When proposing a crane for a job or creating a lift plan, there are somethings that can’t be controlled, but need to be accounted for.  One of those items is wind.  Wind can play a large factor in your lifting operations. 

A great deal of judgment is required when assessing whether prevailing wind conditions are great enough to prohibit and / or discontinue heavy lifting operations.  Most crane manufacturers specify maximum permissible wind speeds for conduct of a lift, or how to de-rate the lift crane under windy conditions.  Their published load charts make no account for prevailing wind conditions on the lift crane or on the lifted load.

The following factors should be considered in assessing the effects of wind loading on a planned lift as well as the manufacturers’ wind charts.  These wind charts are specific to certain crane configurations and careful attention must be taken to ensure the correct chart is being used.  Table 1 shows an example of one of these charts comparing the wind speed to the length of boom installed, and giving the percent reduction to be taken.

• The first factor is the geometry and shape of the load and how they play a large factor in what is known as the ‘sail area’ effect.  If you are lifting a large flat sheet, then it can catch the wind like a sail.  This can cause the load to be pushed out of plum resulting in adverse effects on the crane.  Provisions should be made to adequately control the load if it is subjected to a gust, and the lift crane’s ability to withstand any imposed wind loading shall be confirmed.  The sail area of the piece can be used to calculate the maximum wind speed allowed during lifting operations that would result in the piece staying plum on the crane.

• A second factor is the maximum height to which the lifted load must be raised.  Wind speed generally increases with height above grade and therefore the effect on the load increases.  Knowing how high you are lifting will let you know where to check the wind speed.  If you are lifting a piece 400 feet into the air, then just checking the wind speed at grade isn’t sufficient.  It helps to have an anemometer installed on the top of the boom or jib tip to give accurate wind speed readings.  These reading should be conservative in evaluating whether or not to go forward with the lift since the readings would be taken higher than where the lifted piece would be ending up.

• A third factor to consider is the backward stability of the lift crane.  The backward stability of the lift crane should be confirmed for the condition of prevailing winds acting from the front, when the exposed tower is high.  This is especially true when the crane has a luffing jib installed on it.  Again a manufacturer will provide wind speed charts for when operations shall cease or be de-rated for the particular setup of the crane.  Careful attention should be made when selecting the length of boom and luffing jib.  Just changing the boom or jib length by 10 feet could jump you into a different de-rating wind speed chart and effect your lifting operations. 

• A fourth factor is the wind acting from behind the lift crane.  The wind loading imposed from the rear of the crane can cause the lifted load to be blown away from the crane so as to increase the operating radius and hence decrease the available lifting capacity.  This definitely needs to be looked at when lifting at 85% of chart or more.  When reading the range diagram of the crane or the load charts, you can check to see if you have enough capacity on the next part of the load chart.  If not, then attention needs to be paid as to how you will control the load during a wind gust to prevent the crane from going out of chart.

• Another major factor to consider is having lift crane operations between structures.  Lift crane operations between buildings or existing plant infrastructure under windy conditions amplifies the hazards of secondary loading due to the “wind tunnel” effect.  As air blows around obstructions, local areas of increased wind velocity may exceed safe limits even though the prevailing wind conditions are generally acceptable.  This is where it definitely help to have a wind speed indicator (anemometer) fixed to the tip of the jib to monitor the potential for excessive wind gusts.  Care should also be taken when lifting the load around a building.  As the load comes around the building this wind tunnel effect can create an excess load on the piece instantaneously.  It helps to have additional monitors along the lifting path where wind speeds are known change.

It is necessary to plan out every lift and wind speed is critical to proper planning.  Even the most standard lift needs a simple lift plan to make sure that the lift is within the limits of the equipment and is accomplished safely. 

We want to make sure you have a successful lift and that starts with proper planning.  We can help in this process by helping to create a lift plan with you whether it is a standard lift, non-standard lift, or an engineered (critical) lift.  Here at The Dothea Group, we will do our best to meet your schedule needs and we will provide efficient, engineered solutions to better your project, improve safety, and increase your bottom line. 

If you are interested in learning more about our engineering services, contact us today.  We can partner with you to ensure a successful bid or project.