Views:2 Author:Site Editor Publish Time: 2018-01-24 Origin:hse
This Technical Information Note applies to top slewing tower cranes (both saddle jib and luffing jib) and self erecting tower cranes
The wind forces exerted on a tower crane and any load suspended from it, may well be quite large and affect the safe handling of the crane and the load. It is not always appreciated that these forces are due to wind pressure, not wind speed, and that wind pressure varies as the square of the wind speed.
Consequently if the wind speed doubles, the wind pressure increases by a factor of four times. This means that a small increase of wind speed can have a significant effect on the safe operation of the tower crane.
The tower crane’s manual will specify the maximum wind speed at which the tower crane must be taken out of service. This is normally 45 mph (20 m/s, 72kph) and is based on the requirements of the tower crane design standards. It is however, a maximum value and does not take into account the time required to take the crane out of service or the difficulty of lifting large area loads in high winds. Following a review of inservice wind speeds by the CPA Tower Crane Interest Group, involving tower crane suppliers, major contractors and the Health and Safety Executive, the industry recommended maximum wind speed at which tower cranes operating in the UK must be taken out of service is 38 mph (16.5 m/s, 60 kph).
It must be emphasised that the operator may decide to take the crane out of service at a lower speed due to the type of load being lifted or difficulty in controlling the crane. The operator has the primary responsibility for making the decision, in conjunction with the appointed person or crane supervisor. The operator’s decision to take the crane out of service should not be overridden by site management under any circumstances.
Measuring Wind Speed
It is essential that tower cranes are fitted with anemometers or other wind-speed monitoring devices. These should have their indicators located in clear view of the tower crane operator. It may also be helpful to have visual indicators located outside the cab showing wind-speed warning and alarm levels.
The correct operation of these devices should be determined regularly and they should be maintained in good working order. The sensor of the indicator should be positioned so that it can measure air flow uninterrupted by the tower crane or adjacent structures. Sensors are often positioned on the highest point of the tower crane.
In cases where a number of wind-speed monitoring devices are located on a site, the device fitted on a specific crane must be used for assessing the wind effect on that crane. Devices located on other parts of the site will not give an accurate wind-speed for that crane.
The Effect of Wind on Suspended Loads Strong winds may swing suspended loads (crates, panels, etc) out of balance and radius, making the tower crane unstable. If the operator feels that he cannot maintain full control of the load, it should not be lifted. For large, light loads such as shutters, this situation may occur some way below the tower crane's design wind speed. For example, with a wind speed of 14 m/s (31 mph) the wind load on an 8' x 4' sheet of ply will be 38 kg. If the wind speed increases to 20 m/s (45 mph) the wind load will rise to 76 kg!
The effect of wind on the load may well require the limiting in-service wind speed of the crane to be lowered when lifting loads with a low mass and large wind area.