We all have our own “fidgety” fixes: whether it is clicking a pen, tapping our feet, or ladies-twirling our hair. Well, have you ever been fidgeting with a paper clip and pulled the outer wire straight and then started bending it back and forth? What happens? It snaps. This is called metal fatigue. It's the damage or premature failure of metal due to repeated application of load below the yield point.
Dampening of Arms
Dampening of steel transmission arms helps to prevent an arm from vibrating in the wind, and typically only needs to be done when conductors and insulators have not yet been installed. As the air blows over an arm, it creates vortices, which essentially are just little swirls of air. When those swirls happen, the arm almost acts like a wing that can move in an alternating upward and downward direction, forcing the arm to start vibrating. This phenomenon is something referred to as vortex shedding.
This can be detrimental because the welds that hold the arm on to the structure can undergo metal fatigue cracking. The arm has a natural frequency that it wants to vibrate at, and if the wind is blowing at the speed that coincides, the arm will just continue to bounce.
Dampening of the arms should always be considered especially on projects that:
a.) Are located in areas known to be susceptible to constant smooth winds like wind farms, coastal regions, etc.
b.) Have unloaded arms during construction for extended periods of time.
c.) Have long, slender arms which could be at a higher risk of vibrating.
Without proper dampening, the arm welds can start to crack, causing an arm failure.
The point at which yield stress becomes a factor is a function of how high the load is and how many times it’s being applied. So if it’s a relatively low load, it takes many more applications, or reversals, of that load before fatigue becomes a problem. However, if you’re loading something up to 90 percent of its capacity repeatedly, it will take much less repetitions for it to become a problem.
This can sometimes be an issue with the pole columns (vertical part of the structure), though rare because pole columns are large enough to withstand more wind speed. But with arms, it’s more of an issue because they are a smaller diameter, and it’s much more likely to have a constant 15 or 20 mph wind speed as opposed to an 80 mph wind speed.
There are different ways to dampen an arm, but they all have the same goal: to reduce, restrict or prevent oscillations, or in other words, prevent vibrating.
One method is to hang insulators on the end of the arms if they are heavy enough. Other options, depending on configuration and application, are hanging sand bags at the end of the arms or straking the arms. These could help prevent vibrations that can lead to arm failures.
Arms that are the most susceptible are smaller arms in areas with a constant wind speed. But the wind speed has to be resonating with the physical dimensions of the arm. More specific calculations can be found in Fluid Dynamics.
So without proper dampening, will the arms fall off every time?
The answer is no, not always. It’s going to take a specific set of circumstances to bring this about. There has to be a constant wind speed blowing over the arm at the right angle.
But the main issue is that proper dampening tends to get overlooked or not thought about. What runs through our minds is, “ok, good, the arms are on the pole,” but you have to remember that it could be weeks or months until that line actually gets strung.
*Content contributed by Robert K. Craig, PE, Senior Civil Engineer at DIS-TRAN Steel, LLC, 17 years of engineering experience.