Although it can’t be seen directly, wind can have a massive effect on the safe operation of cranes. In this article, we will look at some issues relating to cranes and wind, writes Stuart Edwards.
Permissible wind speed
The safe wind speed to operate a crane can be subject to much conjecture and discussion, especially when there is undue “pressure” to complete a job. However, determining a safe wind speed can be broken down to three simple things.
Firstly, the wind speed at which the crew feels safe operating. This should be judged by experience and risk assessment. The risk assessment may include how much a load may move about (and therefore increased risk of injury), the ability to control the load by tag lines, site restrictions and other factors. It is very important the decision of the crane crew is respected. There are a number of high profile incidents where this is reportedly the cause of catastrophic failure.
Secondly, the load chart wind speed that the crane manufacturer specifies as the maximum wind speed. The default load chart wind speed is often cited as 10m/s but typical values vary from 9-20m/s and can be as higher. The load chart wind speed may vary by configuration, boom length, sequence, etc. or may be a set wind speed for the entire chart. Some manufacturers may allow a higher load chart wind speed if the lifting capacity is reduced.
Thirdly, the permissible wind speed may need to be reduced below the load chart wind speed due to the surface area of the load. In this article, we will focus on the requirements detailed in AS 1418.5 and EN 13000, as this is the standard/load chart generally adopted for mobile cranes in Australia. For this standard, loads that have a surface area greater than 1.2m2/tonne (including drag coefficient) require the permissible wind speed to be reduced.
In concept, let’s say you can just hold onto a kite that has a surface area of 1m2 in 10m/s wind. If you double the size of the kite to 2m2, you wouldn’t be able to still hold onto it at the same wind speed. But if you reduced the wind speed to the right level, you would still be able to hold onto the larger kite.
The same applies to cranes. The load chart wind speed acting on the load chart allowable surface area equates to a certain allowable force on the crane. If the surface area is greater than the load chart allowable surface area, a lower wind speed is required to produce the same or lesser force on the crane.
The calculation for permissible wind speed is simply based on three things: the load mass, the load chart wind speed and the surface area of the load. These three variables are entered into the formula below to determine the permissible wind speed.
We will cover in detail the calculation of surface area and drag coefficient in the next article. However, when in doubt request this information from the supplier of the load or, if this is not available, contact a competent person to perform this calculation for you.
Measuring wind speed
Wind speed increases with height above the ground so it is recommended that a wind speed meter be fitted to the tip of the boom for real time monitoring.
Gust versus average wind speed
The load chart wind speed and permissible wind speed are based on the instantaneous maximum wind speed inclusive of any gusts. This is the so called “three second gust” measured at the highest point of the boom system, and not the average wind speed measured at a 10m elevation over a time period of 10 minutes as given by most weather stations. The three second gust wind speed can easily be higher by a factor of two and more i.e. taking into account the average wind speed at 10m elevation may significantly underestimate the real conditions.
Lay down / weather vaning
Consideration should be given to weather vaning and/or requirements to lay down the boom. Laying down the boom also gives a good opportunity to inspect the boom tip. Where the boom cannot be laid down, consideration can be given to boom rests or other control measures.
Outrigger / track loads
Outrigger and track loads provided by most manufacturers are based on static loads only; and do not account for dynamics from crane motions and/or wind. FEM 5.016 Guideline “Safety Issues in Wind Turbine Installation and Transportation” indicates that up to 35% should be added to outrigger loads due to wind.
One potentially overlooked area relating to wind speed is calculating the potential productivity for a project in advance; and selecting a crane based on this. The example below, based on real wind speed data from a site over a period of just over a year, shows a massive 342% increase in available crane time by selecting a crane with a 20m/sec load chart wind speed versus a 12m/sec load chart wind speed. Whilst the cost of the more capable crane may be greater, the cost savings on a project may be massive.
Stuart Edwards is principal of specialist engineering consultancy, Edwards Heavy Lift and a Cranes and Lifting columnist.