Strength of Timbers - Carpenter's and Builder's Math, Plans, and Specifications - page 96

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Figure 4-6 The actions of vertical and horizontal shears.

If the depth of a wood beam is greater than one-tenth to one-
twelfth of its span, horizontal shears (and not bending strength) of-
ten govern its ability to carry loads. Shears are usually not dangerous
in wood beams unless they are relatively deep and heavily loaded.

Transverse or Bending Stress
This is the kind of stress present on numerous building timbers (such
as girders, joists, or rafters) that causes a deflection or bending be-
tween the points of support. What takes place when these or similar
members are subjected to bending stress is discussed in the section
titled Beams later in this chapter.

Stiffness
By definition, stiffness is that quality possessed by a beam or
other timber to resist the action of a bending force. The action
of the bending force tends to change a beam from a straight to
a curved form (that is, a deflection takes place). When a load is ap-
plied, the beam (originally assumed straight and horizontal) sags or
bends downward between the supports. The amount of downward