Neutral Axis of Full-size Lumber With Multiple Knots

Abstract

The neutral axis (NA) for a beam with isotropic, homogeneous material coincides with the geometrical centroid of the cross section. For wood beams, it is often assumed that the NA is at the centroid of the beam as well. Wood, however, is neither isotropic nor homogeneous. A multitude of anatomical features such as grain direction, grain pattern, knots, and so on provide wood with its orthotropic properties and in turn cause deviations in the location of the NA from the centroid of the beam. Knowledge of the true location of the NA would help one gain a better understanding of the mechanical behavior of wood beams. The objective of this study was to continue developing a protocol to optically analyze the NA in wood beams. Moreover, this study characterized the changes in the location of the NA with the presence of single and multiple knots in the beam. To characterize the location of the NA and its subsequent movement due to the presence of knots, a digital image correlation technique was applied to full-size lumber beams, which were loaded in flexure. The NA location was determined using longitudinal strain plots. The location and move- ment of the NA varied greatly between specimens and was affected by different patterns and types of knots in those specimens. In this testing, the average location of the NA in a nominal 2 � 4 clear beam was 50.7 % of the depth above the tension side. With multiple knots, the results revealed more complex, qualitative behav- ior. In some beams the knots in tension and the knots in compression had very nearly equal effects on the NA and, in fact, offset each other with very little movement in the local location of the NA. In other cases, when the knots were placed close together longitudinally, the NA showed little to no movement. This work marks significant progress in developing a protocol for optically understanding the effect of knots on the NA and can be used for other beam sizes and species.