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Which part of a log makes the strongest beam?

Started by wesdor, September 08, 2013, 08:32:58 PM

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wesdor

Not sure this is the correct place to post this question, so if it belongs somewhere else, please point me in the right direction.

I am up in the Cascade mountains of Washington in a very remote place.  We have limited internet and even more limited phones.  I will be sawing for another 4 days and hope to get to some 30" Douglas Fir logs.  One of the fellows in charge of a major remodel project has asked for 8x8 beams 16' long.  We have the length and size to do it, but he wants them cut with the pith centered in the beam (1 beam per log).  He says this is the strongest possible beam.

Others are saying that the 8x8 beams outside the pith would be stronger?  Can anybody weigh in on which beam would be strongest for structural support?  I probably won't get to the logs until Tuesday, but would like to leave the best possible cuts for the project before I leave on Friday.

Thanks in advance.

rendezvous

Hi; Wesdor, sounds like your are in Holden.  I don't know for sure but have heard if the beams are heart center they wont twist as much.
Greg

Carpenter

I'm going to preface this statement with, I Don't Know.

With 30" logs you could really quarter those and get 4 8x8's free of heart center.  I don't know, but it's just a gut feeling that the free of heart center beams would actually be stronger.  Boxed heart beams will check and usually those checks run to the heart, is what I've read and observed.  I've sawn some free of heart center 8x8s from ponderosa pine and of course they checked some but nowhere near as much as the boxed beams.  But, I haven't sawn enough of them to say that this is always the case.  I don't get a lot of ponderosa big enough to do that with.  As far as actual strength?  Several thoughts on the subject, but as I really have no idea what I'm talking about I'll leave it there.

   Oh, and I would saw it any way the customer wants it.  After all he's the one paying the bill.

beenthere

D.fir will behave some different from other species, so sticking just to D.fir, I don't see one, single answer to the question. Strongest will be dependent upon where the knots and defects are in the logs you get, the growth rate of the logs, as well as the size of the small end.
If you need to satisfy the "fellow in charge", then you have the "answer" how to saw. IMO. ;)

And a log can't be sawn the two different ways for you to make a comparison (even if you had a machine to do the bending). And near impossible to get two logs that match and saw each one by the different methods and compare.

Likely for the design of the building getting the 8x8 beam, it will make no difference. If the 8x8 is pith in the center, then there will be a lot of volume in jacket boards or lumber. And centered may make the best overall yield possible.

Keep the boss happy with your work. ;)
south central Wisconsin
It may be that my sole purpose in life is simply to serve as a warning to others

Brad_bb

Strongest based on milling?  Really the milling is done to get good stability.  Depending on how it's milled, what species is is etc, will affect it's stability as it dries.  Also how it's treated after mililng, whether it's left out in the sun or kept in the shade(much preferred). If in winter and it goes into a heated shop, you're going to get quick drying out of the outer layers of the timber creating internal stresses.  You want drying to occur cool and over time.  Not quickly.   If it can be cut green and assembled in the frame before it really dries out, being held in the frame will help keep it more stable as it dries too. 

For Doug fir, you're probably best off boxing the heart for stability and cutting boards off the outside if there's enough material. 

When it comes to strength, the timber framer has to look at where the timber will go first.  You need the best timbers for horizontal beams like tie beams.  Posts can afford the most defects as they are in compression.  A framer needs to also evaluate the particular timber for defects, and where they occur in the length and location on the timber.  I can't go into all the details here, but sufficed to say that you have to understand where stresses will occur on a timber and select an acceptable timber based on it's individual characteristics.  So the strength of a timber has less to do with how you mill it versus the individual characteristics and defects of that timber.  You won't know until you cut them.  Even once on site, the timber framer may cull several timbers that may not work for that frame.  It's nice to have extra to choose from, or be able to order more.

Anything someone can design, I can sure figure out how to fix!
If I say it\\\\\\\'s going to take so long, multiply that by at least 3!

Piston

When my wife and I were planning on having a timber frame home custom built for us in NH, we visited a very reputable builder in the area, Davis Frame.  I toured the shop and spent quite a bit of time with one of the salesman, I was very impressed with their work.  I asked one of the shop guys if they used all boxed heart timbers.  He said they use ONLY free of heart Douglas Fir timbers, unless specified by the client. 

I was surprised to hear that as I've always thought you should use boxed heart timbers. 

They're reasoning was that only one face will check badly, which will be easily hidden in the frame, as opposed to all 4 faces most likely developing checks in boxed timbers. 

My understanding is that boxed timbers will be more stable when drying, but check on more faces.  Free of heart centers may dry in a parallelogram shape, and come "out of square" after drying. 

Strength wise, I can't comment on.  I believe either would be sufficient, and if the method of sawing is what determined whether an 8X8 would be strong enough or not, then in my opinion you are cutting it way too close, and the timber size should be increased. 
-Matt
"What the Lion is to the Cat the Mastiff is to the Dog, the noblest of the family; he stands alone, and all others sink before him. His courage does not exceed his temper and generosity, and in attachment he equals the kindest of his race."

giant splinter

Wesdor,
If you have enough log to get a verticle grain beam out of it or perhaps you can get 4-8X8's out of a big enough doug-fir, the lumber grade for a verticle grain beam is SEL-STR or select structural and with doug-fir it is the strongest when the grain is oriented correctly. Here is a link to the grade type and helps explain the use.
Boxed heart is fine if the loading is not heavy and it is stable as far as twisting, SEL-STR is also stable and has superior weight carrying capacity. http://www.bearcreeklumber.com/species/fir_selstruct.html
roll with it

Carpenter

Yes, I was going to mention the orientation of the grain.  It doesn't show on lumber strength charts that I know of but, any baseball player will tell you that it's important.

wesdor

Rendezvous - You are correct, I'm up at Holden.  How do you know about Holden?

Talked with a fellow last evening who spent 17 years as a structural engineer.  He gave it a lot of thought and agreed that a boxed heart would be strongest.

Yes Giant Splinter, vertical grain would make a much stronger log.

We have a small DF on the deck to be sawed first thing in the morning.  We will for sure have a boxed heart 8x8 and it has mostly VG.  After reading all your comments, I'm thinking of measuring again to see if we can get 2 more 8x8 beams (perhaps 4). 
If we can't get more beams, then we need to cut CVG flooring (1 1/8 x 3) 

Some of this wood is very impressive and fun to work with.  The beams will be used for the remodel of porches on the chalets.  They won't have quite as much load bearing as something holding up the main floor.

Thanks all for your excellent input.  Internet up hear is really slow but once I return home, I'll try to post photos.  Most of all that means I have to stop sawing long enough to take photos.

Only 3 more days to saw, so we are trying to get the most production out of each day.

RayMO

Beams will be more rigid with less compression with the grain oriented up and down under the load verses a boxed heart .
Look at the grain orientation on any properly installed wood handle ....
Father & Son Logging and sawing operation .

rendezvous

Wesdor: Just a good guess. I bought the Mobile dimension mill they use to have.
The clue was no phone and slow internet.  I remember trying to communicate with them.
  Good luck on your job ,beautiful country.
Greg 

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