Signs of the Times - July 2017 - 54

SEAN MCFARLAND
Sean McFarland owns
McFarland Engineering
in Advance, NC. He is the
firm's primary engineer
and a registered professional engineer in 46 states
and the District of Columbia.
Sean has engineered
signs - monument, pylon,
roadway arches, channel
letters, light structures,
billboards, national sign
projects and more - in nearly 3,000 US cities.
For more, visit signstructures.com.

values are commonly expressed in pounds per square
inch (psi), or thousands of pounds per square inch (ksi),
thus 42 ksi is equal to 42,000 psi. His advice philosophically
relates to the above designer and opportunist's narrative;
the former would specify the higher strength steel whereas
the opportunist might prioritize price over strength.
McFarland also said you should consider wide-flange
beams, sometimes called I-beams, for multiple-support
signs, especially when you're installing pole covers. The
distribution of greater strength - to both flanges in the
wind load direction - positively affects their strength on
a pound for pound basis, compared to round pipe which
distributes material in the non-wind load direction as well.
Again, note that McFarland said "for multiple support
signs." However, he also said single-support signs are
always more economical than dual support, considering
support and foundation requirements. "It's a much more
economical solution," he said.
Jones wrote that wide-flange beams as sign columns
should not be used as a single support unless a torsional
analysis is performed. He said wide-flange beams, when
54

SIGNS OF THE TIMES

JULY 2017

used in structures comprising two or more columns, provide
each other a degree of redundancy, depending upon the
stiffness of the sign cabinet frame.
McFarland added that any vertical-sign support should
pass through the sign cabinet, attaching to both the top
and bottom and thereby adding stiffness and support to
the cabinet structure. However, he advised that signmakers
always use Neoprene (synthetic rubber) to separate steel
from any aluminum and said the Neoprene will discourage
galvanic corrosion and allow steel sign supports within an
aluminum sign cabinet.
Galvanic corrosion occurs when two dissimilar metals are
in contact with one another, often in an outdoor environment. With continued exposure to an electrolyte (such
as water), one of the metals experiences a preferentially
accelerated corrosion rate through a process called electrolysis. In this situation, the more chemically "noble"
(i.e., corrosion-resistant) of the metals becomes the cathode
and is protected against corrosion, while the less noble
becomes the anode and corrodes. As McFarland referenced,
aluminum, when subjected to steel, presents a risk of
galvanic corrosion. The Neoprene insulation forestalls
most corrosive action.
McFarland also cautioned that vortex shedding - aerodynamic effects - are a significant problem for signs in the
mid-height range, where sign pipe failure occurs because
of constant buffeting conditions. He said his firm provides
a simple and inexpensive solution for this problem.
While this article touches only on a small element of
an elaborate design and engineering challenge, hopefully,
by discussing these key factors, sign builders will work to
educate themselves, ensuring their sign structure designs
are well engineered.

http://www.signstructures.com

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