Cm for Pressure Treated Wood

[DTS419]

Pressure treated wood is typically kiln dried to moisture content of 19% or less like most lumber before it is treated. After treatment, the moisture content can be up to 75%. We've probably all seen fresh pressure treated wood from the lumber yard that practically bled water when cut. However, this applied moisture content is a bit different from green moisture that exists in the cells of the fiber; the moisture content from chemical treatment purportedly dries much faster. The question is, does this wet condition count as moisture content >19% for the purpose of determining Cm per NDS Table 10.3.3? Or is this condition ignored since it is not the same as green moisture?

 

[phamENG]

C_m has nothing to do with green vs seasoned/kiln dried. C_m is based on in service moisture. So no, moisture content due to treating has nothing to do with determining your wet use factor.

 

[DTS419]

Yes, the moisture content matters "At Time of Fabrication" per Table 10.3.3. When the wood shows up from the lumber yard freshly saturated with treatment chemicals, does this count as >19% At Time of Fabrication?

 

[phamENG]

Oh, I missed the table reference. Sorry about that. What version are you using? I'm guessing something that predates CLT, since chapter 10 is now Cross Laminated Timber. I'm going to go with 11.3.3 from the 2015 and 2018 versions, Wet Service Factors for Connections.

As I understand it, fabrication means when the connection is made. Which is why I have prohibitions in my spec from using sopping wet lumber from Home Depot. So if it shows up dripping wet and over 19% and the cut it, drill it, and bolt it up that day - yes, it would be greater than 19% at fabrication.

 

[ChorasDen]

the moisture content matters "At Time of Fabrication" per Table 10.3.3

Not sure I'm following here, Chapter 10 of NDS is CLT, and there is not a Table 10.3.3 that I am aware of. Did you mean to quote section 10.3.3? If so, why are you looking at pressure treated CLT?

 

[ChorasDen]

I'm going to go with 11.3.3 from the 2015 and 2018 versions, Wet Service Factors for Connections.

Thanks Pham, that likely clears this up. Back to the original question, water bourne treatments are most prevalent, but not the only treatment carrier used for wood products in the US. Oil based treatments are also in use, particularly in places like Hawaii, so make sure you know what treatment is being applied to the wood you are sizing. This is doubly important as different chemical treatments have different required minimum retentions, and this can affect the time in the pressure cylinder and the level of pressure used to impregnate the treatment into the wood fiber, which would indirectly affect the moisture level due to treatment time.

My suggestion is that unless you are quite confident that the treated wood will reach a kiln dried state during framing, I would use the appropriate adjustment shown in the table. Secondly, the table shows the same Cm factor is applied for at fabrication time as well as in-service moisture content, and in that case, why are you working with pressure treated lumber if it's not expected to get wet?

edit: I see, it's been a while since I've looked at this table, I forgot about the significance of the fabrication moisture content on the connection design.

 

[DTS419]

Sorry, I should have clarified my table 10.3.3 is from NDS 2012.