Log Beams 1

[Kentfish1966]

I have been asked to help the Amish design log beams for a log home. They are using locally cut white pine trees for the logs. Does anyone know what design values I should be using for the bending strength, shear strength, and modulus of elasticity? I thought to use the values of #3 visually graded lumber but this would fall more into the beams and stringers category. In Wisconsin, because this is ungraded lumber, we are supposed to use values for #3 material. If you look at beams and stringers there is no #3 grade. Any assistance would be much appreciated.

Thank You,
Kent.

 

[msquared48]

Contact the International Log Builders Association in Lumby, BC, and ask them to send you their small papmphlet entitled "Log Span Tables for Floor Joists, Beams and Roof Support Systems." It has a wealth of information.

Also, if the logs are not machine stress rated, you ususlly assume the stress level one or two grades lower than a visual rating.

Their fax is 250-547-8775, and the address is:

ILBA
PO Box 775
Lumby, BC V0E 2G0

Unfortunately, I do not have their office phone number, but you can probably Google and get that.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[dhengr]

Kent:

I believe the ICC has some standards for log grading and building with logs. Where in WI are you, that they’re sill cutting nice white pine? It might behoove you to have the logs graded for the better values you might be able to use.

 

[a2mfk]

I once had the pleasure of studying an Amish log cabin up close, built atop a very scenic hill in western Wisconsin. They know some engineering of sorts, though a few things I could not revolve using statics, but overall I was very appreciative of the skill and craftsmanship.

With all that said, I am no help to your problem. I think whatever conservative stress you end up using the size of the members will not control, it will be more about connections and stability concerns?...

 

[RockEngineer]

Timber Product Inspection (TPI) has the best values and info for designing with full round logs. Values for what they classify as wall logs are given in the link below. Their rating of unsawn logs as premium, select and rustic is unique. If using ungraded logs that are nice straight logs without a lot of knots the use of select or rustic values is appropriate in my opinion. You aren't allowed to use the properties for the round log circle for calculating Section modulous and moment of intetia. ASTM spec for milled logs (can't remember the number off the top of my head) requires you to use the properties of a fully inscribed rectangle. This is good for milled logs but overconservative for full round logs. I usually use NDS requirement for round piles which lets you use the properties of a square with equal crossectional area as the log.
If you do need the logs graded and there is a TPI inspector in your area, they will usually come on site and do an inspection and grading of all the logs for a very reasonable price. Separate out your beams and girders which need higher values from the wall logs to make the inspection easier.

http://www.tpinspection.com/download/LogProgramTechnicalGuide.pdf

The log span tables Mike McCann referred to by ILBA don't break items down by grade at least in my old copy. In talking to them they assumed similar to a WWPA #1 for the species even if it isn't graded.