Tentative Grading Rules for Timber

TENTATIVE GRADING RULES FOR TIMBER The following tentative rules are for the purpose of strength classification only, and do not take into account requirements of a general nature such as conformity to dimensions, proportion of sap, or other requirements made necessary by peculiarities of certain species.

Grade 1 Timbers (a) Must contain only dense wood.

(b) Must not have Class 2 or large Class 1 knots in volume 1.

(c) Must not have large Class 2 knots in volume 2.

(d) The aggregate diameter of knots on any face within the center half of the length shall not exceed the width of the face.

(e) Must not have shakes or deep checks.

(f) Must not have diagonal grain with a slope greater than 1 inch in 20.

Grade 2 Timbers (a) Must contain only dense wood.

(b) Must not have large Class 2 knots in volume 1.

(e) The aggregate diameter of knots on any face in the center half of the length shall not exceed two times the width of the face.

(d) Must not have shakes which extend along an annual ring a distance greater than the width of the piece.

Classification of Timbers

As the result of the application of the proposed grades to the species tested, the Forest Service classifies them in order of strength as follows: Class 1 Timbers—To include Grade 1 timbers for longleaf pine, shortleaf pine, loblolly pine, and Douglas fir. Shortleaf pine and loblolly pine, however, generally contain quite a large proportion of sapwood, which is not nearly so durable as the heartwood. Therefore unless these species were treated with a preservative, they should be excluded from this class and put into Class 2.

Class 2 Timbers—To consist of Grade 2 longleaf pine, Grade 2 Douglas fir, Grade 1 western larch and hemlock, and Grade 1 tamarack.

Class 3 Timbers—To include Grade 1 redwood, Grade 1 Norway pine, Grade 2 shortleaf pine, Grade 2 loblolly pine, Grade 2 tamarack, Grade 2 western hemlock.

This classification is based entirely upon the strength developed by the timbers tested, and does not take into consideration other properties which may be desirable for any particular use. For example, the durability of the different species is influenced greatly by the amount of sapwood which the timbers contain. Shortleaf pine, loblolly pine, Norway pine, and tamarack usually contain a considerable proportion of sapwood. All of the other species mentioned can be secured, as a rule, in dimension sizes practically free from sapwood. If, however, the timbers are to be given a preservative treatment, sapwood may be an advantage, since it readily absorbs creosote and other preservatives.

The tests of the Forest Service upon structural timbers lead to these conclusions: (1) The mechanical properties of timber beams are dependent upon: a, The quality of the wood irrespective of defects; b, the character and location of defects.

(2) The mechanical properties of wood free from defects vary directly with its dry weight. The relative dry weight of the different pieces of wood of any species can be approximated by comparing the proportion of summerwood in each.

(3) The only defects which materially decrease the breaking strength of timber beams are the more serious ones, such as large knots and cross-grains occurring where fibers are subjected to comparatively high stresses.

(4) All the species tested seem to be subject to the same general laws regarding the relation of mechanical to physical properties.