Pressure Treated lumber 5

[WARose]

I got a project where I will be using Pressure Treated lumber [PT]. I have done some wood design in my career......but never with PT.....so i have a few questions:

1. I assume the same allowables (for bending, compression, shear, etc.) are the same as the base wood right (i.e. treated #2 pine has the same allowables as untreated #2 pine)?

2. What kind of shrinkage/expansion % should I use? I've heard PT is bad about shrinkage.

3. If I have to (say) notch a post.....does that surface need to be painted.....or does the treatment protect it even at it's depth?

Thanks.

 

[JAE]

1) Yes
2) Not sure about this - would have to research it.
3) Yes - further treatment is required. I cut a stair stringer from treated lumber for my OWN rear yard deck and 4 years later had to deal with rot along the cut edges.

 

[EngineeringEric]

1: Yes, unless the PT results in incising (incised lumber Ci factor i believe gets applied)

2. PT lumber when purchased is wet. like saturated wet so it will shrink a decent amount. usually it isn't a significant concern and if it is, then i'd recommend letting it dry first down to 15% or something.

3. yes. you need to treat all cut edges and drilled holes. big orange stores sell products that are most likely compatible with your PT unless you are doing something unique for a treatment.

 

[phamENG]

2) Yes - shrinkage is bad. The wood can arrive on site literally dripping wet from the treatment plant. A couple options here. You can specify Kiln Dried After Treatment (KDAT) if control of moisture is critical. That should keep you less than 20% MC. I believe 19% is the target for KDAT. Alternatively, you can direct the contractor to receive the material and stack it, under cover, and air dry it until it comes down to whatever MC you specify.

Here's a related read: Link

 

[Ceinostuv]

WARose,

Regarding 1) - Are you in the PNW? Most PT in this area ends up being Hem-Fir instead of the usual DF we see used for framing.

Judgement-In-Training

 

[phamENG]

Ceinostuv - FYI: if you click on the user name the last login location is shown in the profile. Doesn't necessarily mean it's where the project in, but WARose's last login was from South Carolina. PT is southern pine down here.

 

[Ceinostuv]

Thanks for the tip phamENG, I didn't realize that

Judgement-In-Training

 

[phamENG]

You're welcome.

 

[JoelTXCive]

After you make cuts in the pressure treated wood, you can treat with a copper napthenate solution. I think there are several different company's making products with this chemical in it.

In the last 10-15 years, there have been a bunch of changes in pressure treated woods (apparently arsenic and copper chromium are not as friendly to the environment as we once thought). You now have to specify if you want 'ground contact' or 'non-ground contact' rated materials. I think I have some homebuilding articles on the different treatment levels. They have good background info.

Would you like me to post?

 

[STrctPono]

I'm assuming this is for an interior application using a borate type treatment, but can you clarify? Because, if exterior, you're looking at a whole different type of pressure treating and implications that go with it.

1. As EngineeringEric pointed out. Incising would be the one parameter that would affect your allowable stresses. You may have seen this wood before. They make very small closely spaced indentations over the entire surface of the wood. This helps get the pressure treating to impregnate the wood at greater depths and concentrations. This, however, is typically only an issue with denser woods like Douglas Fir when using some oil-borne preservatives that don't penetrate as well as the water-borne preservatives. My understanding is that pine is not as problematic as DF.

3. Yes, coating the ends of cut pieces should be called out for on your notes. (But you'll be lucky if any Contractor actually complies). Most lumber yards or Big Box stores sell either borate based preservative (interior) or a Copper Naphthenate (if exterior treated) to brush on your cut ends.

 

[WARose]

Thanks for the replies guys.

After you make cuts in the pressure treated wood, you can treat with a copper napthenate solution. I think there are several different company's making products with this chemical in it.

In the last 10-15 years, there have been a bunch of changes in pressure treated woods (apparently arsenic and copper chromium are not as friendly to the environment as we once thought). You now have to specify if you want 'ground contact' or 'non-ground contact' rated materials. I think I have some homebuilding articles on the different treatment levels. They have good background info.

Would you like me to post?

You can.....does either one make any difference in terms of treatment after notching?

WARose,

Regarding 1) - Are you in the PNW?

Nope. South Carolina.

 

[JoelTXCive]

WARose - I do not think the 'ground contact' versus 'non-ground contact' makes any difference on how you treat cuts in the wood.

Attached are some good articles / background info. (I'm more of a hoarder than Oldestguy)

Article 1: How to Specify per Western Wood Preservers Institute

 

[JoelTXCive]

Article 2: Pressure Treated Use Category Summary

 

[JoelTXCive]

Article 3: How to Specify Per AWPA

 

[JoelTXCive]

Article 4: Pressure Treated Wood-What's the Difference?

 

[JoelTXCive]

Article 5: Pressure Treated Wood - The Next Generation

 

[EdStainless]

And the newer treatments make the wood much more corrosive toward fasteners. One reason that you see special coated screws and stainless steel screws a lot of place.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy

 

[Ron]

Older treatment chemicals (ACC-Acid Copper Chromate and CCA-Chromated Copper Arsenate) are no longer allowed. Those came out of treatment at a lower moisture content than present chemicals. Further, as EdStainless noted, the treatment is more corrosive....not only to fasteners but to any metal accessory. Take a look sometime at joist hangers (buckets) in an off-grade wood structure.....older one had little corrosion. Newer ones show white residue (zinc oxide) on the galvanizing seemingly much sooner. Generalizations to be sure, but there have been changes in the PT industry in the past 20 or so years.

1. Watch for brittleness in PT lumber
2. Excessive moisture not only creates more shrinkage but also more warp, wane, checking, etc.
3. Not sure if there's any info out there on this, but changes in structural efficacy are likely considering the changes in material properties

 

[STrctPono]

Older treatment chemicals (ACC-Acid Copper Chromate and CCA-Chromated Copper Arsenate) are no longer allowed

They were voluntarily removed by the industry from use in residential projects. They are still available for commercial projects.