LIGHT STANDARD DESIGN
Light standards (pole) are designed to accommodate certain specific environmental, load and aesthetic requirements. Various standards, guidelines, and codes govern their use like the American Association of State Highway and Transportation Officials (AASHTO), American National Standards Institute (ANSI) and many local building codes. These standards and codes are based on theoretical analysis, research and current industry practice. The standards and codes take into account direct wind pressures on the pole and luminaire; bending, shear, axial and torsional stresses; secondary moment effects (the pole and fixture being off center of the pole base when the wind deflects the pole) and the effect of heat on the base material in the area adjacent to the weld. Pole design required consideration of field conditions. All variables must be taken into consideration with selecting the pole.
Poles, which perform satisfactory in many installations all over the country, may experience destruction vibration for no apparent reason at a select location. Typically poles are designed or selected based on the 50-year mean wind map found in AASHTO, ANSI or local building codes. These indicate a minimum wind load of 70+ mph, but do not take into account certain wind conditions that can create damaging vibration. Vibration is a local site-specific condition, which is many times overlooked by those selecting a pole because it is difficult to accurately predict. Vibration can be caused by steady relatively low speed wind (10-30 mph), topography or the structure the pole is mounted to can also have impact. Studies indicate that the natural turbulence of the air stream at higher wind velocities, above 30 mph, inhibit vibration. Destructive vibration is not an indication of substandard material, workmanship or design of the pole.
Each job site has different variable that may contribute to structural fatigue vibration. These pole variables should be taken into consideration, along with environmental and structural factors, to determine if the potential for vibration exists.
- Total Load (EPA) and Shaft Length: Light loading, less than 2.0 EPA and shaft length at or above 25 feet. These two factors when combined can be key ingredients for destructive vibration.
- Shape: Straight Square Poles have historically experienced more effects of destructive vibration over other shapes, but no shape is exempt.
- Installation Procedures: Poles are designed to carry a load. Never install a pole without the intended luminaire being installed.
ENVIRONMENTAL & STRUCTURAL FACTORS
The presence of special wind conditions in an area can be attributed to various factors. There can also be factors generated by the structure that the pole is mounted to. These factors can create the destructive conditions over an entire site or can be isolated affecting only specific pole locations on the site. If the following factors are present, be aware the conditions may exist for structural fatigue vibration to occur.
- Parking Deck Installation: Influences from surrounding structures and transferred vibration generated by moving vehicles.
- Near or at airports: Little or no objects to break the wind currents and the presence of turbulence created by aircraft.
- Bridge Installation: Little or no objects to break the wind currents and the transfer of vibration generated by moving vehicles.
- Mountain Foothill areas: Air currents traveling from the higher elevations can create steady damaging winds.
- Large expanse of flat ground: In tandem with little or no structures, the wind currents will not be disrupted which sets up the possibility for low steady winds and destructive vibration.
- Steady low level winds: The upper mid-west and plains states have shown this trend
Note: This is not a complete list, other factors can influence the effects of wind.
To minimize the effects of structural fatigue vibration, a device can be factory or field installed to absorb or dampen the vibration. Another option is to initially design the pole to withstand the fatigue effects if it is in an area with historical problems.
The poles designed can be based on the infinite fatigue life of the materials. This option is more expensive and may not meet the budget restraints of a project, consult factory for assistance.
A more economical option is the use of factory or field installable damping devices. The field installable version is more economical than the factory installable damper and provides the flexibility of installation where and when necessary, see below.
Another option is the shape. Even though all pole types can experience vibration, straight square shafts seem to be more susceptible. Round tapered shafts tend to disrupt the vortex- shedding resonance state. The use of a vibration damper in conjunction with the round tapered design may be the best solution, but is not a guarantee to prevent destructive vibration.