Historical Timber Values?

[Archie264]

I'm looking at a document I pulled off of Slide_Rule_Era's website, "Pole Building Design". In an example on pg. 13 it lists the design value of wood poles using Douglas fir or Southern Pine as "8,000 psi, USA rating." The document was first published in 1957 and revised several times, the last being 1969.

So, my question is, were design values that high really used then? Or is the "USA rating" some specific criteria of which I'm unaware? Or am I missing something?

 

[Lion06]

I'm not sure what was used then, but what is also important is the E value. I typically find that dimension lumber is controlled by strength and engineered lumber is controlled by deflection. With an Fb of 8000 psi, all lumber would almost certainly be controlled by deflection (and you wouldn't get to take advantage of that strength) unless E were much higher as well.

 

[msquared48]

Yes, but with a pole structure, depending on the useage, you can get away with larger deflections.

I wonder if that 8000 psi value was an ultimate value, with no safety factor for actual design. Seems awfully high to me.

Mike McCann
MMC Engineering

 

[XR250]

Could be ultimate with a short load duration.
That is certainly plausible.

 

[BAretired]

No such high stress was used in the Canadian code at that time. I no longer have my 1955 NBC but there was no sudden change in permissible stress between 1955 and 1970 that I can recall. The allowable flexural stress for Light Framing members in 1970 was 2200, 1850, 1500 and 850 psi for Select Structural, No. 1, No. 2 and No. 3 respectively. For Joist and Plank sizes, it was 1900, 1600, 1300 and 750 psi for the same grades.

BA

 

[Archie264]

It is indeed ultimate; good call. The exact reading is, "Douglas fir or Southern pine with an ultimate fiber stress of 8,000 psi, USA rating."

And good call on the short duration as well. At the end of that page and on the beginning of the next one the author discusses load duration. An excerpt: "Wind load, on the other hand, is a transient, fluctuating load. Wen its maximum occurs, it is an extreme that lasts for only a breif period. These extremes seldom occur in conjunction with maximum live loads."

Y'all are good!

 

[Archie264]

PS: it still seems high to me but in the context of what y'all wrote at least now I can wrap my mind around it. Thanks.