Abstract
The paper deals with the development of grading rules for structural timbers, the principles involved, effect of structural features on strength, and the application of the principles in commercial practice. Structural grading rides are based on the provisions of the American Lumber Standards, developed by the combined efforts of lumber producers, distributors, specifiers, and consumers, under the auspices of the U. S. Departments of Commerce and Agriculture. Structural material is divided into classes according to size and use: horizontal load-bearing members such as joists, planks, beams, and stringers, and vertical compression members, as posts and columns.
The strength of wood members is influenced by the density of the wood, moisture, seasoning, and the size, location, and number of strength-influencing factors present, such as knots, shakes, checks, and slope of grain. Heartwood, sapwood, and wane are more important from other points of view than for their influence on strength. Principles of structural-material grading have been applied in a practical manner by lumber-manufacturers’ associations in drawing up commercial grading rules.
Safe working stresses for various grades and different conditions of exposure have been established in accordance with strength of clear wood, effect of strength-influencing factors permitted, the effect of seasoning, time of loading, and the characteristics of each species. Basic principles can also be applied to general construction material carried in retail lumber yards, thus insuring maximum strength and efficient utilization.
