Sign Structure Safety

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.

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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.

Although double-pole signs sometimes incorporate similar structural connections, failures of this specific type generally haven’t been observed on them. According to UCSD analysis that compares single- and double-pole structures, the former are more susceptible to wind-induced vibrations due to their inherently low damping properties and corresponding modes of vibration.

One important caveat, however, applies to double-pole structures located at closed or abandoned business sites. If sign cabinets or other structural members that connect twin poles have been removed, then each pole becomes susceptible to damage from wind-induced vibration. Consequently, when such signs are removed, the customer should be informed that satisfactory connection of the poles is essential to prevent possible damage.

Fatigue test findings
ISA’s follow-up research, detailed in the October 2011 report Fatigue Tests of Welded Connections in Cantilevered Steel Sign Structures is based on tests performed at UCSD’s Charles Lee Powell Structural Systems Laboratory from March 2010 to May 2011. Pursuant to findings in the original 2008 report, ISA and UCSD developed various alternative concepts for single-pole sign structures as the basis for test comparisons reported in this study. Specimens incorporated the common lap-splice connection (Fig. 7), which is widely used for single-pole, telescoping sign structures. The UCSD report indicates the average fatigue life of the tested lap-splice specimens is less than 20,000 cycles. Based on standards developed by the American Assn. of State Highway and Transportation Officials (AASHTO), the fatigue life of the common lap-splice connection corresponds to category E (“E-prime”), which represents the lowest fatigue category for structural details.

Because of the limited number of test specimens, and the normal range of variation in tests of this type, results of ISA’s fatigue-testing research primarily indicate general durability trends and don’t reflect specific (or average) lifetimes for each type of connection. Additionally, the loads imposed on the test specimens don’t correspond to natural wind effects on outdoor sign structures. Thus, this report suggests alternative, promising design approaches. The concepts tested and described in this report, however, don’t represent specific design recommendations by ISA.

Research results indicate tapered poles (Fig. 4) represent substantial design improvements. Corresponding UCSD lab test results support the current, successful usage of this type of pole for lighting, power trans-mission and telecommunications. A tapered pole’s “slip-joint” connection differs fundamentally from the geometry of the common lap-splice connection. Because tapered-pole sections are connected by gravity and friction rather than welding, potential failure points susceptible to fatigue don’t exist.

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Additionally, both anecdotal evidence and results from the UCSD fatigue-testing project indicate field installation of gussets (Fig. 5) is ineffective for making durable repairs to damaged sign structures. UCSD’s lab tests indicate fillet welds that attach gusset bases to the horizontal “cap plate” are equally susceptible to fatigue damage. Furthermore, if the vertical welds that attach gussets to the pipe extend around the gussets’ upper tips, this creates additional points susceptible to crack formation.

The research also addresses the critical importance of slot-welds (also known as “plug-welds”) for the integrity of common lap-splice connections (Fig. 6). Results of computer modeling reported in the 2008 UCSD study indicate missing or detached slot-welds (sometimes due to incomplete or “tack welding”) substantially increase the maximum forces exerted on the primary lap-splice connection.

Thus, proper field-welding must be performed by a certified welder. Similarly, this stresses the critical nature of accurate project specifications and fabrication of the pole sections. In particular, fit-up between the inner wall of the lower pipe section and the inserted “guide ring(s)” (attached to the upper pipe section) must be close enough to facilitate slot-weld placement. An inadequate fit-up might preclude properly executing these welds.

Also, weld slots should incorporate rounded ends and a sufficient front (outside) bevel to facilitate electrode access for executing root-pass welds in the field. Thus, fabricators must verify, prior to shipment, acceptable fit-up of pole sections and proper alignment of weld slots with the guide ring(s) on the inserted pole sections.

Additionally, test results indicate significant durability improvement is possible by altering the common connection design that incorporates two, circular fillet welds that connect the cap plate to the pipe (Fig. 7). Specifically, fatigue tests indicate durability improves when the top (exposed) fillet weld is eliminated, and only the bottom (concealed) fillet weld is specified. This alteration, however, requires an appropriate sealant to prevent water from collecting in the gap created by eliminating the top fillet weld.

The research also provides guidance for repair or rebuilding of existing structures. For example, the UCSD testing evaluated a retrofit device and a structural repair material known as Fiber Reinforced Polymer (FRP). Each alternative demonstrated substantial durability improvement, compared to common lap-splice connections.

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However, each method requires careful adherence to specific installation procedures, and each must be periodically inspected as part of scheduled sign maintenance. FRP repairs are available strictly through qualified providers and contractors. When structures are repaired accordingly, FRP manufacturers provide corresponding warranties.

ISA’s purpose in evaluating these repair options, however, is neither to suggest nor recommend any particular approach for correction of damaged sign structures. These decisions must rest with sign companies, end users and their professional engineers. Due to local jurisdiction, sign-code restrictions and other factors, not all damaged structures are likely to be replaced, but ISA-tested repair options appear to represent significant improvements to some prior methods.

In connection with the sign-structures research, ISA previously published guidelines for installing and inspecting all types of freestanding sign structures.

Most significantly, this research speaks to the immediate need for all sign structures to be inspected regularly for possible damage and related defects. Thus, for safety, ISA recommends that regular structural inspections should accompany all scheduled maintenance programs that involve freestanding signs, regardless of the sign’s size, type, overall height or number of supporting poles.

With these findings and corresponding safety guidelines, we trust the ISA research will enable engineers, manufacturers, installers and end users to enhance the safety and quality of freestanding signs. By emphasizing the key roles of structural safety inspections and continuing education of all stakeholders, we believe that the integrity of sign structures will be permanently enhanced for the benefit of all.