Log Shear Walls with Chink Joints 2

[TTUengr51]

I've read through some of the previous threads pertaining to log shear walls (thread726-101643 and thread337-25324) and have gained some good insight. However I would like to take this question one step farther. The log home client I deal with uses rectangular shaped logs (6x12) with a 4" chink joint between each log course. The chink joint consists of wood squash blocking at intervals no greater than 96" on-center with styrofoam block filling the remainder of the joint.

In regards to the horizontal shear capacity of the shear wall, we have been utilizing the steel pipe method of anchoring the logs together for horizontal shear while using all-thread rod at wall ends to combat uplift due to overturning.

A question has been posed by a contractor who has been building log homes on the west coast for years who has been utilizing another system that uses only 1/2" threaded rods for shear and uplift resistance. He had this system engineered several years ago by a Arizona PE.

My concern with this system is the ability of the threaded rods to resist bending that would be created at the chink joint. I understand that if the logs were on top of each other, the dowel-bearing yield states would govern. However with the chink joints, wouldn't the 4" gap create a moment arm on the threaded rod? I have used this reasoning with the pipe system I am currently using and it governs the allowable load versus the dowel-bearing check.

Are my concerns valid, or is my approach overly conservative? Any ideas or comments would be greatly appreciated.

 

[DaveAtkins]

I understand what you are thinking, but perhaps you will get better results if you assume the 1/2" rods are CLAMPING the logs together--they only take tension (no shear or bending), and the shear is transferred via friction through the squash blocking.

DaveAtkins

 

[TTUengr51]

The clamping thought makes sense, but if the squash blocks are the only element you are counting on for shear resistance, wouldn't there have to be a substantial amount of squash blocking to resist the shear in friction?

My biggest concern with this would be quantifying the amount of friction you could count on. With the clamping action, I would imagine you could count on a bolt pre-tensioning force along with the dead weight of the wall. However with wood shrinkage/settlement over time, the advantage created by the pre-tensioning would diminish.

Any ideas on a coefficient of friction?

I could see this idea working for a log system with a log to log connection with no chinking in a low shear application, but I'm still a little skeptical in using it with the chinked system I have. Especially in high wind regions where I'm getting of 400 plf of shear on my walls.

 

[RockEngineer]

I agree that with the 4" wide chinking strips you will get some bending in the all thread. I have used the pipe concept before in log trusses when I needed more bearing area or had bending that couldn't be handled otherwise. With the squash blocks 8 ft apart you don't get a lot of shear resistance out of them. The system you talk about probably also has dovetailed corners which give you some locking but are hard to put a number to. I haven't engineered this type of log home although I have engineered many other types. I would think the pipe even though it is a pain for the builder is a fairly good solution.

 

[dik]

The system likely functions by the shear force being transferred as tension through the bolts in much the same way that shear friction works for concrete. I'm not sure how to quantify this.

 

[RARWOOD]

Although I've seen some engineers add in friction caused by the clamping action of the bolt in ledger connections, it is not something I would do.

The problem with assuming friction action, is the fact that wood shrinks as it dries out. I would think in most applications, wood shrinkage would relieve what ever friction that was intially present.

 

[dik]

Sorry... to clarify. I wasn't trying to imply that you should use the clamping friction... too variable due to shrinkage properties. Shear friction in concrete works by 'transforming' the shear into tensile forces in the reinforcing.

Dik

 

[RARWOOD]

dik

Actually utilizing friction maybe a common assumption. While looking for information on another topic I stumbled upon an article about log shear walls.

Still not sure that I would use friction. Then again I don't design any log structures.

If one does a Google search for Wood Design Focus, they can probably find several articles on log structures.