Operating tower cranes and any other heavy lifting equipment like crawler cranes bring clear financial and personal responsibility. Cranes are exposed to a vast amount of hazards that make them prone to accidents.
Wind speed is critical for cranes and lifting operations, whether the crane is in operation or in standby mode. Cranes should not operate in high wind speeds. In order to know if it is safe to operate a crane, a wind speed sensor or anemometer is required. The anemometer should be installed at the highest point of the crane possible.
Between 2000 and 2010, there were 1125 tower crane accidents reported worldwide, resulting in over 780 deaths.
One of the main culprits behind these tragedies was exposure to wind, which caused directly 23 percent of all accidents and probably more as a contributor factor of other primary causes.
The primary concern when operating a crane is wind pressure or force of the wind load. The wind pressure quadruples as the wind speed doubles, let’s repeat that: Every time the wind speed doubles, the wind load pressure increases by a factor of four.
Image from WINDCRANE management platform
Even that the average wind speed is 5m/s, the Wind gust is over 10m/s, this means that the wind load on the crane would have increased by a factor of 4 in just a few seconds. Now imagine this force pushing and pulling the crane as the gusts of wind reach the structure, we enter Wind Turbulence territory, if you want to know more see this Blog about how wind turbulences affect cranes.
Wind speed is air moving due to changes on atmospheric pressure and temperature.
Colder air has a higher density (pressure) than hotter air and this creates a flow of air moving from the higher pressure areas to the lower pressure areas, and therefore wind.
This wind has creates a pressure, the stronger the wind speed the stronger the pressure. For cranes this is referred as Wind Load:
Where F = Wind Load, A is the area of the object, P is the wind pressure and Cd is the drag factor (shape of the body)
In general major contractors, crane manufacturers and the Heath and Safety Executive recommend a maximum wind speed for tower cranes at 38mph (16.5m/s or 60kph) and completely prohibited over 20m/s (45mph or 72kph)
To determinate if it is safe to operate the crane, an anemometer (or wind speed sensor / sender) should be installed on the crane to prevent hazardous lifting situations. In some countries is required to record the wind speed and have means for alerting site operators on the ground of the hazardous conditions via wind speed alarms and alerts.
A review of in-service wind speeds by the CPA Tower Crane Interest Group concluded that the maximum recommended wind speed for towers in the UK should be 18.5 m/s. (41mph)
Ultimately the crane operator may decide to take the crane out of operation at lower wind speeds due to the type of load being lifted or difficult to control it under wind pressure. The crane operator has the primary responsibility in conjunction with the crane supervisor to make that decision. The crane operator’s decision to stop the crane should not be overridden by site managers under any given circumstances.
Very calm or still conditions - wind speeds below 1.99 m/s (4.47mph)
Calm - wind speeds between 2 and 4.99 m/s (4.48 to 11.16mph)
Low - wind speeds between 5 and 9.99 m/s (11.17mph to 22.35mph)
Medium - wind speeds between 10 and 14.99 m/s (22.36mph to 33.53mph)
High - wind speeds between 15 m/s and 20 m/s (over 34mph)
To measure wind speed on a crane, an anemometer or wind sensor device is required. An anemometer is a device used to measure the wind speed. A typical cups anemometer has a rotational body with 3 cups that capture the wind and rotates at the same speed as the wind speed, therefore measuring the speed of the wind.
There’re other types of anemometers like ultrasonic or propeller ones. The anemometer should be positioned on the highest possible point of the crane, sometimes when using a luffing crane it may be required to measure the wind at two different points simultaneously.
The correct operation of these devices should be determined regularly and they should be maintained in good working order. The sensor's crane anemometer should be positioned so that it can measure airflow uninterrupted by the tower crane or adjacent structures. The correct operation of these devices should be determined regularly and they should be maintained in good working order.
The anemometer should have a display for the operator to see the wind speed while operating the crane. It should also be possible for project and site managers to be able to have access to the wind data, thus sharing the lifting responsibility in order to increase safety.
BSI EN 13000 and IEC-61400 declare a wind gust is the average wind speed measured for a duration of 3 second. The Wind Turbulence should also be measured. This means that the anemometer signal need to be measured faster than 1Hz (once per second) in order to comply with EN 13000 and IEC-61400.
The WINDCRANE system samples the anemometer faster than 1Hz, for this rapid sampling rate a wired anemometer is always preferred over a wireless anemometer.
The accuracy of WINDCRANE is better than 20ppm (parts per million), that is an accuracy better than 0.007% for wind speed measurement.
WINDCRANE is a reliable fully Wireless wind speed system, with a global reach using the GSM network.
Wind forecasting is essential when working at heights and a safety requirement for operating cranes and heavy lifting equipment. It is always advisable to measure the site specific wind speed and height, however you can take standard weather forecasts (often measured at 10m height over the ground) and apply the following rule of thumb for extrapolating at higher heights, or even better use your existing WINDCRANE system to compare.
In city centre locations, the wind speed and in particular the wind gusts can be easily more than double of the ones measured at a ground level. Also nearby buildings can have a strong impact on the wind turbulence and therefore increasing the wind loading and variance pressure on the tower cranes.
Height above ground
Level = Multiplier