The CICA Technical Committee explains recent changes to wind load calculations.
Mobile crane operations are subject to wind speed limitations during lifting, erection, dismantling, and also when out-of-service. Excessive wind forces acting on loads and cranes have led to several serious accidents around the world. If not controlled properly, wind can be a major risk for crane operations.
To provide assistance to mobile crane users, the CICA technical committee developed a guidance note on wind load calculation, six years ago. This guidance note covered the manufacturer’s permissible wind surface area, the calculation of the dynamic wind pressure and the permissible wind speed. Following the calculations in the guidance note, wind effects can be assessed to determine reduced permissible wind speeds based on load surface area in accordance with load charts.
For example:
- If the wind surface area of a load is less than the total allowable wind surface area (specified by the crane manufacturer) multiplied by the chart capacity, no further calculations need to be made.
- If the wind surface area of a load is greater than total allowable wind surface area, further calculations need to be made to determine the permissible wind speed.
Wind speed on site should be measured and compared with the load chart wind speed or the calculated permissible wind speed, to see if it is safe to operate the crane under the site condition. (It should be noted that the wind speed can vary between ground level and the boom tip or load height. The use of anemometers attached to a pendulum arrangement on a boom tip, can be an excellent means to assess the conditions.)
When the CICA technical committee developed the guidance note six years ago, it was recommended by the manufacturers and the industry that the crane load chart capacity be used to calculate the allowable wind surface area, so for that reason, the calculation formulas used in the guidance note are all based on crane load chart capacity.
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Most of the CICA technical committee members are heavy lift planners themselves and they have noticed that over the past few years, the practice recommended by the crane manufacturers and some international guidance on wind load calculation has changed. Instead of using the crane chart capacity for the allowable wind surface area calculation, it is now widely adopted by the crane manufacturers that the hoisted load (mass of the load and the hook) should be used to calculate the allowable wind surface area. To provide guidance for the Australian crane industry with the latest industry practice, the CICA technical committee have decided to revise the CICA wind guidance note to reflect this change.
This revision of the guidance note is underway, and we expect to finalise the updated guidance in two months’ time. The CICA technical committee is currently working with the crane manufacturers to obtain their support for the new guidance. Manufacturer’s recommendations on wind load calculation will be attached in the appendix for reference and CICA are also evaluating the international guidance on this topic, to capture the best industry practices.
Calculating wind load impact on crane operation can be complicated and for special cases, the lift designer should seek manufacturer’s instruction on wind load calculation as the crane manufacturers have a better knowledge of the crane limits.
Other than the calculations, the new guidance will also consider practices on site for the dogger/rigger who are controlling the load, for example – topics on tag lines, recommendations on assessing forces and rotational effects acting on a load and load controlling skills for different load cases will be discussed.
Once completed, the guidance note will be available to CICA members, please contact Alice Edwards (projeng@cica.com.au) if you would like a copy.