A crane is essentially a metallic structure, and like all structures it has elements and joints that can only tolerate certain force before they fail. Of course, the forces required to cause failure are extremely high for a metallic crane, which is built to hoist equipment weighing many tonnes. However, there is another form of structural failure that can occur at much lower effort levels, called fatigue failure.
All structures, including tower cranes, have a natural frequency at which they tend to vibrate in response to external forces. If turbulent wind produces a variable force close to the natural frequency of a crane, vibration increases dramatically. This behaviour is called resonance, and it can occur even with relatively light loads.
Since turbulent wind can cause resonance and fatigue failure in tower cranes, it must be kept under watch in construction projects where these cranes are used. Wind monitoring solutions tend to focus only on wind speed and direction, overlooking turbulence in many cases.
In simple terms, we can define wind turbulence as the degree of randomness in wind movement. We tend to associate turbulence with a high wind speed, but this is not always the case:
Turbulence in the wind is invisible, but it would appear as random vortexes in all directions if it could be seen, as represented in the image below. Any wind that displays this movement pattern is considered turbulent, regardless of its strength.
When wind speed is measured, it is possible to calculate an average speed value. Then, if wind turbulence is present, it appears as ongoing variation above and below the average speed. A turbulent 30 mph wind feels very different from a stable 30 mph wind, even if the average speed is the same in both cases. This is why a fast and accurate measurement system is required to detect turbulence.
As discussed above, the average wind speed value does not tell us anything about turbulence. Instead, we must analyse how much the wind speed changes above and below the average value:
Turbulence intensity is the percentage variation of wind speed above and below the average value. Let’s continue with the example above:
Also note that turbulence intensity is relative. The variation of 3 mph is only 10% at 30 mph. However, if we have another site where the average wind speed is 12 mph, the 3 mph variation represents 25%.
Construction companies must manage multiple projects with different site conditions, and this includes the wind. Although weather monitoring is always important, it is critical when tower cranes are in operation, since they are susceptible to harsh weather.
When it comes to safe crane operation, average wind speed only tells part of the story. Keep in mind that cranes are vulnerable to strong winds, but they can also be affected by turbulence even at moderate wind speeds.
Safe operation is extremely important when dealing with tower cranes, since accidents are almost always severe. Consider all the damage caused if an operational tower crane collapses due to turbulent wind:
It is not an exaggeration to say that a crane accident can mean bankruptcy for a construction company. This article does not intend to scare the reader, but being aware of the potential consequences of crane accidents is important. Prevention is the best solution: with a reliable wind monitoring system that measures turbulence intensity, construction managers can suspend crane activities under dangerous weather.