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RE: Log Home to be constructed in Seismic Zone 4

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I've trained in building with logs, have built log homes/structures in New York and Alaska and have consulted on many others.  Just a word of caution on the drift pins and all-thread rods.  Both methods work well to transfer shear between logs, but each has problems related to shrinkage.    

Rule of thumb for shrinkage is 3/4" per foot, adjust as necessary for the species and moisture content of you logs.  I recommend that you have your wood tested to determine the shrinkage before starting construction.  This allows you to put a leash on the demon.

Drift pins will transfer shear, but allow for shrinkage.  For 11 inch logs, allow at least ½" above and below the bottom of the pin.  The drawback to drift pins is that they do not prevent vertical separation of the logs.  I prefer regularly spaced steel rods from  top  to bottom of wall.

I recommend using smooth rod in place of the threaded rod.  The problem with threaded rod is that as the logs shrink, the hole through the wood becomes smaller and the logs twist.  This produces a very effective clamping force between the wood and all-thread.  The logs cannot slide down the all-thread as required which leaves a gap between the logs.  The threaded rod becomes a steel column in the wall, holding the logs apart.  I've seen instances where people have stripped the threads as they try to tighten the all-thread to close the gaps.

The slack in the rods can be taken out at the foundation or top of wall depending which detailing you prefer.  My experience has been that it takes about 7 years for the walls to shrink down.  You will probably need to re-adjust twice in the first year  and then at yearly intervals.  

The scribe fit seems to provide a more effective structure than using flattened logs.  Custom fitting the logs together is like aggregate interlock in concrete.

B. Allan Mackie from British Columbia is a recognized leader in log construction.  He has several books out on log construction  which should be available at you local book stores.

Bruce Hopper, P.E.
USKH, Inc.

***     Previous Message      ***

The method I use to transfer lateral load between log
courses is drift pins with a hole drilled through the
course being laid and the pin driven into the course 
below.  The drift pins are sized and spaced based on 
AITC Timber Construction Manual values.  The drift pins 
are usually spaced 4 to 6 feet oc and are staggered in 
each successive course.  This means works for both
round log with saddle type end construction and two and
three sided logs.  The first course of logs is connected 
to the foundations using the mud sill anchor bolts, 
couple, and short threaded rod installed through the 
first course.  

For round log construction using the saddle end, I use 
an all-thread rod running completely through the full 
stack of logs with the rod installed in the overhanging 
end that extend beyond the foundation wall.  At least one end of the all-thread rod is available for access.  
The all-thread rod is used to tighten the wall up over time as the logs shrink.  Normally the weight of the 
wall and the structure above is enough to consolidate 
the logs as they shrink but a little help is needed 
sometimes.  While access to the ends of the all-thread
is more difficult in two and three sides log 
construction this same detail has be used successfully.

Shrinkage of log construction is a major concern and the 
details of construction used must be able to acommodate
relatively large settlements.

Forrest T. Braun, P.E.
BBFM Engineers Inc
(907) 274-2236