Signs of the Times - July 2017 - 52

force is the action of one body against the other, that
"tends to change the shape or state of motion, including
the static equilibrium."
Think billiards.
WHAT'S THE FREQUENCY?
Benjamin Jones, PE, vice president of YESCO (Salt Lake
City), structural engineer and author of Engineering Sign
Structures: An Introduction to Analysis and Design, said
wind gust factors can either be simple or maddeningly
complex. He said a sign structure's sensitivity to shortterm gusting depends upon the sign stiffness and mass,
as indicated by its natural frequency. Mass, he said, is the
material property acted on by gravity to produce a weight
force. Stiffness is the function of the structural material and
its cross-section shape. Jones notes that all structures
possess a natural frequency at which they vibrate when
set in motion by force. Think in terms of harmonics - it's
why you sound better singing in the shower. This natural
frequency is the number of complete back and forth repetitions (in cycles per second) the moving sign makes per
second, and Jones notes that a structure may have many
natural frequencies that correspond to different modes,
each mode representing an overall shape that the sign
structure may take while vibrating. Obviously, a sign
structure enduring wind battering takes different shapes
than when at static equilibrium; those new shapes may
further affect any forces railing against the sign.
I'll also note the additional problems of gravity and
entropy (leading to component deterioration), both of which
are constant with the latter, deterioration, as increasingly
relentless. Remember, metal breaks down over time and
extended use - and metal particles weaken when bent
or twisted (such as during wind loads). Once weakened,
metal deteriorates at a faster rate. Thus, metal fatigue and
cracks can eventually cause the metal to fail, even when a
small force is applied.
interpretation. Forces that affect signage may originate
through impact by other objects (think careless cars or
angry birds), natural forces (wind, gravity, or additional
weight caused by rain, ice or snow) and causal heat or cold
expansion or contraction from temperature variations.
H. W. Morrow, in his book Statics and Strength of Materials,
defined "statics" as the branch of mechanics which involves
the study of forces and their effects acting on bodies in
equilibrium. To describe force, he said, you need to specify
the force magnitude, direction and point of application.
He said the examination of statics can answer questions
concerning the loads that bodies in equilibrium support.
Morrow expands my above definition of force by saying
52

SIGNS OF THE TIMES

JULY 2017

PRACTICAL ENGINEERING ADVICE
For some real-world advice, I telephoned Sean McFarland, PE,
structural engineer and owner of McFarland Engineering,
Advance, NC, and asked him about sign engineering.
McFarland has designed thousands of signs and his firm
contracts with many nationwide franchises to engineer
their signs. I asked, "What basic but important things
should sign builders know?"
McFarland put support pipes first. He said, "Go with
a higher-yield steel pipe when you can," and added, "On
a strength per dollar basis, you get the best bang for the
buck going with, say, 42-ksi steel pipe versus 35-ksi steel
pipe." (A metal's yield strength and ultimate tensile strength

Table of Contents for the Digital Edition of Signs of the Times - July 2017