Wood as a Building Material - Carpenter's and Builder's Math, Plans, and Specifications - page 52

Shear
If two or three planks are placed one upon the other between two
blocks, and a person were to stand in the middle, the planks would
bend (see Figure 2-8). It will be noted that at the outer ends the
boards tend to slip past each other.

Figure 2-8 Illustrating shear in lumber.

If the planks were securely spiked through from top to bottom,
the slipping would be in a great measure prevented and the boards
would act more as one piece of wood. In very solid timber, there
is the same tendency for the various parts of the piece to slip past
each other. This tendency is called horizontal shear .Adefect (such
as a check, which runs horizontally through a piece of a timber and
tends to separate the upper from the lower part) is a weakness in
shear.
Suppose that the planks were spiked through at the center of span
only (that is, halfway between the blocks). Such spikes would not
increase the stiffness of the planks. It is clear, therefore, that there is
no horizontal shear near the center of the span (see Figure 2-9), and
that the shear increases as one approaches either end of the beam.
This will explain why, as most carpenters have doubtlessly observed,
steel stirrups are used in concrete beams (weak in shear), why there
is usually none near the center, and why they are put closer and
closer together near the ends of the beams.