Pressure Treated Stress Losses 3

[LPPE]

Anyone have these values?

 

[redhead]

There is no reduction in allowable stresses when wood is treated with preservatives. However, this is not true for fire retardant treatments. The amount of reduction, if any, must be obtained from the process manufacturer.

 

[LPPE]

OK. Thanks.

PT engineered wood products do have a reduction in strength. Do you think the preservative treatment weakens the glue used in engineered wood products?

 

[Taro]

If the wood is incised as part of the PT process, there is an incising factor that may decrease your allowable stresses. See NDS section 2.3.11.

 

[redhead]

I think you may be correct in your assumption about the glue in engineered wood products. Some years ago "Trusjoist" produced pressure treated "Microlam" beams, but then they stopped producing them. I believe they were having delamination problems.

They are now producing pressure treated "Parallam" beams with reduced strength factors, probably to allow for reduction in the strength of the glue, as you presume. It will be interesting to see how these products weather in the next ten years.

 

[boo1]

In the US 80% of PT wood is Southern Pine. Southern Pine is one of the few wood species that does no require incising. Design values, apply to both treated and untreated Southern Pine. Design values for dimension lumber are based on normal-use conditions (Moisture Content =19%. These values are intended for use in covered structures, or where the moisture content in use does not exceed 19% for an extended period of time.

The use of proper fasteners and connectors for treated wood framing is also very important. Standard carbon steel nails and fasteners may rust and corrode with time causing unsightly stains and possibly an eventual failure to hold securely. When building with pressure-treated, use hot-dipped galvanized or stainless steel nails and fasteners that are manufactured to resist corrosion. (Note: mechanical-galvanized nails may be adequate for enclosed interior treated framing applications only). All connecting hardware such as joist hangers, hurricane straps, lag screws, and bolts should also be galvanized.