Signs of the Times - October 2017 - 32

NUTS + BOLTS

Tension

Welded T-joint

Welds oriented perpendicular to the direction of applied
stress (tension) can become sources of fatigue cracking.

This telecom tower incorporates a tapered-pole design
with overlapping "slip-joints" that eliminate welding.
structures, these signs are characterized by low-damping
properties. But a twin-pole sign, like that on page 30,
represents an arch configuration that incorporates
heightened damping by virtue of structural connections
between the two poles. Another key factor which affects
damping is the tightness of anchor bolts fastening sign
poles to their concrete foundations. These bolts must be
properly tightened during installation and periodically
inspected during a sign's lifetime for evidence of loosening
or corrosion which can allow excessive movement and
related damage to the entire structure.
Engineering studies, however, have not suggested that
double-pole structures are inherently superior for signage
applications. These studies emphasize that fatigue design
represents an important consideration for structural
connections joining sections of single-pole structures. In
this vein, extensive testing conducted both by the sign
industry and various state highway departments verifies
the superior durability of tapered-pole designs which have
been widely used for many years in applications such as
high-mast roadway lighting and telecommunications
towers.
Two principal factors are responsible for the durability
32

SIGNS OF THE TIMES

OCTOBER 2017

of this design. First, engineering studies demonstrate that,
by virtue of their geometry, tapered poles do not transmit
wind-induced vibrations as readily as non-tapered poles.
Secondly, the slip-joint connections between sections of
tapered poles depend entirely on gravity and friction, thus
eliminating the need for welding.
WELDING CONSIDERATIONS
Even the highest-quality welding incorporates tiny internal
flaws which may serve as the initial sites of cracking due
to metal fatigue. Welding produces stress concentrations
in the base metal directly adjacent to the weld toes. For
this reason, in steel sign structures, welds oriented perpendicular to the direction of applied stress may become
sources of fatigue cracking. By contrast, welds oriented
parallel to the direction of applied stress are substantially more resistant to this type of damage.
Given the central role of welding in fabricating or installing sign structures, it's critically important for sign companies to employ welders having proper certifications in
accordance with applicable standards of the American
Welding Society (AWS). Welders should also be familiar with
the AWS visual inspection guidelines which determine
acceptability of various types of welds. These measures
support proper workmanship and provide the sign company with documentation of employee qualifications.
FATIGUE DESIGN STANDARDS
The American Assn. of State Highway and Transportation
Officials (AASHTO) develops and publishes standards
governing the designs of various highway and transportation

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