Timber Tests by the Forest Products Laboratory

2,3, and 4, For the same grades, the working stresses ari in the relation of 7, 6, 5, and 4.

The use for the difierent grades are as follows: (Sl) for use where highest strength requirements obtain in exceptional places in railway and mill construction; (S2) {or general use in railway and mill construction, and wherever i select grade of strirctual material is desirable; (S3) fo'r general ule iir huilding construction and to a large extent in mill construction; (S4) for general use in small houses, false work, and ordinary construction where strength requirements are not so critical.

- In order to put into use data which may lead to the building of beiter, and more economical structures of wood, the Forest Products Laboratory conducts a short course of instruction in the uses of wood in buildings. This course is designed for the especial benefit of architects and engineers.

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The photograph reproduced here shows a beam test of a structuial timber at the Forest Products Latboratory of the LI. S. Forest Service. From tests of this sort on timbers containing defects, it was found that the factors of greatest importance affecting the strength of a timber are the number, size, and location of the knots, shakes, and checks and the amount of decay present. Based on the defect limitation, four'basic grades for structura'l timbers have recently been evolved by the federal laboratory together with accurately determined working stresses for the important commercial species in all four grades.

In the numerous tables of working stresses now in use there is a lack of agreem,ent in the values assigned to a given species, and uniform structural timber grading rules do not exist. The result is apt to be either poor construction or poor utilization. When a timber too small for its load is used an unsatisfactory structure, if not actual failure, is the outcome. When a; timrber is larger than need be, the result is, a waste of tim'ber and needless expense.

The relation of the size of defects permissible in the Forest Service grades, now named Extra Select (S1), Select (S2), Standard (S3), and Common (S4), is that of l,

Sash and Doors

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Glass and Panels

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