Here are some of my opinion;
1. Corner Cracking
Beside the concern for wind that Roger Turk brought up, there are other
factors that cause cracking in URM walls. Foundation problems are one of the
most common that can offset a wall vertically and cause considerable
cracking of the mortar in the bed and head joints. This is visible as a
stepped crack - which follows the mortar or in extreme cases shears a number
Impacts and vibration are another cause of cracking - again mostly in the
bed and head joints and of greater concern where the mortar is weak.
You mentioned that the owner grouted the collar joints. Depending on how the
grout was installed, it is not uncommon to blow out sections of the walls
from the pressure of the grouting if placed in lifts exceeding a few feet at
a time. The pressure causes a number of problems including complete blowouts
as I mentioned.
2. In seismic zone 3 and 4, URM buildings are considered hazardous but they
account for the majority of low income multi-residential (apartment)
buildings in major cities. The purpose of the UCBC code as well as the state
requirement for retrofits was to mitigate the potential to loss of life -
NOT to condemn these buildings. The code is explicit as there is no
guarantee how the building will perform. Still, the retrofit codes are
intended to improve performance and lower the risk of death.
The buildings are still considered hazardous - retrofit or not.
3. The UCBC is applicable to bearing wall buildings. Knowing this, I would
be hesitant to add anything structural to a URM wall without providing some
sort of secondary support for the object. You may want to consider this to
protect those on the site should the wall fail.
The epoxy anchors that you refer are generally good for between 750-1000
pounds in shear for an 8" penetration (partially of which is in a collar
joint) and 600-1800 pounds in tension. These values are for manufactured
anchors such as the Hilti "Hit" adhesive system and depending of factors
such as the thickness of the wall and the capacity of the existing mortar.
4. Most URM buildings in California that I have inspected did not make
ledger/joist connections but embedded the rafters into the URM wall
maintaining a fire-cut (angled cut) at the end of the joist to prevent
damage to the wall if the joist fails and drop.
Where tension connections had been made, I have found that the contractors
used a minimal "Governor Anchor" which consisted of a piece of conduit bent
into the wall and hooked through the side of the joist/rafter to prevent
pull-out. These were placed anywhere from 48" to 72" on center - if used at
You may wish to search out documents from SEAOSC (seaosc(--nospam--at)aol.com) or the
state SEAOC regarding Division 68, Division 88, ABK Methodology and the City
of Los Angeles RGA 1-91 (which became, with some, modification the UCBC
Appendix Chapter 1. There are, I am sure, other documents created by SEA of
Northern California - especially related to the Anchors Plus program
established in the city of San Francisco. Unfortunately, I stopped
retrofitting URM's when I moved to the desert almost seven years ago -
having been involved in only two or three since then. In particular, read
the sections of the UCBC related to Special Procedure which, in multi-story
structures, tends to be less restrictive than the "General Procedure".
Good luck with your project.
Dennis S. Wish, PE
From: David Hall [mailto:Dahl(--nospam--at)deainc.com]
Sent: Saturday, February 12, 2000 11:37 AM
To: RickLennen(--nospam--at)AOL.COM; seaint(--nospam--at)seaint.org
Subject: RE: Unreinforced Masonry Building
If you are in a seismic zone Zero, why worry about retrofitting the
Dennis S. Wish, PE
This is an option I will give the client because I thought the same thing
However, they are hanging equipment off the walls which are being epoxy
bolted into air space. I know 2184.108.40.206 states there are no special
provisions for seismic risk in zones 0 and 1 where as there is a minimum
vertical reinforcement requirement of 4 foot centers for the rest, the
client paid to have these voids filled with grout and reinforced and he
should be compensated for lack there of.
I will look into the stresses involved per the code you stated and let the
client know about this. However, there are corner cracks already in the
building. And don't they have hurricanes down there.
Another concern I would have is what if a couble of years from now, cracks
start showing up in one of these buildings, the client hires another
engineer who reviews my recommendations and states that these walls should
have been reinforced vertically.
And lastly, I don't feel comfortable about stating these walls are fine as
built. If someone else would like to buy off on these buildings as is, let
me know and I will put you in contact with the contractor. :)
Thank you all for your responses, this forum is great.
David A. Hall, S.E., P.E.
Senior Structural Engineer
David Evans and Associates, Inc.
2828 SW Corbett Ave
Portland, OR 97201