MECHANICAL PROPERTIES OF WOODS GROWN IN UNITED STATES. 17 
formulae are discussed in the various textbooks on mechanics and 
strength of materials.) 
COMPRESSION PERPENDICULAR TO GRAIN. 
In the compression perpendicular to grain test, a block 2 by 2 
inches in cross section and 6 inches long is laid upon its side and 
pressure applied to it through a cast-iron plate 2 inches wide laid 
across the center of the piece and at right angles to its length. Hence 
but one-third of the surface is directly subjected to compression. 
The only strength value obtained in this test is the fiber stress at 
elastic limit. It represents the maximum stress which can be applied 
to the timber without injury. It is important in computing the 
bearing area for beams, stringers, joists, etc., and in comparing 
species for railroad ties. 
Two-thirds of the fiber stress at elastic limit, as given in the table 
for tests on green material, may be used as a safe stress in dry interior 
construction. 
HARDNESS. 
Hardness is tested by measuring the load required to embed a 
0.444-inch ball to one-half its diameter in the wood. This test is a 
modification of one originated by Janka. 1 
The hardness test is applied to end, radial, and tangential surfaces 
of the timber. There is no consistent difference between radial and 
tangential hardnesses and they are averaged and tabulated as "side 
hardness." End hardness is usually greater than side hardness. 
The quality represented by these figures is important in woods for 
paving blocks, railroad ties, furniture, flooring, etc. 
SHEARING STRENGTH PARALLEL TO GRAIN. 
The shearing test is made by applying force to a 2 by 2 inch lip 
projecting from the side of a block. The shearing stress is the maxi- 
mum force required to shear off the projection divided by the area 
of the plane of failure. 
Shearing strength parallel to the grain is a measure of the ability 
of timber to resist slipping of one part upon another along the grain. 
Shearing stress is produced to a greater or less degree in most uses 
of timber. It is most important in beams, where it is known as 
horizontal shear — the stress tending to cause the upper half of the 
beam to slide upon the lower. It is also important in the design of 
various kinds of timber joints. 
Only about one-eighth of the values given in the table for green 
material (Table 1) should be used as allowable stress in horizontal 
1 " Die Harte des Holzes," by Gabriel Janka, k. k. Forst- und Domanenverwalter: Mitteilung der k. fc. 
forstlichen Versuchsanstalt in Mariabrunn, Wien, 1906. 
91728°— Bull. 556—17 3 
