Extreme Fiber Stress in Bending of 2 x 12 Lumber from 1911
Extreme Fiber Stress in Bending of 2 x 12 Lumber from 1911
(OP)
I am in need of making a conservative estimate for the extreme fiber stress in bending (fb) of non-visually graded lumber that is estimated to have been installed circa 1911.
Application:
Estimate what load roof framing originally designed for (Dead, Live, and Snow).
Determine if roof can handle additional load.
Knowns:
One-story commercial building, built 1911, NJ, 16 ft. Flat roof, minimal pitch.
Built-up Bitumen Roofing with reflective coating membrane.
4-bays of 24'wide X 40'deep.
Each bay spanned by 24'long 2 x 12 lumber rafters.
I have good information on the current loading and dimensions; the only thing missing is a conservative estimate for the extreme fiber bending stress of wood typically used for that type of construction in 1911 in New Jersey.
Thank you.
Application:
Estimate what load roof framing originally designed for (Dead, Live, and Snow).
Determine if roof can handle additional load.
Knowns:
One-story commercial building, built 1911, NJ, 16 ft. Flat roof, minimal pitch.
Built-up Bitumen Roofing with reflective coating membrane.
4-bays of 24'wide X 40'deep.
Each bay spanned by 24'long 2 x 12 lumber rafters.
I have good information on the current loading and dimensions; the only thing missing is a conservative estimate for the extreme fiber bending stress of wood typically used for that type of construction in 1911 in New Jersey.
Thank you.






RE: Extreme Fiber Stress in Bending of 2 x 12 Lumber from 1911
Spruce & White Pine: fb = 750 psi
Oak: fb = 1000 psi
Yellow Pine: fb = 1250 psi
You can download the book (free .pdf) from this page of my website:
http://www.slideruleera.net/miscellaneous.html
See page 234.
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RE: Extreme Fiber Stress in Bending of 2 x 12 Lumber from 1911
That's a great start. What an impressive collection of great engineering thinking and data from what I perceive to be an era of peak engineering deftness, genius, creativity, and clarity. Reading further into the collection I realize I have a bit more to think about, for instance the natural effect that age and duty exact on the the wood fibers and the weakening thereof.
Thanks again.
a
RE: Extreme Fiber Stress in Bending of 2 x 12 Lumber from 1911
RE: Extreme Fiber Stress in Bending of 2 x 12 Lumber from 1911
RE: Extreme Fiber Stress in Bending of 2 x 12 Lumber from 1911
Make an estimate of what part has been elastic deflection and what part creep.
Add 150% to 400% additional deflection to the instantaneous elastic nominal deflection from creep?
See how your estimate/s of the creep effect corresponds with your estimate of the elastic nominal modulus in sight of the standing load and actal current deflection. Select the most likely model or bracket between two models of creep.
Project then the deflection behaviour under the attempted loadings according to your estimates. See if structural strength seems adequate.
RE: Extreme Fiber Stress in Bending of 2 x 12 Lumber from 1911
RE: Extreme Fiber Stress in Bending of 2 x 12 Lumber from 1911
"A Grading Protocol for Structural Lumber and Timber in Historic Structures"
http://w
"Structural Values of Old Lumber"
http:/
"History of Yard Lumber Size Standards"
http://ww
"Effect of Damage on Grade Yield of Recycled Lumber"
htt
"Evaluation of Lumber Recycled From an Industrial Military Building"
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RE: Extreme Fiber Stress in Bending of 2 x 12 Lumber from 1911
Standing deflection (not in the drawing) from levels taken at support and center and maybe auxiliary line.
This way one can measure reasonably enough without complicated means with some aluminum rule with no offset to 0.
RE: Extreme Fiber Stress in Bending of 2 x 12 Lumber from 1911
RE: Extreme Fiber Stress in Bending of 2 x 12 Lumber from 1911
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Extreme Fiber Stress in Bending of 2 x 12 Lumber from 1911
RE: Extreme Fiber Stress in Bending of 2 x 12 Lumber from 1911
In the US, as I understand it, the older wood came from original growth trees, so much of that lumber was much denser and stronger. And much of that may have been taken near the center of the tree. Today's forests are grown like agricultural land, with fertilizer and other factors fostering faster growth. They often are cut down at a much earlier age then the original growth (yeah, I know, obvious).... My experience with older wood, even 75 year old S Pine here in FL (which is nothing special), is it is much denser and difficult to nail or cut as Mike said.
Sorry if this is not very technical, I am no forester or wood expert...
RE: Extreme Fiber Stress in Bending of 2 x 12 Lumber from 1911
One thing I would ask is using the density and/or hardness of a wood as a measuring stick of its strength? When I look at the allowables for oak or maple (which are certainly harder and more dense than others), they are actually lower than the allowables for Doug-Fir, Hem-Fir, etc.
RE: Extreme Fiber Stress in Bending of 2 x 12 Lumber from 1911
Wood is such a special beast since it is composed of a plant cellular structure versus all of our other engineering materials. But if a dense wood was not strong, ie, poor in tension, that would be analogous to concrete- good in compression, hard and tough to drill through, but poor in tension. For wood it must be explainable on a cellular or fiber level.
One interesting property of wood is trying to cut along the grain. I once tried to run a chainsaw parallel to grain on a big log of pine, and albeit my chainsaw and my skills are not that good, but I quickly gave that up.