ct of a single
factor, such as moisture, is the object of experiment, it is
necessary to use small pieces of wood in order to eliminate so
far as possible all disturbing factors. If the specimens are too
large, it is impossible to secure enough perfect pieces from one
tree to form a series for various tests. Moreover, the drying
process with large timbers is very difficult and irregular, and
requires a long period of time, besides causing checks and
internal stresses which may obscure the results obtained.

On the other hand, the smaller the dimensions of the test
specimen the greater becomes the relative effect of the inherent
factors affecting the mechanical properties. For example, the
effect of a knot of given size is more serious in a small stick
than in a large one. Moreover, the smaller the specimen the
fewer growth rings it contains, hence there is greater
opportunity for variation due to irregularities of grain.

Tests on large timbers are considered necessary to furnish
designers data on the probable strength of the different sizes
and grades of timber on the market; their coefficients of
elasticity under bending (since the stiffness rather than the
strength often determines the size of a beam); and the manner of
failure, whether in bending fibre stress or horizontal shear. It
is believed that this information can only be obtained by direct
tests on the different grades of car sills, stringers, and other
material in common use.

When small pieces are selected for test they very often are
clear and straight-grained, and thus of so much better grade
than the large sticks that tests upon them may not yield unit
values applicable to the larger sizes. Extensive experiments
show, however, (1) that the modulus of elasticity is
approximately the same for large timbers as for small clear
specimens cut from them, and (2) that the fibre stress at
elastic limit for large beams is, except in the weakest timbers,
practically equal to the crushing strength of small clear pieces
of the same material.[57]

[Footnote 57: Bul. 108, U. S. Forest Service: Tests of
structural timbers, pp. 53-54.]

MOISTURE DETERMINATION

In order for tests to be comparable, it is necessary to know the
moisture content of the specimens at the zone of failure. This
is determined from disks an inch thick cut from the timber
immediately after testing.

In cases, as in large beams, where it is desirable to know not
only the average m