Testing historic timber for strength

[tpe1973]

I am working on a balloon framed, masonry veneer warehouse structure from around 1910 that the client would like to reuse as retail/office and residential space. My initial analysis using modern design strengths for the wood members indicated that quite a bit of strengthening would be required for the intended use.

The building is constructed of southern pine 2 x 12 joists and beams (beam is 4 laminated 2x12) with white oak 8 x 8 columns. All are planed to approximately modern dimensions. Visual inspection led me to believe that the wood was stronger than what modern code allows due to the density of the members and the overall stiffness of the floor system.

The owner was concerned about the cost of the originally proposed remediations and agreed to remove members to be tested. I had samples prepared and tested according to ASTM D143. Using the results from these tests, I calculated allowable loads considering defects (Grade of wood) and safety factor according to ASTM D245. The results I obtained were approximately 2-3 times higher than modern code allowable stresses. Now that I have these very high results I am reluctant to use them even though it seems that the AF&PA allows it.

Has anyone ever done something similar? Did I miss something in my analysis or are these indeed the loads I can use?

 

[hokie66]

That is not surprising. I have experienced the same thing. But while the allowable stresses may be much higher than you are accustomed to, deflection and thus the Modulus of Elasticity is probably the most important thing. Just curious, what allowable stresses and E did you find from the testing of the old SYP?

 

[msquared48]

I posted a question asking for similar information re testing on old timber a couple of months ago in another string. This is very interesting, and quite frankly, what I expected.

Mike McCann
McCann Engineering

 

[msquared48]

As hokie66 says, please post your results.

I have felt the same way for years after the situation I described in that other string.

Personally, for the axial and bending I would have no problems with 1.5X bsased on your 2 to 3 times finding.
The E valus isvery important too.\


Mike McCann
McCann Engineering

 

[tpe1973]

Hi guys, thanks for the input.

Results are as follows:

SYP:
Measured ultimate bending stress = 13,000 psi
Allowable bending stress = 3500 psi

Measured MOE (from midspan deflection) = 4.0e6 psi
Allowable MOE = 4.3e6 (I was a little surprised there is no reduction and the FS is 0.94)

White Oak (This stuff had more than 12 growth rings per inch)
Measured comp. parallel to grain (allowable) = 6820 (2550) psi
Measured comp. perp. to grain @ d=0.04" (allowable) = 3250 (1400) psi

So according to my results your well founded concerns about stiffness do not play out in the results.

The bending tests were interesting though. The wood had a moisture content less than 10% and they all ruptured suddenly shortly after the proportional limit.

Does anyone have any insight into what the NDS allows regarding this?

Thanks, Patrick

 

[OlafTheBlack]

Check the USDA Wood Handbook. You will see the strength properties of wood ranging by a factor of two. The code allowable stresses factored for safety are based on the low end of the range. Most likely your old building is made from old growth wood having strength from the upper range. You could use the tested samples to justify a higher loading capacity. You just need to have sufficent samples and use the ASTM procedures for determining allowable stresses.

 

[JAE]

Patrick,

Honestly, I don't have a lot of familiarity with the two ASTM documents you referenced. But in the ACI code, the "abrupt" failure mode of shear in beams and slabs results in a lower [φ] factor to account for the lack of long term warning of failure that you find in the concrete beams failing in flexure.

I would suggest, if the ASTM documents do not consider the abrupt failures of your test samples, to account for this in your allowable stresses.

Also, did your samples represent an average type of quality in the joists? What I'm getting at is that your samples only represent a very small percentage of the total joists that are there in the structure.

The level of safety factor prescribed by the NDS may be due to
the variability of properties of lumber produced by the major timber organizations

....and NOT due to
the variability of the quality/properties of your in-place joists.

This is an important distinction to consider....again - don't know if the ASTM docs cover this.

 

[jheidt2543]

I found similar results in two 1896 and 1907 heavy timber industrial buildings. I would suggest contacting the U.S. Forest Products Lab in Madision and discussing values with them. They get pretty excited discussing wood used in really old buildings! If needed, they have some pretty nifty non-distructive testing equipment too.

In my two buildings, the moisture content of the heavy timber trusses (6x8 & 8x8 members with up 70' span) and columns was under 5%. But there were some areas of old water damage that had caused "dry rot" with mushrooms the size of softballs - really!

Just think, they built buildings like this by hand without computers!

 

[dik]

Old timber/lumber is significantly stronger and you should research to determine values or determine the stress level and work backwards.

Older timber/lumber has closer spaced annual rings... the trees were more mature, they are better cured/aged and they often have a greater concentration of extractives within the wood itself.

Dik