huangyhg

log wall strength
how do i calculate the bearing capacity of a stacked log wall?
while the main run of a typical log wall is probably not a problem i am concered about concentrated loads from beams or dormer trusses coming down on a short section of a stacked log wall which has no bracing walls attached to it.
should i just calculate the slenderness based on the log contact area and screw the logs together with lag screws to handle my worst calculated moment?
because the logs shrink the lag screws could loosen but perhaps this is negligable on the length of a lag screw.
try
dr. edwin j. burke
wood science laboratory
school of forestry
university of montana
missoula, mt 59812
(406) 243-5157 fax (406) 243-4845
short sections of stacked logs can be a problem because they really don't act like a column. they want to buckle and roll under compressive load. usually you will try to have at least two continuous logs above such openings to provide lateral restraint. these two logs need to be connected together with lag bolts or some other means to make them act together laterally and possibly vertically as a beam. you may also want to have inset posts at these openings that provide lateral resistance but have vertically slotted connections to the logs to allow settling. there are standard details for these window and door bucks that most log home builders have. what you do depends on the construction method. there are as many ways to build log homes as there are log home builders and new ideas come up every day. good luck. engineering log homes can be fun and challenging.
log homes can be rather confusing and defy most conventional fea programs. i don't know the exact situation you are reviewing but most walls have little moment on them because second floor/roof trusses bear directly on the wall with a simple pin-type connection. your concern about buckling is legitimate, although i have very seldom seen a log wall buckle or bow. (i used to manage an engineering dept. for a log home company.) it really depends on the method on construction and log profile as to the actual bearing area. i generally use the actual contact area between the log courses for bearing area. if the logs are full length and extend from corner to corner (not spliced in the middle of the wall) you really don't have much to worry about, because as stated by rockengineer the courses above the opening are full length and act as giant header for the point loads. plus the wall acts more as a diaphragm when all the logs are screwed together than individual logs.
different companies solve (or say they did to the customer) this problem in many different ways. some use 3/4" rebar rods or bolts drilled through the log courses to stiffen the wall. (and the buckling capacity of a 3/4" 8 foot tall rod is?...i don't think that really does much except to keep the logs from warping out of position. the key is to reduce any eccentricities from loading and make sure the logs are stacked directly on top of one another. "rough bucks" or a tongue and groove system in the window frame is mandatory to allow log settling without crushing the window. oly-log or log hog brand screws work very well and are usually used every 2-4 feet. i wouldn't recommend using traditional lag screws. the ones mentioned have a small head and actually countersink themselves about .25 inch allowing for log shrinkage without inducing stress on other logs.
i hope this helps.
alex

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