Return to index: [Subject] [Thread] [Date] [Author]

RE: Unreinforced Masonry Building

[Subject Prev][Subject Next][Thread Prev][Thread Next]
If you are in a seismic zone Zero, why worry about retrofitting the
building. The masonry mass should be sufficient to resist the wind loads -
which if at 80mph is not very much (we design for 70-80 mph in Exposure B
and C in our area's. I suspect that the wind load is resisted well by the
weight of the masonry alone as it has throughout California.
What is the reason for retrofitting the buildings?

Finally, if you do want to tie the diaphragm to the walls - you can follow
the provisions of the Uniform Code for Building Conservation Appendix
Chapter 1 as published by ICBO. This is the provisions written in California
for Hazard Mitigation for Unreinforced Masonry Buildings.

With some variation in requirements in various municipalities, there are
provisions in the UCBC for the retrofit of URM's in all Seismic Zones.
However, I believe that most buildings - other than essential facilities -
are exempted from retrofit provisions if they exist in Zone's 0 and 1.

The provisions of Collar Joint coverage and minimal reinforcement that you
indicated is not uncommon of URM buildings in California. The amount of
Collar Joint coverage helps to determine the walls slenderness ability - the
minimum height before out-of-plane bracing is required. It has less to do
with in-plane shear requirements which is controlled more by the capacity of
the mortar in the bed and head joints. As I mentioned, it is fairly common
to see walls with almost zero collar joint coverage but this has never been
a great concern and I certainly would not want to try and fill the cavity.

In the old days (the mid 80's) we used to stuff rags into the collar joint
before grouting anchors in place to help control the flow of grout. However,
with the new Epoxy Anchors, tension anchors tool advantage of the lack of
collar joint coverage by using a screen to "spaghetti" the epoxy out into
the collar joint which helps it plug the cavity and increase tensile
strength.

In shear, the anchors capacity is generally calculated from on the area of
the anchor that penetrates the face bricks). The strength of the anchor is
actually based on the strength of the mortar joint rather than the
resistance it can obtain from the collar joint. In brick buildings this can
be easier understood where the walls are more than 2 wythes. The inner wyth
is locked to the two outer wythes at approximately ever 7th course. The
inner wythe is assumed to have almost no mortar strength as it was rare to
take the time and cost to make the wall solid. Therefore, by nature of the
URM wall using interlocking courses, the strength of the bonds comes from
the outer wythes of brick at the mortar joint.

Hope this helps put your project in perspective.

Dennis S. Wish, PE

-----Original Message-----
From: David Hall [mailto:Dahl(--nospam--at)deainc.com]
Sent: Friday, February 11, 2000 9:42 AM
To: RickLennen(--nospam--at)AOL.COM; seaint(--nospam--at)seaint.org
Subject: Unreinforced Masonry Building


A few weeks back I sent out a request asking if anyone knew how to conduct
non-destructive testing to determine if reinforcing and grout was placed in
the voids of an existing masonry building.  I appreciate the responses that
I received from many of you.

Well guess what.  There was only grout and reinforcing in the horizontal
bond beams which were placed at 4 foot centers horizontal, in the lintels,
and vertically at each side of the expansion joints and openings.

Now heres another one for you, there are 15 of these building designed by
our office and from what I recently heard 12 more that were designed by
another.

These buildings are telecommunication building in Texas which is in seismic
zone 0 with a 80 mph wind speed.  I haven't checked yet but I believe that
#4's at 4 feet on center will work vertically.

Now to retrofit this building I am considering having them saw cut and
remove the inner shell of the block from the inside, embed #4 bars into the
upper and lower bond beams with epoxy grout ,lapping the bars 30 inches,
then gluing the shell back up to the course just below upper bond beam, then
filling the void with grout continuing up carefully and adding the last
shell and pointing the remainder.

What a mess, If anyone has a better idea I would sure like to hear it.
Adding shotcrete with a grid of steel to the interior is not an alternative.

I would appreciate your suggestions.


David A. Hall, S.E., P.E.
Senior Structural Engineer
David Evans and Associates, Inc.
2828 SW Corbett Ave
Portland, OR 97201
503-499-0280
Fax 503-223-2701
E-Mail dahl(--nospam--at)deainc.com