The major defining characteristic of lumber cut from trees that have been infected with the mountain pine beetle is the extent of fungal bluestain in the sapwood. To determine whether this bluestained lumber differs in its strength properties from non-stained lumber, small clear wood tests and a test on a truss connector were conducted.
Fourteen mills were approached and asked to provide an equal number of samples of bluestained and non-stained 2 x 4 in. lumber. Approximately 270 pieces each of bluestained and non-stained samples were collected and delivered to the Forintek Vancouver laboratory for conditioning and processing into test specimens. Small clear bending and toughness test specimens, meeting the general requirements of the standard test method ASTM D143, were prepared from an equal number of bluestained and non-stained lumber pieces. A subset of the bluestained and non-stained lumber sample was also selected and used to prepare metal plate-connected tension splice specimens. The three tests and the measured mechanical properties were judged to be sensitive indicators of any possible effects of bluestain on the structural performance of full-size lumber. For bluestain, an impact on the clear wood strength or the strength of the connector could be considered a precursor to a possible reduction in the structural performance of full-size lumber. Direct tests on full-size lumber tend to be confounded by the presence of strength-reducing growth characteristics such as knots or slope of grain, and are therefore more suited for quantifying a particular effect once it has been confirmed to exist.
The following results were found:
Wood with beetle-transmitted bluestain and non-stained wood have comparable clear wood bending properties and truss plate grip capacity.
The observed lower mean toughness of bluestained wood compared to non-stained wood was found to be only marginally significant (p = 0.05). There does not appear to be any difference at toughness levels below the lower quartile of the strength distribution.
The small differences that appear to be associated with bluestain (5% decrease in mean toughness, and 5% increase in mean truss plate connector grip capacity) are more likely to be masked by differences in the mechanical properties of the heartwood and sapwood, and, in the case of full-size lumber, by the presence of strength-reducing growth characteristics such as knots and slope of grain.
Insects - Attack on trees
Stains - Fungal
Pinus contorta Dougl. var. latifolia - Mechanical properties