Justine (a Hansen Buildings Project Coordinator) recently needed to order some glu-laminated columns for a pole building kit package. One of our vendors recommended she instead order Parallam® Plus PSL columns instead, insisting they would be as strong as or stronger than glu-lams.
Parallam® is the brand name for an engineered wood product developed by Trus Joist MacMillan which is made from veneer strands laid in parallel alignment and bonded with adhesive. It is used for beams, headers, columns, and posts, among other uses.
The rated strength of Parallam® is greater than the wood from which it is made. This is because knots and other imperfections are removed so strength variability is less than in solid sawn wooden beams.
When I served on the board of directors of the MSRLPC (Machine Stress Rated Lumber Producers Council), we had the opportunity to tour MacMillan’s Parallam® manufacturing plant. Truly an amazing process, as the product is continuously formed – it is extruded, not unlike plastic pipe! This means the maximum length of the beams are limited only by the maximum length capacity of the plant.
The product is produced as a huge rectangular “slab”, which is then cut down to popular sizes – widths of 3½”, 5¼” or 7″ and depths between 9¼” and 18″.
Parallam® Plus PSL columns are typically manufactured in three sizes: 3-1/2” x 5-1/4”, 5-1/4” x 5-1/4” and 7” x 7”. For use as a structural building column, Weyerhauser has simplified product design and specification by grouping the AWPA (American Wood Preservers Association) Use Categories into Service Levels. As such, the Service Level for exposed posts in contact with the ground would be SL 3.
In this application, the Parallam® Plus PSL columns would have a Fb (fiberstress in bending) value of 1344 psi. Multiplying this value by Sm (section modulus of the column) gives a value of 32,413.5.
Most typically, Hansen Buildings utilizes glu-laminated columns as manufactured by companies such as Timber Technologies, LLC, which have a Fb of 1897.5 psi. With a standard 3 ply 2×6 column measuring 4-1/8” x 5-3/8”, Fb x Sm for the glulams equals 37,688.7.
While the Parallam® Plus PSL columns are a great product, the vendor was incorrect in his advice to Justine, as the glu-lam would be over 16% stronger in bending. This is where it pays off to do investigative research on a product, rather than just relying upon what may perhaps be errant information from a vendor.
What you fail to understand or convey to the reader, PSL Plus being an extruded product, is that treatment of the lumber is through and through. There is no need to dress any incisions and the product will never decay. It is specifically designed for burial, therefore only a footing with stainless steel connections are required.
Thank you for your input, there would typically be no nails or screws used below grade, so stainless steel connectors would not be required. Above grade, HDG (hot dipped galvanized) would be recommended.