Slope of Grain and Its Measurement
One reason knots have such impact on the strength capacity of a timber is because of the distorted grain angle that occurs as the tree grows around the branch. When logs are milled into lumber, the areas of distorted grain can be cut so that segments of grain “run out” at one or several locations along a board’s length rather than extend parallel along the entire length of the board. The same effect can occur if the board is milled at an angle that is not parallel to the grain, or if the entire log is twisted due to spiral growth patterns in the tree. Areas of cross grain, or where the grain runs out, create deviations in the way stresses are transmitted throughout the piece and concentrate stresses where the wood fibers have been discontinued and significantly weaken the element.
Slope of grain is generally measured as a ratio of rise to run, that is, the number of inches the grain slopes upward or downward within a given distance (generally 8, 10, 12, or 15 inches) that is parallel to the long axis of the member (See Figure 6). As with knots, while the location of slope of grain along the length of a bending member (within the middle third of the length or in the outer two-thirds) also affects the performance of a beam, this was not taken into account to provide a level of simplicity to the field measurements. This again results in a conservative limitation on knot size for the outer third of beams.
Typically, slope of grain must extend for 10 inches or more to be considered a grade-limiting defect. Only the most severe slope of grain needs to be checked; however, localized grain deviation around a knot should not be measured to determine slope of grain. To measure slope of grain, a tape measure (Figure 6) or an acetate sheet with a printed ½-inch grid is needed (Figure 7). The length of the area that appears to have significant slope of grain should be measured first from an axis parallel to the long axis of the member. If the length of the slope-of- grain area exceeds 10 inches, then the acetate grid or tape measure can be used to establish the rise over run ratio. To do so, one edge of the acetate grid or the end of the tape must be aligned parallel to the long axis of the member. The total number of inches for the rise can be determined along the vertical axis by counting from the lowest point of the rise to the highest point of the rise (or wherever the grain crosses the edge of the acetate sheet or triangulated from the tape as seen in Figure 6). The total number of inches in the run can be determined along the horizontal axis by measuring with the tape or by counting across from the lowest point of the rise to the highest point of the rise (or wherever the grain crosses the edge of the acetate sheet). This ratio can then be reduced to represent the actual slope of grain over a given length (generally 8, 10, or 12 inches) to determine the appropriate grade.
Since seasoning (drying) checks in timber generally follow the slope of grain, determining the slope of grain on painted timbers can be achieved relatively easily simply by examining drying checks. All visible faces of the timber should be examined for slope of grain, as not all faces will exhibit the same extent of the slope of grain. While not always conclusive, this approach closely correlates with results achieved using laboratory methods for measuring slope of grain that are not practical for field use.
Seasoning checks, which are separations between wood fibers that do not fully penetrate the width or thickness of a member, are common in structural timbers and rarely affect the performance of a wood member. Splits are separations of wood fibers that extend completely through the width or thickness of a wood member. Short splits typically do not affect the performance of a wood member but long splits should be evaluated by the engineer if there are concerns about shear strength in beams or buckling in columns.