Strength of Timbers - Carpenter's and Builder's Math, Plans, and Specifications - page 100

Both strength and stiffness are greater in dry timber than in green
or wet wood of the same species. A piece of long-leaf yellow pine
is 30 percent to 50 percent stronger, and 30 percent stiffer, when
in an air-dry condition than when green. In general, both strength
and stiffness are proportional to densities, or dry weights, although
this is not always true. Edge-grain pieces are usually stronger and
stiffer because the tangent to the rings runs horizontally, but not
appreciably so. There is little or no difference in the sapwood and
heartwood of the same species if the densities are the same. The tool
handle of red heartwood is as serviceable as the handle of white
sapwood, although white sapwood handles are still called premium
grade.
Modulus of Elasticity
Since it is desirable (and, for many purposes, essential) to know be-
forehand that a given piece with a given load will bend only by a
given amount, the stiffness of wood is usually stated in a uniform
manner under the term modulus (measure) of elasticity . For good
grades of Douglas fir and yellow pine that are stress-rated, the mod-
ulus of elasticity is 1,760,000 pounds per square inch.
Beams
A beam is a single structural member (usually horizontal or nearly
so) that carries a load or loads over a given space. At their supports,
beams may be:
Freely supported —This merely means the beams are resting
on their bearings.
Restrained, or partially fixed at their bearings —Although
some designers choose to consider such restraint in their de-
signs, the actual degree of restraint can never be accurately
determined, so restrained beams are more often considered as
being freely supported.
Fixed at their supports— In wood beams, this condition is
rarely found. In steel frames, it is not unknown. In reinforced
concrete frames, it is quite common. Attempts to fix the ends
of wood beams are rarely permanent. Building the ends of a
beam, any beam, into a wall or casting it into concrete for a
short distance does not fix the beams at their supports.
Allowable Loads on Wood Beams
The allowable loads on freely supported wood beams of any species
are readily calculated if the timbers are stress-rated and the allowable