In our previous article we looked at how to calculate the permissible wind speed to operate a crane based on three simple things – load mass, load chart wind speed and surface area of the load, writes Stuart Edwards.
This is calculated using the formula:
The load mass and load chart wind speed are pieces of information that are relatively easy to obtain. This article focuses on calculating the surface area of a load.
The surface area is made up of two components, where the total surface area is calculated by the following formula:
The projected area is the net area of the load perpendicular to the load. For example, for a 6.1m long (12’) shipping container the projected area is 6.1m x 2.6m = 15.86m2.
The drag coefficient is more or less a measure of how easily the object “slips” through the wind. For example, the most aerodynamic production car in the world, a VW XL1, has a drag coefficient of 0.189. Conversely, the “boxy but good” Volvo 244 has a less flattering drag coefficient in the range of 0.47.
The drag coefficient can be incredibly complex to determine accurately. Various standards and publications provide different calculation methods (with various differences in results that are borne out of the need to simplify something that is truly complex). In this respect, wind tunnel testing may also be relevant but typically out of the scope of a typical crane hirer or contactor.
In the first instance, is it recommended that surface area information be requested from the manufacturer of the load. The mxanufacturer may have complete detailed calculations or have conducted wind tunnel testing. Wind turbine companies are increasingly able to provide this sort of information, for example, but more commonly this information is not available.
FEM 5.016 Guideline
“Safety Issues in Wind Turbine Installation and Transportation” indicates that if a drag coefficient is not known, it is permissible to use a drag coefficient of 2.4. If in doubt, it is recommended that an appropriately qualified and competent engineer be engaged.
FEM 5.016 Guideline
FEM 5.016 – Guideline – “Safety Issues in Wind Turbine Installation and Transportation” provides the following general drag coefficients
Further detail is included in ISO 4302 as per the extract below and other standards and publications.
Using this table, we can determine the drag coefficient of a 6.1 m container, using the ratios of b/c and f/b we obtain a drag coefficient of 1.4.
Therefore, the overall surface area of the load is as follows:
Don’t forget the rigging, crane hook and hoist rope is considered by most crane manufacturers to be part of the load surface area. These can be calculated separately and added to the total surface area.
Stuart Edwards is principal of specialist engineering consultancy, Edwards Heavy Lift and a Cranes and Lifting columnist.
working on drag coefficient for a man basket lift