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Epoxy Repair of Wood

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One of our other offices is trying to get some information on repairing wood
with epoxy. Following is a description of the application from one of our
offices in the Boston area. Any thoughts on the questions raised would be
appreciated. If you have experience related to this topic, please copy my
colleague at Bill.Blue(--nospam--at)Earthtech.com.

Jim Lutz, P.E., S.E.
Earth Tech, Inc.
10800 NE 8th St - 7th Floor
Bellevue, WA 98004
425-455-9494

We have a project that has some heavy timber trusses in the attic of a very
old fire station. The trusses were added at some point to provide support
for the second floor when it become necessary to eliminate columns
supporting the second floor when they converted from horse drawn engines to
powered engines. Steel hangers support the second floor beam and extend up
into the attic space. The trusses were built in the attic space but do not
support the roof. Recently, an electrician working in the attic noticed that
one of the trusses kicked over at the top, about 2 feet. One half of the
truss rotated until it achieved internal stability. My feeling is that the
bracing was inadequately nailed to the roof rafters and failed, allowing the
truss to displace.

Because the hanger rods are offset from the truss joints, shear cracks,
together with shrinkage cracks, have developed in most members. Also, the
joint that rotated developed fractures of the wood in the vicinity of the
joint. I am going to reinforce the truss with steel channels but before
doing that I want to repair, or replace, the damaged wood members.

I have the ASCE book on evaluation and repair of timber structures. The book
is 20 to 25 years old and discusses the use of epoxy to repair wood, and
truss members in particular. I also have several papers off of the internet
describing similar repairs. The papers make it sound as though you can do
almost anything with epoxy. I have found that a lot of companies that
produce epoxies are small local companies. You can inject cracks, add sand
as an aggregate for wider cracks or remove the damaged material and create
forms and create a missing section of the member out of epoxy. You can add
fiberglass reinforcing rods to reinforce the joint. I have talked with a
number of these smaller suppliers and they have differing opinions as to
what they recommend can be done. I found a paper describing truss repairs
and the engineer used Sika products. I gave Sika a call and they said that
epoxies have been used for lots of different applications but there is not a
lot of technical information, such as codes, that offer guidance. They said
that epoxy repairs in concrete have been well researched and there is a lot
of info available. They said that there might be some info out of Canada but
I did not find anything.

There are issues such as heat, associated with use of epoxies, and Sika
would not offer cost information, since they only supply the product and
don't deal with the end use. So, it seems that the use of epoxy for
structural applications is more art than science, and design and cost
information is not readily available.

I am not relying upon the repaired wood to carry the final design loads,
only as required to transfer the loads to the new steel channels. So I want
to make repairs but don't want unnecessary repairs that would drive up the
cost or make repairs that are so extensive that they might cause unforeseen
problems, due to the Hi-Mod nature of the epoxy, such as excess heat and
shrinkage.

So, I am just trying to see if anyone has experience that might be germane
to my application.



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