ISA’s four-year university study yields recommended changes.
By Bill Dundas
Bill Dundas is ISA’s Director, Technical & Regulatory Affairs. He installed signs for more than a quarter century, and he also served as ST’s Technical Editor for several years.
Four years after research began to address isolated, sign-structure failures, which created a potentially serious issue for the sign industry, solutions have been suggested to prevent subsequent problems. Responding to reports of unusual and sporadic failures of single-pole, high-rise signs, the Intl. Sign Assn. (ISA) Mechanical & Structural Subcommittee recently concluded extensive research at the University of California, San Diego (UCSD).
This research first sought to identify the incidents’ root causes, and then evaluate alternatives. Although the overall percentage of sign structures affected is relatively small, the unusual nature of these failures, and the large number of such signs currently in use, prompted ISA to sponsor this project and to support signs’ safety, quality and acceptance.
-->Heart of the matter
The initial, ISA-sponsored research report, Evaluation of Sleeve Connection of Cantilevered Steel Sign Structures (October 2008), identifies the root cause of these incidents as wind-induced vibration and related, fatigue-type damage. This report finds that, under certain conditions, substantial forces may be imposed on the welded connections that join segments of these telescoping structures (Fig. 1).
When the wind speed is relatively constant (typically 30-50 mph), a dynamic wind effect known as vortex shedding (Fig. 2) can produce intense and sporadic vibrations. This wind-speed range, of course, is substantially lower than the maximum wind speed these structures are designed to withstand. Although wind vibration from vortex shedding certainly isn’t the only potential cause of sign-structure failures, ISA research indicates this phenomenon can damage existing, single-pole structures, even when all installation and welding procedures have been carefully executed according to project specifications.
When the frequency of wind-induced vibrations matches a sign structure’s natural period of vibration, a lock-in effect known as harmonic resonance occurs. For brief, intermit-tent periods, this effect greatly multiplies the number of cycles imposed on structural connections.
Over time, these cyclic stresses can create cracks at the “toe” of the exposed fillet weld, which connects the upper and lower telescoping pipe sections (Fig. 3). As the number of cycles accumulates, these cracks penetrate the pipe wall and propagate around its circumference, typically in alignment with the weld toe. If undetected, this condition worsens and ultimately may cause the structure to collapse.
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