The formula for the dimensions of any beam to support a strain
transversely is

4 bd²
S = ----
l

[TeX: $S = \frac{4 bd^2}{l}$]

S = the ultimate strength in lbs.
b = the breadth in inches.
d = the depth in inches.
l = the length in inches.


=Detrusion.= Detrusion is the crushing against some fixed point, such
as obtains where a brace abuts against a chord, or where a bridge
rests on a bolster; and the shearing of pins, bolts and rivets, also
comes under this head.


=General Abstract.= The resistance to the above mentioned strains
varies as the area of the cross section; so that by doubling the area
we double the strength. Any material will bear a much greater strain
for a short time than for a long one. The working strength of materials,
or the weight which does not injure them enough, to render them unsafe,
is a mooted point, and varies, according to the authority, from 1-3
to 1-10 of the ultimate strength. The ratio of the ultimate strength
to the working strength is called the _factor of safety_.