log homes shearwalls
log homes shearwalls
(OP)
How do one calculate the shear strength (plf)of a 6" log wall. Each course of logs are lagged together with 3/8" dia x8" screws @ 24" o.c.. What about the openings (doors and windows).An adjustment factor can be applied for perforated shearwall (framed).
What about holddowns at the end of it shearwall segment. There's not a corner post to attach a simpson connector.
What about holddowns at the end of it shearwall segment. There's not a corner post to attach a simpson connector.






RE: log homes shearwalls
I would check the lags at the lowest log for shear and pullout. Use all the lags along the entire length of log, taking into account butt joints between logs.
You are correct that you cannot use Simpson holddowns, so 60% of the dead load on the shear wall must be able to prevent overturning of the shear wall due to lateral loads.
DaveAtkins
RE: log homes shearwalls
I've also seen details where logs are connected together with pipe 7/8" x 19 1/2" spaced at 96" o.c. and the shear is stated to be equal to 270(max.). Does it means that the pipes, determine the value of the shear for the log wall?
A 3/4" through-bolts c/w nuts and timber washer is install at each end of the logwall. Will this bolt take care the overturning moment acting at the wall?
thanks
RE: log homes shearwalls
Even in framed housing there is usually a lot more problem with racking of the wall than there is with overturning unless you have a high aspect ratio where the wall is much taller than it is long.
RE: log homes shearwalls
RE: log homes shearwalls
RE: log homes shearwalls
thanks
RE: log homes shearwalls
Some designs have the thru bolts on springs or other automatic tensioners to allow for wall shrinkage.
Having the bolts near the corners and on both sides of openings takes care of the uplift similar to the Simpson ties on framed construction.
If you look at a lot of old log structures they have weathered many wind storms and earthquakes and many have never even been tied to the foundation. As a engineer you need to follow your numbers though. It can give you a little piece of mind to know that there are forces at work which hold your structure together that you are not taking into account in your calculations. It is difficult to account for friction between logs, notching and all the other things that go into a well built log home.
RE: log homes shearwalls
I would just like to clarify again about the 4 ft spacing of the through-bolts, it seems to me that the bolts are so closely spaced. Is this because your also designing the bolts to carry also the shear forces? Or are you placing this together with your lag bolts,or maybe dowel or pipes?
RE: log homes shearwalls
If you are using lag bolts or spikes or pounded in rebar you usually will not include thru bolts. You will just attach the bottom log to the foundation and let your lag bolts, etc, provide your shear resistance between logs and the uplift resistance.
There is another thread in eng-tips.com about log home shear wall design which may help. Thread337-25324 I believe there are some others too. In this thread I think I was a little confusing about dowel bearing. If you use the bolt bearing strengths and all the additional factors for wind and seismic you will get a reasonable spacing for you bolts, screws, spikes or rebar. Look at the NDS for bolting and the formulas gives you some extra shear for having relatively long bolts and thick members connected together.
RE: log homes shearwalls
RE: log homes shearwalls
RE: log homes shearwalls
I laugh when I talk to engineers for big companies when they tell me they can't make any changes to their design or provide engineering without full scale tests to see how it would perform no matter what their numbers tell them. I have very seldom had the luxury of full scale tests. Log home builders don't have that type of money and aren't that organized. They also use too many methods which react differently. Use your brain and a little common sense. Look at how things really work. Take advantage of all the additional factors the codes and NDS give you. If you get too conservative you won't have any clients and it will be too difficult to build.
Good Luck and enjoy.
RE: log homes shearwalls
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RE: log homes shearwalls
Remember with all log homes there is a great deal of shrinkage (up to 1-1/2” in 8’ high wall) and the design has to allow for this. So if you have a thru bolt it has to be tightened as the wall settles. There is a builder in Pigeon Forge TN. That has a great idea. The threaded rod has a high strength spring between the top log and the Nut. This maintains tension on the rod even after shrinkage. I believe they space the rods at 48” o/c.
RE: log homes shearwalls
The Canadian log building standards published by the International Log Builders Association recommend allowing for up to 1/2" per foot settlement when building with green logs. That would be 4" of settlement in an 8' wall. I have had some builders who recommend designing for up to 3/4" per foot settlement.
There is one method of log home building I know of that doesn't have the wall settlement. The Log Home Builders Association of North America teaches a butt and pass method where 1/2" rebar is pounded from one log into the log below every 24". The logs shrink toward their centers so the gap between the logs gets bigger but the wall height does not change. It is very labor intensive and must be used with chinking. It has lots of shear resistance due to the rebar. This method is popular with people building their own log homes because it doesn't require a lot of skill just lots of hard work.
For those engineering log homes the link Boo1 gave to the draft ICC log home standards is one of the few places you can get the allowable stresses for TPI graded logs without going directly to TPI. TPI is one of the few timber grading societies that grades full logs for log homes or other uses so engineers have an allowable stress to work with. Using sawn timber values from WWPA or NDS is not always practical for logs because WWPA and most grading societies don't grade the raw logs. They grade the sawn timber.