Wood as a Building Material - Carpenter's and Builder's Math, Plans, and Specifications - page 50

a year's growth forms two types of wood: the springwood (char-
acterized by softness and openness of grain) and the summerwood
(characterized by hardness and closeness, or density, of grain). The
springwood and summer wood growth for one year is called an
annual ring .
Density and Strength
There is one ring for each year of growth. This development of
springwood and summerwood is a marked characteristic of prac-
tically all woods that grow in a temperate climate. It is evident in
such trees as the yellow pines and firs and less so in the white pines,
maple, and the like. Careful examination will reveal this annual
ring, however, in practically all species. It follows, therefore, that
a tree in which the dense summerwood predominates is stronger
than one in which the soft spring wood predominates. This is a
point that should be kept in mind when selecting material for im-
portant members such as girders and posts carrying heavy loads.
The strength of wood of the same species varies markedly with
the density. For example, Douglas fir or southern pine, carefully
selected for density, is one-sixth stronger than lumber of the same
species and knot limitations in which the springwood predominates.
Trees having approximately one-third or more of cross-sectional
area in summerwood fulfill one of the requirements for structural
timbers.
Estimating Density
It must be remembered that the small cells or fibers that make up
the wood structure are hollow. Wood substance itself has a specific
gravity of about 1.5. That means it will sink in water. It is stated
that wood substance of all species is practically of the same den-
sity. Strength of wood depends upon its density and varies with its
density. The actual dry weight of lumber is a good criterion of its
strength, although weight cannot always be relied upon as a basis
for determining strength, because other important factors frequently
must be considered in a specific piece of wood.
The hardness of wood is also another factor that assists in estimat-
ing the strength of wood. A test sometimes used is cutting across the
grain. This test cannot be utilized in the commercial grading of lum-
ber because moisture content will affect the hardness and because
hardness, thus measured, cannot be adequately defined. The annual
rings found in practically all species are an important consideration
in estimating density, although the annual rings indicate different
conditions in different species. In ring-porous hardwoods and in the
conifers, where the contrast between springwood and summerwood