.

Dennis S. Wish PE


-----Original Message-----
From: ITSEKSON SASHA [mailto:itsekson(--nospam--at)eichleayca.com]
Sent: Tuesday, June 16, 1998 2:27 AM
To: seaoc
Subject: RE: Screen Anchors in Unreinforced Masonry


Isn't the diameter of the hole for the screened anchor 1 inch.  Considering
that the thickness of the mortar joint is 3/8 inch to 5/8 inch there will be
some damage to the brick itself if you drill into the mortar joint.
Obviously not as much as if you drill into the brick itself.

Is this an application of the tension-only anchors or combination anchors?
I would seriously consider anchoring into the brick if the anchor is to
resist in- plane shear loading, unless there is testing showing the shear
capacity of the anchor in the mortar joint for the wall discussed.

Regards,

Sasha Itsekson, PE


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
color/texture?

Thanks for your thoughts,

Fred Turner

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