Structural Composite Lumber In Trusses
The lumber used in most metal plate connected wood trusses is either visually- or mechanically-graded solid sawn dimensional lumber. However, the Truss Plate Institute's ANSI/TPI 1 allows the use of structural composite lumber (SCL) products such as laminated veneer lumber (LVL), laminated strand lumber (LSL) and parallel strand lumber (PSL). These engineered wood products can conceivably compete with sawn lumber and complement it in truss designs. The qualification in ANSI/TPI 1 is that the truss designs incorporating these products must be reviewed and approved by the truss designer. Even though the option is available, some structural composite lumber may not be practical in metal plate connected truss applications. As a truss manufacturer, how can I incorporate structural composite lumber (SCL) into my truss designs?
In theory, any type of lumber product can be used in wood trusses as long as the lumber and plate design values for that product are available. In reality, you should check first with your software provider to see if they have values for the product you are considering and second with your engineer to see if this is something that can be readily incorporated into the design process.
We are aware of several products that have been tested and evaluated by software suppliers. Because these engineered products can have significantly higher strength properties than standard lumber grades, it makes them a good choice for girders, attic frame bottom chords or tail-bearing top chords. Some other advantages to higher strength structural lumber products are that longer lengths reduce the number of chord splices, greater strength-to-size ratios allow smaller or custom member sizing, no knots, and less shrinkage and splitting. These advantages do come at a cost – SCL is generally more expensive per board foot than sawn lumber – but if a larger attic room can be created or a girder ply can be eliminated, SCL adds value. The overall value must be calculated as part of the entire cost equation: inventory, design, manufacture, installation, site performance and customer satisfaction.