Log building shear walls 3

[mavmerl]

I am designing a log home, and would like some informaiton on the allowable shear strength of the walls. The logs are lagged together. I have used the formulas in the NDS lag bolt section to get values. These values are lower that the numbers that I have seen used on other projects, but I have not been able to justify higher values. Does anyone have any more information about using log walls as shear walls?
Thanks

 

[Trussdoc]

Have you considered the friction between the upper and lower planes of the logs as part of the shear resistance?

Also, isn't there a log home manufacturers trade association that may have done tests? One would think that with the popularity of log homes and the need for seismic design in many areas, there would be some test or study of the subject.

 

[RockEngineer]

I have been engineering various types of log homes for 8 years. There are several methods that engineers use to develop the shear in log walls. There are also a lot of particular things you need to watch out for in log homes.

You stated that you were using lag bolts to connect the logs and trussdoc suggested you could use friction between the logs. This is an option but their are some problems. Initially you tighten down the lag bolts to get a good compression between the logs. Depending on whether you are using green or dried logs you will get shrinkage in the logs that will loosen your lag bolts. If properly installed you countersunk you lag bolts so that as the logs shrink they will not lift up the log above them. (The shrinkage can be up to 1/2" per foot of wall height. The problem in counting on friction is what to use for the weight (normal force) especially in a seismic evaluation. If you really look at ground motion the weight of the logs above may not be there to give you the large friction resistance you are looking for especially since the lag bolts are loose due to log shrinkage and the logs above can be lifted up slightly. The larger numbers you saw other engineers use were probably because they used the dowel loads on the lag bolts rather than lag bolts in single shear. The rational for this is that you have lag bolts which are a minimum of 1-1/2 logs long which can be from 12 to 15 inches. These are spaced on an average of 24" to 36" apart depending on the loads you are resisting. In this situation they don't act like a bolt in single shear they act more like a dowel and so engineers use the dowel bearing from NDS 97. This is a lot higher value. They do the same thing when they spike the logs or use thru rods. It may not be the most scientific method but with all the things you have working for you that you can't quanitify it is still conservative.

If you are not familiar with all the problems that can go along with engineering and building log homes, the Canadians have a book called "Log Building Standards for residential, Handcrafted, Interlocking, Scribe-fit construction". It is available through the Canadian and American Log Builders Association International. They have lots of books and a monthly journal for members. They have a web sight

http://www.logassociation.org.

The standards are there for a free download under resources. They don't help much on engineering but they do give you things to watch out for.

Trussdoc suggested there may be some shear wall tests by some of the log building organizations. I have looked for this but have not found any. The log builders are usually too loosely organized to have the money for shear wall tests and there are too many methods of fastening the logs together. The engineers are left to their own ingeniuty. Do you like engineering by the seat of your pants? Welcome to My World. It's a good thing that log homes are so strong compared to framed construction. The worst thing is their mass tends to work against you in seismic loading and there are a lot of unknowns like what resistance do you get from notching. Some old log homes with nothing but corner notching have resisted some pretty good earthquakes.

Good luck. If you have more questions I'll be around. Log homes can be fun and challenging.

Richard Rock
Rock Engineering
Port Orchard, WA

 

[LarryGlickfeld]

RockEngineer... I think I've spoken to you before re. log construction. Anyhow, two issues here:
1) Not clear how/why you justify dowel bearing strength vs. tab values for bolts in wood. Would still seem that the failure mode is the wood giving out.
2) With the thru rod construction, the rods are generally not snug, e.g., the holes are over-sized, so I would think the only shear resistance would be thru friction created by tightening the rods. Of course, this is a problem as logs shrink or settle. I know that spring loaded washers are often used to compensate.

Amazing that there still seems to be no industry standards for log wall analysis. By the way, I have observed some failures, or buckling action, of log walls, but generally due to sloppy construction, or occasionally foundation settlement.

 

[RockEngineer]

The only justification I have come up with for using dowel strength in the pins is that there is a complex interaction. I have looked at the final failure mode. Even though the holes on the thru rods are oversized, before the wall can completely fail you have to either shear the rod or destroy the log with the rod. I realize that by the time the rods bear on the hole that the wall has had significant movement but the rod is the mechanism that stops the total failure. This is why some engineers always insist on the spring loaded rods or other methods that ensure friction between the logs. Some may disagree but I feel in this situation my goal with the rods is to stop complete failure if there is a loss of friction between the logs. The walls may move under this very unlikely and unusual situation but they will not fall down. This is similar to my reasoning on dowel strength rather than the lower bolt strength. It is an engineering judgement based on experience and one I feel comfortable with and am willing to take the liability for. I feel much more confident with the strength of most of the log homes I have engineered than with the framed homes.