Signs of the Times - February 2012 - (Page 58)

Sign Structure Safety ISA’s four-year university study yields recommended changes. By Bill Dundas our 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 singlepole, 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. F This sign structure failed due to cracking that propagated entirely around the fillet weld “toe.” 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 windinduced vibrations matches a sign structure’s natural period of vibration, a lock-in effect known as harmonic resonance occurs. For brief, intermittent 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 58 SIGNS OF THE TIMES / FEBRUARY 2012 / www.signweb.com http://www.signweb.com

Table of Contents for the Digital Edition of Signs of the Times - February 2012

Signs of the Times - February 2012
Contents
ST Update
Technology Update
Software Update
Vinyl Apps
Strictly Electric
LED Update
Technology Review
Sign Museum News
New Products
Palisades Center Mall’s Retrofit
Sign Structure Safety
Picture Them Rolling
Sign the Vote
Taxing Times
The Value of Signs
See Ya Real Soon!
Industry News
Advertising Index
Editorially Speaking

Signs of the Times - February 2012