Figure 51
Diagram of specific gravity apparatus, showing a balance with container (c) filled with water in which the test block (b) is held submerged by a light rod (a) which is adjustable vertically and provided with a sharp point to be driven into the specimen.
Air-dry specimens will be dipped in water and then wiped dry after the first weighing and just before being immersed for weighing their displacement. All displacement determinations will be made as quickly as possible in order to minimize the absorption of water by the specimen.
STRENGTH VALUES FOR STRUCTURAL TIMBERS
(From Cir. 189, U.S. Forest Service)
The following tables bring together in condensed form the average strength values resulting from a large number of tests made by the Forest Service on the principal structural timbers of the United States. These results are more completely discussed in other publications of the Service, a list of which is given on [pages 157]-159.
The tests were made at the laboratories of the U.S. Forest Service, in cooperation with the following institutions: Yale Forest School, Purdue University, University of California, University of Oregon, University of Washington, University of Colorado, and University of Wisconsin.
Tables [XVIII] and [XIX] give the average results obtained from tests on green material, while Tables [XX] and [XXI] give average results from tests on air-seasoned material. The small specimens, which were invariably 2" × 2" in cross section, were free from defects such as knots, checks, and cross grain; all other specimens were representative of material secured in the open market. The relation of stresses developed in different structural forms to those developed in the small clear specimens is shown for each factor in the column headed "Ratio to 2" × 2"." Tests to determine the mechanical properties of different species are often confined to small, clear specimens. The ratios included in the tables may be applied to such results in order to approximate the strength of the species in structural sizes, and containing the defects usually encountered, when tests on such forms are not available.