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Scheduling Crane Lifts Safely: How Does Each Load Interact with the Wind?

As a “rule of thumb”, crane operations must stop when wind speed reaches 20 mph (8.94 m/s). However, each load interacts with the wind in a unique way depending on its shape and size. There are many cases where proceeding with a lift is dangerous due to load geometry, even at speeds much lower than 20 mph.

As you might guess, wind speed is one of the main factors that determines the wind force exerted on an object. If speed is doubled, the wind force is multiplied by four (quadratic relation). The resulting wind force also depends on the physical features of the load:

  • Shape of the load.
  • Cross-sectional area facing the wind.

The concept of area facing the wind can be confusing at first, since not all loads have flat surfaces. You can think of this concept as the area of the projected shadow if you expose the load to lighting from a single direction.

In general, larger loads experience higher forces because the wind can apply pressure on more area. However, the shape of the load also has an effect, and loads of similar size can experience very different wind forces due to their geometry. Assuming the same projected area, the force on a square load will be higher than the force on a spherical load, and the force on a cylindrical load will be intermediate.

  • The wind resistance factor of an object describes the effect of shape on wind force
  • Assuming the same load area and wind speed, an object with a wind resistance factor of 1.4 will experience twice the force compared with an object having a factor of 0.7

The wind cannot be controlled, but you can determine which loads are more susceptible to high winds depending on their geometry and size. To reduce risks, these lifts can be scheduled on days where low wind speeds are expected. Actually, there is a formula you can use to estimate the wind force on a load:

F = 0.5 C ⍴ V2 A

Where:

  • F = Wind force
  • C = Wind resistance factor
  • ⍴ (Greek letter rho) = Air density
  • V = Wind speed
  • A = Area facing the wind

Air density is a constant value and wind speed cannot be controlled, but you can choose a suitable schedule for each lift based on the wind resistance factor (C) and projected area (A) of the load.

You also need to consider the physical capabilities of the crane being used for the lift. Crane manufacturers will provide detailed wind force diagrams to help you with this task.

How Wind Forces Interact with a Crane

Wind direction is very important when a crane is operating. Winds from behind the crane tend to push the load away and increase its swing radius, increasing load on the boom. Lateral winds should also be monitored carefully, since they cause side loading. Winds coming from the front of the crane are generally the least dangerous, since they can actually reduce the load on the boom. However, high winds should not be underestimated regardless of their direction.

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Crane manufacturers provide wind force diagrams that specify the maximum wind speed for the corresponding crane assembly. No lifts should be attempted above this value, regardless of the load being lifted. At wind speeds below the maximum value specified for the crane, the chart tells you the maximum speed for a specific load based on three factors:

  • Wind resistance factor
  • Projected area
  • Weight, normally specified in tonnes

 

As a quick example, assume the maximum speed for a crane assembly is 9 m/s. The crane should not operate above this speed with any load, but having a lower speed does not mean you can proceed. For instance, if the wind speed forecast is 7 m/s, you need to ensure the load can be lifted safely based on its properties: weight, wind resistance factor, and projected area.

Planning crane lifts is a process that requires careful calculations and judgment - this article only provides a quick summary. In an actual project, you must take the time to gather detailed information about each load, and determine the maximum allowable wind speed before proceeding with a lift.

Knowing the maximum wind speed at which you can lift each load safely is very useful for planning. Loads with large areas facing the wind or high wind resistance factors should have priority on days where low wind speeds are expected.

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