Peeled Log Design Stresses 5

[JAE]

We've got an upcoming project which involves a museum-like space. The owner is interested in using a combination of Glue-laminated beams and roof deck as well as other areas with round, peeled logs used in trusses.

We can handle the GLB work OK. We've also found a few entities out there (in Sweets and from other sources) that supply these logs as well as their design. However, they are all quite expensive and we may have a local source for these logs...just want to know how to verify/specify/determine, etc. a set of allowable stresses for them. NDS provides values for kiln dried wood of various species and grades but these would probably be non-kiln dried and we may not actually know the source.

Would it make sense to either test the logs, or to assume a very low value? The log trusses will most likely be quite oversized anyway for aesthetic reasons.

Any thoughts?

 

[dison]

JAE,

I think it would be OK for this design to use a conservative value of allowable stress based on the species of the source tree, especially if aesthetics governs the member sizes. It may be worth a trial run to see what allowable stress would be required with the member sizes necessary for aesthetics.

How would the timber be dried if not by kiln? In order to get the best structural properties of the wood, it would need to be dried. If the drying process cannot be controlled, I think testing samples for moisture content might be appropriate. Sorry, I don't have a source for correlating strength & moisture content though.

I am far from an expert in timber design, so don't over-estimate the value of my opinion. ~dison

 

[Qshake]

JAE, I'm not an experienced timber designer either but have experience in dealing with performance specification of structural material/systems that are uncommon. In light of that, I recommend your approach with the specification and testing for the local material. Unfortunately this will increase the cost of the local wood but it certainly would eliminate much haggling over material produced by a source not necessarily familar with the safety critical needs of our industry.

In the meantime, I will search our library for some material on peeled timber - sounds interesting and I like to get lost in our library on Fridays!

 

[Ron]

JAE..I, too, would take the testing approach. The tests are not that difficult (bending(modulus of rupture, modulus of elasticity), compression perpendicular and parallel to grain, ) and also not that expensive (a set of same can be performed in a three-specimen set for about $1500), though there would be some sample preparation (sawcut to size, etc.) that would run the ticket up just a bit.

Just recently did this series for evaluation of 100-year old timber structure.

 

[phuduhudu]

JAE,

I did some of this recently and the critical areas are the joints. It is very difficult to develop anywhere near the full strength at the joints. Expecially since with this type of construction people normally want to see a single bolt connection. You can improve things with toothed ring connectors but your joints are still likely to be critical.
Carl Bauer

http://www.bauerconsultbotswana.com

 

[JAE]

Thanks for the replys!

I agree that some type of testing would offer assurance against my assumptions. Just not sure if our local testing labs know how to get the values from say, a 12" diameter log.

The connections are definitely a concern per your comment, Carl. A local conference center (home of the Arbor Society) has a facility that uses logs with single gusset plates knifed into the log with through bolts. One bolt will not occur on any of my connections.

While the trusses are fairly small and low-load - I still would like to have some kind of handle on what I'm doing.

 

[Ron]

JAE...would be glad to talk your local lab through the process. Let me know.
Ron

 

[JAE]

Ron, is there an ASTM test process developed or is it more a matter of a simple set-up and procedure learned through experience?

dison: regarding the kiln dried - most log homes are not kiln dried for the mere fact that the logs are too big to fit in a kiln and they typically design the home to shrink (i.e. door frames, cabinets, etc. are designed for the vertical contraction that is expected). Thus, I assumed that for a log truss, the logs would not be kiln dried but perhaps only partially dry through time and exposure. This, I'm sure, affects the allowable stresses but not sure how much.

 

[Ron]

JAE..this is where to start...
ASTM D4761 Standard Test Methods for Mechanical Properties of Lumber and Wood-Base Structural Material

Also, check ASTM D-198 (similar but for dimensional lumber)

Both standards reference others for calculation of properties

Ron

 

[Ingenuity]

Just as a side bar re testing...there are rapid load testing techniques available today that are full scale, portable, and very cost effective. I do not think it would work real well in your situation JAE where you are trying to get a "handle" for a new design (unless you want to do a prototype), but for existing strcutures, or prototype testing it works well. We have done a few on existing concrete structures in a period of several hours - not days - with virtually instant results, and lots of $ savings.

 

[BuddyShowalter]

Here's an article that might save you the expense of testing. Grading agencies, like the one list in this article could provide visual inpection and certification of the logs.

Also, the ASTM standard for logs is shown below. The problem with testing is that you need a much larger sample size than three to get good results.

"Visual Stress Grading of Wall Logs and Sawn Round Timbers Used in Log Timbers", by Edwin J. Burke, Ph.D., Univ. of Montana, BOCA Magazine, 31(2): 28-34.
Abstract: The author of this article, a professor of Wood Science at the Univ. of Montana's School of Forestry, explains the need for, history of and process of stress grading of logs used in the construction of log homes, and offers practical information for code officials encountering this type of construction system.

Dr. Edwin J. Burke
Wood Science Laboratory
School of Forestry
University of Montana
Missoula, MT 59812
406-243-5157
406-243-4845(F)

ASTM Standard D3957-90(1996)e1 Standard Practices for Establishing Stress Grades for Structural Members Used in Log Buildings distinguishes between two types of sawn or machined timbers with different grading rules for each. Sawn Round Timbers and Wall Logs are defined.

Hope this information helps.

Buddy Showalter, P.E.
AF&PA/AWC

 

[JAE]

Thanks much!!!

 

[sven]

JAE,

One problem with "peeled" logs is that peeling removes the strongest timber from the log. It also destroys continuity in the outer fibres of the timber.

If you decide to go with tabulated values for your species of timber, I would recommend applying a capacity reduction factor of 0.7 to allow for peeling.

I agree that the only worthwile testing would be full-size samples and this can be difficult in practice.

There is a useful Australian Standard, which give design values for "natural" and "shaved"(peeled) timber poles, assumed green (unseasoned) and based on species. I assume that something similar is available for your local timbers?