Tensile strength classes for hardwoods

Generally, for structural timber the main properties are listed in a strength class system. In Europe EN 338 (2016) lists available strength class and their respective characteristic properties. Depending on the product application the bending or the tension properties are of main interest. For glulam lamellas, tension classes are preferred as through tension classes the mechanical properties of glulam can be better predicted. Also, machine grading allows for a better prediction of tensile properties compared to bending properties. So far, tension grades were regulated in EN 14081- 4. Due to the increased demand on glulam products, the grades were recently analysed, and a tensile class table was introduced in EN 338 (2016). The new strength profiles are the preferred option for glulam production. The requirements for softwood glulam are regulated in EN 14080 (2013). So far, no tension classes exist for hardwoods. The EN 338 lists for hardwood specimens only so-called “D” classes based on the bending strength. The other properties, like tension strength, are assigned by the default on the safe side using the equations listed in EN 384. For those classes, the ratio tension strength to bending strength is declared on the safe side 0.6. However as shown by Burger & Glos (1997) for higher quality timber higher ratio can be expected. New tensile classes for hardwoods would allow utilising the properties of hardwoods more efficiently. Whereas the old European standards EN 1194 and EN 14080 (2005) did not regulate species, the most recent version of EN 14080 (2013) is restricted to softwoods only. This means that hardwood glulam producers in Europe face additional costs for obtaining approvals for their products.