~Safe load~ is the load considered safe for a material to
support in actual practice. It is always less than the load at
elastic limit and is usually taken as a certain proportion of
the ultimate or breaking load.

The ratio of the breaking to the safe load is called the factor
of safety. (Factor of safety = ultimate strength / safe load) In
order to make due allowance for the natural variations and
imperfections in wood and in the aggregate structure, as well as
for variations in the load, the factor of safety is usually as
high as 6 or 10, especially if the safety of human life depends
upon the structure. This means that only from one-sixth to
one-tenth of the computed strength values is considered safe to
use. If the depth of timbers exceeds four times their thickness
there is a great tendency for the material to twist when loaded.
It is to overcome this tendency that floor joists are braced at
frequent intervals. Short deep pieces shear out or split before
their strength in bending can fully come into play.

_Application of Loads_

There are three[12] general methods in which loads may be
applied to beams, namely:

[Footnote 12: A fourth might be added, namely, ~vibratory~, or
~harmonic repetition~, which is frequently serious in the case
of bridges.]

(1) ~Static loading~ or the gradual imposition of load so that
the moving parts acquire no appreciable momentum. Loads are so
applied in the ordinary testing machine.

(2) ~Sudden imposition of load without initial velocity.~ "Thus
in the case of placing a load on a beam, if the load be brought
into contact with the beam, but its weight sustained by external
means, as by a cord, and then this external support be
_suddenly_ (instantaneously) removed, as by quickly cutting the
cord, then, although the load is already touching the beam (and
hence there is no real impact), yet the beam is at first
offering no resistance, as it has yet suffered no deformation.
Furthermore, as the beam deflects the resistance increases, but
does not come to be equal to the load until it has attained its
normal deflection. In the meantime there has been an unbalanced
force of gravity acting, of a constantly diminishing amount,
equal at first to the entire load, at the normal deflection. But
at this instant the load and the beam are in motion, the
hitherto unbalanced force having produced an accelerated
velocity, and this velocity of the weight and beam gives to them
an energy, o