Civil Engineering Reference
In-Depth Information
Depth
Length
Width
Figure 3.5. Terms for a beam's three dimensions, from an observer's point of view.
plank greater than its depth), they would flap up and down with the passing
load. But if enough planks were available and were laid next to each other
while resting on their narrow edges (width now much narrower than depth),
the structure would become much stiffer—it could resist far greater loads.
A stiff beam exhibits a kind of stress behavior that most of us would
not think of. Called
shear,
it occurs when forces push in opposite directions:
not at directly opposed points on the same cross-sectional plane (that would
just be compression) but at opposite points on adjacent planes (figure 3.6).
For example, consider the effects of a pair of pliers on a sheet of metal as
compared to a pair of shears. As you press together with the pliers on the
sheet, forces converge from opposite directions on the same area in the
metal, causing ordinary compressive forces. By contrast, if you press with
the shears on the sheet, the area the downward force presses is separated
by a very small space from the area the upward force presses. The sheet of
metal will be cut—or sheared—apart.
But even a beam on which no scissors act, one that bears only its
embodied dead load, undergoes shear strain. This is important, so we should
try to picture it. Imagine that the beam is an assemblage of cards glued
together and placed horizontally across two bricks, directly in front of us,
left to right. As we follow the cards left to right, we see some cards that
are resting directly on the left brick, until we reach the first card suspended
