Span Table for Actual-Size Lumber
I’m constantly working with 80-100 year old ACTUAL dimension pine floor joists. 2×8 are actually 2×8, the 2x10s actual 2x10s.
Compared to their slimmer decendants, these beefier joists can certainly carry more weight over longer spans. The problem is, inspectors (and many structural engineers, sadly) treat these as nominal framing.
I’ve never seen even a meager attempt at a span table for actual size lumber. I realize that without a structural stamp, its hard to tell the exact attributes of such wood. Still, an actual dimension lumber span table would be useful in planning which floors are sufficient for modern loads, and which are not.
Has any one seen such a span table? if so, please forward.
Cheers,
Aaron
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Replies
Aaron,
I'm not an engineer, but I found this while Web surfing:
"A [planed] 2x8 is actually only 1.5 x 7.25 inches. If you have full-dimension lumber it is about 1.33 times stronger by being 2 inches instead of 1.5 inches (goes directly with thickness), and about 1.34 times for being 8 inches vs. 7.25 (goes as the cube of height)." — http://ths.gardenweb.com/forums/load/oldhouse/msg1021090512629.html
I can't vouch for the information, but I'm providing it for what it's worth. Engineers are invited to respond with more detailed information.
I was looking for this information myself. Luckily I am a Mechanical engineer and have access to finite element analysis code.
My results are as follows.
A nominal 2x10 (1.5x9.25) deflects .009 inches under a 300 kg centered point load with both ends of the beam fixed in all directions.
An actual 2x10 (2x10) deflects .0064 under the same loading conditions.
.0064/.009 = approx 71%
Thus, the modern, smaller, nominal "2x10" is approximately 71% as stiff as an actual 2x10 of the same material.
Invert this and you have that the real 2x10 is approximately 140% as stiff as the smaller beam.
I hope this helps all.
This rough-sawn lumber span table comes from The Visual Handbook of Building and Remodeling by Charlie Wing. It's been published by several publishing houses, including The Taunton Press. My version was printed by Rodale Press in 1990.
The printed version of 'From the Ground Up' by Wing and Cole has everything you need and ought to be required reading. I use it all the time.
An updated version, especially for energy matters would be very useful
The California Building Code (CBC) has a roof rafter span table with true dimension lumber. Though, in our area the older lumber tends to be 2" true by 3.75" true, a bit fatter than the table. It's pretty clear this is all a guess.
I'd much rather have a method where I could apply a specified load to the actual building, and measure actual deflection. This is especially true since in our area these roof structures spent their first 50-90 years supporting a heavy tar and gravel roof, yet now are being asked to hold only a solar array and modern roofing.