There are two common types of wind-induced vibration observed in poles…First Mode Vibration and Second Mode Vibration.
First Mode Vibration
In first mode vibration, sometimes referred to as sway, the maximum deflection occurs at the top of the pole. First mode oscillation typically occurs at a low frequency of approximately one cycle per second. Normal deflection of this shape usually is not harmful to the pole or luminaire, but first mode oscillating vibration will cause damage and failure.
Second Mode Vibration
Second mode vibration can be the most damaging form of vibration. It occurs approximately at the midpoint of the pole with the deflection off center equal from side-to-side. It occurs at a higher frequency, typically three to six cycles per second. Second mode vibration occurs when the wind synchronizes with the pole’s natural frequency of vibration. This is known as resonance. As the steady low level wind moves past the pole, vortices are shed alternately from either side of the structural shaft causing displacement oscillations in a direction perpendicular to that of the wind.
Wind induced vibration can be caused by steady, relatively low-speed wind (10-30 mph), by topography and by the structure to which the pole is mounted or nearby structures. Destructive vibration is not an indication of substandard material, workmanship or design of pole.
Vortices are a swirling motion or pattern of the wind. The most serious situation arises when the vortex-shedding frequency synchronizes with the natural period of vibration in the pole, which can ultimately fatigue the pole to structural failure.
Light Pole Design
Pole design requires consideration of field conditions such as wind speed (sustained/gusts), pole height, appendages and local conditions. Wind induced vibration is a local, site-specific condition that may be overlooked by those selecting a pole because it is difficult to predict accurately. Poles which perform satisfactory in many installations all across the country may experience destructive vibration at a select location for no apparent reason. Typically, poles are designed or selected based on AASHTO (American Association of State Highway and Transportation Officials) criteria. The standards and codes take into account direct wind pressures on the pole and luminaire, the associated bending, shear, axial and torsional stresses on the pole, 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.
