COMPRESSIVE OR CRUSHING STRENGTH


~Compression across the grain~ is very closely related to
hardness and transverse shear. There are two ways in which wood
is subjected to stress of this kind, namely, (1) with the load
acting over the entire area of the specimen, and (2) with a load
concentrated over a portion of the area. (See Fig. 2.) The
latter is the condition more commonly met with in practice, as,
for example, where a post rests on a horizontal sill, or a rail
rests on a cross-tie. The former condition, however, gives the
true resistance of the grain to simple crushing.

[Illustration: FIG. 2.--Compression across the grain.]

The first effect of compression across the grain is to compact
the fibres, the load gradually but irregularly increasing as the
density of the material is increased. If the specimen lies on a
flat surface and the load is applied to only a portion of the
upper area, the bearing plate indents the wood, crushing the
upper fibres without affecting the lower part. (See Fig. 3.) As
the load increases the projecting ends sometimes split
horizontally. (See Fig. 4.) The irregularities in the load are
due to the fact that the fibres collapse a few at a time,
beginning with those with the thinnest walls. The projection of
the ends increases the strength of the material directly beneath
the compressing weight by introducing a beam action which helps