Engineering Timber Columns

[Atomic25]

Has anyone here designed heavy timber columns? I'm talking an amish carpenter cutting down a Southern Pine tree, debarking it, and using it as a 20' tall exposed exterior column for a roof on a house. What issues would this create?

questions in my mind are:
1. What design loads could I use since its not graded by any of the agencies.

2. Does NDS even apply to this?

3. How long would it need to age so it limits shrinkage once in place.

4. How to protect it from rotting

 

[JKW05]

This would fall outside the prescriptive requirements of the IRC, so I would say NDS would be applicable. As a starting point, I guess you could check the lowest allowable stresses listed in NDS (Timbers, not Dimension Lumber). If this is just the roof of a house, the stresses in such a large section may be well below even the lowest allowable design stresses.

You may be able to find someone at a timber supply company or the forestry department at a university that could do the grading.

 

[JAE]

You're also not kiln dried so the NDS tables might not apply very well.

 

[civilperson]

Use only the largest 5 x 7 rectangular section inscribed in the narrowest round section for calcs. Maximum allowable stress in the range of 2000 psi for extreme fiber in bending and 1500 psi for gross area of rectangle in compression or end bearing.

 

[JKW05]

JAE

Footnote No. 1 for Table 4D in the 2005 NDS says:

"For 5" and thicker lumber the GREEN dressed sizes shall be permitted to be used because design values have been adjusted to compensate for any loss in size by shrinkage which may occur."

Would that not apply to this case? Having made that note, I will admit that I conveniently ignored Atomic's question about shrinkage. Is there someplace (and enough time) where this "tree" could be dried? The moisture content can be measured using a meter.

 

[JAE]

KJW05 - thanks for the reference...I was thinking that some of the NDS tables over the past were qualified to be only for dried lumber under controlled conditions. But I think I was thinking of sawn lumber of nominal sizes, not heavier timber.

But even still, I'd be real wary of using green timber like that as too quick of drying, or uncontrolled drying in general, could result in splits and warping that would have a large effect on your structural capacities.

 

[SlideRuleEra]

Atomic25 - IMHO, what you have described is more like an untreated timber piling - the only difference is that it was not "driven". For guidance on how to evaluate it, take a look at the "Laterally Unsupported Timber Pile Study" on this page of my website:

http://www.slideruleera.net/AWPI-Page.html

http://www.SlideRuleEra.net

 

[JKW05]

JAE,

I think your concerns about the shrinkage are appropriate. Does anyone know of guidelines for proper drying procedures? (e.g. "rate of drying", etc.). Or can one just 'bake' it 'till moisture content is reduced?

 

[JJMT]

We do a fair amount of log/timber work and our specifications call for standing dead timber, of a certain species and grade. We use conservative stresses for each species and specify the minmum butt diameter. While the engineer should always consider shrinkage, this typicall becomes a craftmanship issue (log joints opening and closing). If the project is typicall wood framing, i would let it go, longiudianl shrinkage is fairly minimual (and you cant stop the seasonal change anyway). But if its not typical framing (say a W33x130 ridge beam) it is releativly easy to hog out the log and stick in a HSS. For moisture protection make sure the base of the log is elevated above the ground (perferably at the ground snow load height) and we do copper or lead flashing on the base. A note of cation beware of "character" logs, these are the most twisted convaluted sticks in the forest. Make sure these are applied and your structure is provided elsewhere.

 

[Atomic25]

JJMT, can you explain the "hogging" out process a little more? Do they rip the timber down the length and then insert the hss or do they somehow core out the center?

These timbers would be about 25' tall.


SlideRule, that's an excellent paper. thank you

 

[JJMT]

from what i have seen the typical approach is running the log through a saw mill, splitting it in two, then chain saw a cavity for the HSS, finally glue the two pieces back together. It depends on the contractor and the exact usage but sometimes they do all of this in the log yard and ship the whole column to the site. Other times they place it around the steel after erection.