Since some sign structure defects are not visually detectable, the following non-destructive testing methods may be used:
- Dye penetrant crack detection—A penetrating liquid is applied to identify surface-breaking cracks that may not be visible otherwise. This method is not adequate, however, for detecting cracks below the surface of the steel.
- Ultrasonic crack detection—Special portable equipment emits an ultrasonic pulse that passes through the steel and profiles any internal flaws. This method is very effective for identifying cracks below the metal’s surface, but the equipment must be operated by a qualified testing technician.
- Magnetic particle crack detection—The metal is magnetized through the application of electrical current, then magnetic particles are applied to the metal surface that align with flaws below it. Again, the equipment must be operated by a qualified technician employed by an appropriate testing service.
- Radiological testing—Portable X-ray equipment is used to probe the steel’s internal structure and detect any cracks. This is a highly effective method that must be conducted by qualified technicians, taking proper precautions to prevent radiation exposure during testing.
Any sign structure found through visual inspection or non-destructive diagnostic methods to have incurred significant damage should be considered structurally unsound. Corrective measures should then be planned and executed as soon as possible, approved only by a professional engineer.
With regard to future single-pole sign projects, many companies and their customers may prefer to adopt the aforementioned tapered pole design, incorporating the ‘slip joint’ connection as a proven alternative. This design eliminates the welded connection as a focus of structural issues, because the tapered pole sections are joined strictly by gravity and friction. Tapered poles have been successfully used for many years for lighting, telecommunications and power transmission purposes.
For repairs and retrofits of existing telescoping pole signs—and for users who need to maintain them—there are other options based on the independent research tests.
ISA’s key recommendations are as follow:
- Conduct ongoing, regular inspections of telescoping single-pole signs.
- Refer to retrofit and repair guidelines and related options for guidance.
- Consider the tapered pole design for new construction.
These recommendations and the corresponding guidelines are the result of many hours of work by UCSD and ISA’s Mechanical and Structural Subcommittee. While only a handful of failures have occurred, out of tens of thousands of existing sign structures, this research will bolster safety by helping engineers, manufacturers, installers and users understand the importance of regular inspections and continuing education.
Bill Dundas is director of technical and regulatory affairs for the International Sign Association (ISA). For more information, visit www.signs.org/signstructures.