Broadcast Tower Technologies

Tower Vibration

In 1966, when The Beach Boys sang about "Good Vibrations" they certainly weren't referring to the tower industry. Vibration can be a deadly enemy to a tower and anything attached to it. Whether high frequency vibration or low frequency vibration, the effects can be very damaging and in some cases even catastrophic.

Most of the vibration that effects tall towers is Aeolian induced and can be classified as either low frequency, <5Hz, or high frequency, >5Hz. The effects of these two anomalies differ greatly as do the circumstances of their generation. Vibration can be introduced into the tower through guy wires or through appurtenances, usually long cylindrical shaped, that are affected by the wind. Appurtenance vibration usually has far more effect on the devise itself than the tower. Guy wire vibration on the other hand can be induced into the tower and cause adverse reaction in many areas and ways.

All cylindrical devices have a critical velocity, i.e. wind speed, at which vibrations are induced. This vibration is caused by alternating low pressure areas being generated on the downwind side of the device, as shown in the figure above. These alternating low pressure areas cause the device to move towards the low pressure, inducing movement perpendicular to the direction of the wind. When the critical wind speed of the device is achieved, these forces can cause the device to resonate resulting in extreme forces and deflections.

The critical velocity can be calculated as follows:

Low frequency vibration in guy wires occurs far less often than high frequency but has a much greater probability of causing immediate damage with a single occurrence. A classic example of low frequency vibration damage is the 1940 collapse of the Tacoma Narrows Bridge barely 4 months after it was completed. With guy wires, the low frequency vibration causes high amplitude oscillation of the wires commonly called galloping. Galloping guy wire is a condition that requires a special set of circumstances to occur and may only happen once or twice in a tower’s lifetime. In areas where frequent winds usually in the 20-30 mph range occur in conjunction with ice or heavy rains the phenomena is more likely.

The ridged surface of a standard guy cable is believed to help reduce vortex shedding thereby reducing low frequency vibration. When that normally rough surface is coated with a substance that creates a smooth surface vortex shedding can increase. In addition, radial ice, grease or possibly water during heavy rains can create lift much like the effect on an airplane wing and induce the galloping phenomenon.

The forcing function causing vibration is the wind which we cannot control. In the event vibration does occur, the methods we have for control are changing the system mass, changing the stiffness or providing damping. Changing the mass of a tower system is usually not considered a viable option. Methods such as hanging weights or attaching additional wires to guy wires have been used. While somewhat effective, some of these methods are less than desirable since they are less forgiving and can easily come loose or cause additional problems in relation to long term maintenance.

Use of a properly designed snubber system is far more reliable and can be installed relatively easily. A specially designed energy absorbing devise combining springs and dash pot dampers attached to a pulley riding on the guy wire and tensioned to create a downward force on the wire at a calculated angle will virtually eliminate any unwanted vibration induced by the elements.

In this case, Caissons were used but these anchors could just as easily be pad and pier, screw in or any other type of anchor that will hold in the soils available.

These sheaves are made of a non-abrasive nylon that will handle the elements but not adversely affect the guy wire coating. Other materials include stainless steel for the highest quality available, long life and resistance to adverse weather.

At BTTi, each system is designed to meet the specific requirements of the tower on which they are installed. If your tower is located in an area where radial icing occurs in combination with moderate winds these damper systems can significantly reduce stresses to tower members and mounted devices as well as extend the service life of the tower, antennas and feed lines.

If you have observed vibration issues on your tower or feel there is a potential for vibration, contact Broadcast Tower Technologies, Inc. for a comprehensive evaluation and quotation.

(941)359-8833 or jack@tower-technologies.com

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