Extreme Fiber Stress in Bending of 2 x 12 Lumber from 1911 3

[autechre]

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.

 

[SlideRuleEra]

From "Carnegie Pocket Companion - 1903":

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.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[autechre]

Thank you SlideRuleEra.

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

 

[a2mfk]

Make sure you do a good inspection, especially of wood that is 100 years old. It may be in great shape or split and checked all to hell.

 

[autechre]

Thank you a2mfk

 

[ishvaaag]

Measure the additional short term elastic deflection for a small moderate additional load. Estimate current standing elastic modulus.

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.

 

[ishvaaag]

In the second paragraph I refer to the 100 year long loading.

 

[SlideRuleEra]

autechre - The Forest Products Lab has done a lot of work on evaluation of aged lumber. Here are some other documents (free .pdf downloads) that may help you with the project:

"A Grading Protocol for Structural Lumber and Timber in Historic Structures"

http://www.apti.org/publications/Past-Bulletin-Articles/Anthony-Dugan-40-2.pdf

"Structural Values of Old Lumber"

http://ir.library.oregonstate.edu/xmlui/handle/1957/2502

"History of Yard Lumber Size Standards"

http://www.mastercodeprofessional.com/construction_files/History_of_Lumber_Sizes_1964.pdf

"Effect of Damage on Grade Yield of Recycled Lumber"

http://www.fpl.fs.fed.us/documnts/pdf1999/falk99c.pdf

"Evaluation of Lumber Recycled From an Industrial Military Building"

http://www.fpl.fs.fed.us/documnts/pdf1999/falk99a.pdf

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[ishvaaag]

Image attached, mason method to amplify the increase of deflection by 4 for more easy measure. The two upper pulleys or hooks to be installed under the center of the deflecting beam. The descent of the weight is 4 times the deflection.

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.

 

[ishvaaag]

I know, I know, even myself have one (ultrasounds?) measuring device. Laser it is not.

 

[msquared48]

The lumber will also probably be hard as nails, requiring pre-drilling even for nails.

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

 

[ishvaaag]

msquared48, this is not always the case, my family owns a village "tapial" house maybe 200 year old and the pine trunks that span between bearing walls (around 6 m or a bit more where most) are not as hard. You can hammer nails normally with a hammer.

 

[a2mfk]

Ish- probably depends a lot on the species and environment, faster growing conditions I think can lead to softer wood that is not as strong (given the same species).

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...

 

[Lion06]

This thread has been interesting to me.

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.

 

[a2mfk]

Lion- That is interesting and counter-intuitive. I don't know what the correlation with density/hardness and strength is, but I know sometimes they do correlate like with some tropical hardwoods like teak. We have a couple of wood experts and guys who do woodworking on this board that may chime in, I don't have the time to research this. But then bamboo is very strong in tension, but not very tough at all and easy to shear (machete)...

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.