Old Timber Design Values
Old Timber Design Values
(OP)
Hello, I feel like I recently came across a post regarding this but i am unable to locate it.
I have a building that i am working on which was built in the early 19th century and this isn't uncommon from one my firm's large clients. We often analyze existing structures for current loads, residential. Often the structures are abnormal rough hand scrapped lumber. sometimes great quality workmanship and sometimes not.
I was wondering if anybody has any information about the first or early timber property design manuals? old design values are really what i am looking for. I would feel confident with our analysis if I knew how old timber compared to modern. Most of what i see is mixed-red oak and sometimes mixed pine, poplar and cedar finishes.
I know deflection is still an issue but with these structures deflection isn't an issue, it is nearly always bending stress or shear governed. The client doesn't care about occupant comfort (bouncy) or finishes as much as keeping it all original, it is my job to see if original is sufficient for loads.
I have a building that i am working on which was built in the early 19th century and this isn't uncommon from one my firm's large clients. We often analyze existing structures for current loads, residential. Often the structures are abnormal rough hand scrapped lumber. sometimes great quality workmanship and sometimes not.
I was wondering if anybody has any information about the first or early timber property design manuals? old design values are really what i am looking for. I would feel confident with our analysis if I knew how old timber compared to modern. Most of what i see is mixed-red oak and sometimes mixed pine, poplar and cedar finishes.
I know deflection is still an issue but with these structures deflection isn't an issue, it is nearly always bending stress or shear governed. The client doesn't care about occupant comfort (bouncy) or finishes as much as keeping it all original, it is my job to see if original is sufficient for loads.






RE: Old Timber Design Values
BA
RE: Old Timber Design Values
RE: Old Timber Design Values
1. My older mentors (started their careers in the 1927-1947 range) would almost always use Fb' = 1000 psi. Of course they didn't call it F'b but simply the maximum stress that they'd let wood experience (service level loads).
2. If you need to use something to correlate your engineering judgement here ... i.e. what another reasonable engineer would do ... and need to tag to something published, the only thing I can see you could do under the building code would be to do a load test on the floors. This is very expensive (for a residential use especially) but that would at least give you something to depend on as a rational basis for telling a homeowner of a 1800's era house it is "safe".
3. Take a wood sample and test it for flexure and shear. I'm not sure of any ASTM tests for this - and when I ask testing labs about testing samples of wood I've gotten nowhere. There are labs that can take samples and tell you the species...but I'd be afraid that older wood might be more dried out or rotted and a direct species-to-NDS table look up would be meaningless.
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RE: Old Timber Design Values
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RE: Old Timber Design Values
For testing wood properties, I suppose one should look for a lab which performs ASTM D4761
http://www.astm.org/Standards/D4761.htm
A quick google came up with these:
http://www.wpcinfo.org/codes/documents/WMEL_IAS_Sc...
http://www.appliedtesting.com/tests/mechanical-pro...
RE: Old Timber Design Values
RE: Old Timber Design Values
RE: Old Timber Design Values
The next challenge would be to assert a safety factor as the test, I'm sure, only provides an ultimate failure load, not an allowable.
I've always had a hard time finding out what NDS uses for wood safety factors. I guess we could go LRFD in the wood design but even then, you
need to also determine a 90% confidence line for values from the tests.
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RE: Old Timber Design Values
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Old Timber Design Values
First, thank you all.
Second i do mean early 19th (ie 1830 one-eight for this particular project!) I know there were not design manuals for the period so i was mainly looking for information as old as i could go back to see how things have changed (property specific). Testing this wood isn't really an option but i have done that before on different projects.
I think the minimum req't is take 3 samples (I've used 12 so not sure if 3 is truly allowed). test to failure. then make sure you use an ultimate value that is 95% confident it will work. then apply an Omega (factor) of 1.67 for F'b. I think it was 1.67 but now i have to many #'s floating around. Verify the above as now i lost confidence...
I am concerned with the increase of hardness but i am not sure how that would result in a decrease in bending stress... but that is a good valid point.
I will check out the documents that you guys all gave. Thank you so much for thinking about this one for me!
RE: Old Timber Design Values
RE: Old Timber Design Values
http://www.amazon.com/G-P-M-Goss-Structural-Handbo...
Mike McCann, PE, SE (WA)
RE: Old Timber Design Values
RE: Old Timber Design Values
Conjecture here: I think the reduction in bending values, from dealing with older wood materials myself, some as hard as concrete, involves the loss in pliability of the material and increase in overall rigidity, increasing the tendency for a brittle failure of the material. The material also has a greater tendency to crack across the grain, decreasing notches that increases tension stresses.
Mike McCann, PE, SE (WA)
RE: Old Timber Design Values
RE: Old Timber Design Values