From: "Dennis S. Wish" <dennis.wish(--nospam--at)gte.net>
Date: Sun, 21 May 2000 02:52:37 -0700
Allow me to preface with a couple of qualifying facts and I hope that this
does not embarrass Nels too much. Nels credentials are as one of the
foremost authority in the retrofit of Unreinforced Masonry. In addition,
Nels and very few others specialize in the retrofit of unusual materials
like clay tile (structural and non-structural) as well as Adobe. He worked
with John kariotis's office for many years along with Mike Krakower,SE
another of our List subscribers. Both are noted authorities on the subject.
With that said, I had a number of Clay tile structures to retrofit over the
years and Mark's advise is very good. I would only like to add a couple of
points - however, I must add that times have changed and the last retrofit
to a clay tile or other URM building was almost seven years ago. There are
two types of clay tiles - one is a very thin-walled material which is not
intended to be used structurally, however, I did run across a one story
building in Arcadia California which was constructed of this material. The
structural clays have a decent wall thickness of about 1/4" - thin, but as
you will see - it can hold up fairly well.
One of the buildings I retrofit was a one story 4" clay tile structure in
Santa Monica / Venice. I disagree with Mark only on the point of tension
testing the tile. There are no published code values for a tension anchor
(or shear for that matter) in clay tile. The values just by common sense
should be greatly reduced, however, when tested the added few pounds can
save an owner a lot of money as these buildings require a lot of anchors.
There are not many labs with the experience to create jigs to set up the
test procedures. As I remember, it was seven years ago, there the a lab that
would make the jigs special and then submit testing procedures for approval
to the City of Los Angeles that we used - MEC in Glendale. Inasmuch as I am
no longer in Los Angeles, I don't know if they are a player any longer or
not. There are two other testing labs that worked close with SEAOSC during
the retrofit period (I'm sure there are others, but these two had
representatives who volunteered time on the then Hazardous Building
Committee and Anchors committee). These were Tim Foster of Specialized
Testing (a former Hilti employee who started Specialized) and Tecmar Labs
in, I believe, Long Beach. I don't remember Smith Emory having as much
interest in this area as those that I mentioned.
Going back to the building. We did tests for tension and shear and as you
might guess - tension was good for less than 400 pounds per anchor (from
memory) and shear at 600 pounds. As engineer of record, I choose the test
sites and I purposely looked for areas that represented the average
condition of the tiles and mortar. We had already done push tests to get the
head and bed joint shear so this was not an issue at this time.
The H/t or wall slenderness was a nightmare even though it was only a one
story building. Just as a side note, a second floor wall will almost always
require vertical wall braces (diagonals are difficult to use as the brace
must start too close to the floor). I'm not discounting creative solutions
like reinforced stucco finishes or fiber/epoxy applied coverings, but these
were not available when I was doing the work and I don't even know if any of
these are approved today.
The City did allow us to use almost everything we could to brace the walls -
perpendicular wood interior walls - but we still had anchors at the floor
(raised floor, at the ceiling and roof (the roof was a 24" deep carpenter
Was it worth it? The owner called me a few weeks ago because the city wants
him to address the garage for some reason which is also clay tile. I asked
about the building which I retrofit before Northridge. It was located in the
hard hit area of Santa Monica (actually on the Venice border and by the
ocean). It suffered no appreciable damage.
One final note. We used only plated anchors on this building. Tension
anchors governed because of the reduced value and basically every anchor in
the building became a combination anchor for the added cost of the screen
and extra epoxy. I have a high regard for the screen/tube epoxy anchors as I
firmly believe that the area of the "plug" helps in tension as well as shear
on these buildings. Be careful as most retrofit epoxy anchors do not have
reduced values published for this material. We used the Covert system on
this project because we received tremendous support from Covert including
participation in the testing portion of the project.
Hope this helps add a little perspective to the comments.
Dennis S. Wish, PE