Tower Cranes at the Site: :

Maximum unsupported height - 141 feet (43 meters)

Maximum reach - 164 feet (50 meters)

Maximum lifting power – 6.61 tons (6 metric tons)

Counterweights – 17.6 tons (16 metric tons)

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 wind pressure can be approximated by:

-Pressure = ½ x (density of air) x (wind speed)2 x (shape factor)

-The density of air is about 1.25 kg/m3.

-The shape factor (drag coefficient) depends on the shape of the body. It has order of magnitude 1 and is dimension less.

-The wind speed must be expressed in m/s. In that case the pressure has units kg/m/s2, i.e. N/m2.

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.

Eg - A review of in-service 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).

However, 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.

It is important that all lifts are planned and that note is taken of anticipated wind speeds from site specific weather forecasts, to ensure that lifts are not started in rising winds. It should be borne in mind that most weather forecast wind speeds are for a height of 10m above ground and should be corrected for greater heights. In open countryside, wind speed increases with height as shown in the table below:

In city centre locations the gust wind speed at a height of 100m will be approximately twice as strong as the gust wind speed at pedestrian level (excluding effects from nearby buildings). Nearby buildings can have a very significant influence on wind forces. Where surrounding buildings are significantly taller they will often generate increased wind loading on nearby lower cranes.