Views: 29 Author: Site Editor Publish Time: 2018-02-05 Origin: Site
Self Erecting Tower Crane Erection, Dismantle and Jib Folding
The erection, dismantling or jib folding of a self erecting tower crane (SETC) should not be attempted at wind speeds in excess of that specified by the crane manufacturer. The maximum wind speed in which the jib of may be folded back or the crane erected or dismantled is invariably lower than the maximum inservice wind speed. Particular attention should be made to wind direction to ensure that the crane is erected, dismantled or jib folded with the jib in the down wind direction.
If the jib of a SETC has to be folded back before taking the crane out of service to avoid oversailing issues, care must be taken to ensure that the current in-service wind speed is not higher than the permitted maximum for the folding operation. Failure to observe this limit may well result in the crane collapsing. In this situation the maximum jib folding wind speed becomes the maximum in-service wind speed.
Wind speeds should be ascertained with a hand held anemometer prior to the crane being erected and at intervals throughout the day during operation, using either a hand held anemometer or the anemometer mounted on the crane. Measurements should be taken as close as possible to the final erected height of the crane.
High Wind Conditions and Taking the Tower Crane Out of Service
It is important that the operator monitors the wind speed constantly using the anemometer display in the cab. This will give early warning of rising wind speeds and enable him to take action to take the tower crane out of service and descend down the ladders to ground level before the limiting wind speed is reached.
Putting the crane in the out of service condition generally includes ensuring that the jib is free to “weather vane” when out of service so that the minimum wind area is presented to the prevailing wind (see TIN 038). On luffing jib tower cranes it is also important that the jib is left at the correct out of service radius, not the minimum radius, to ensure that there is sufficient wind area to ensure that the crane is able to “weathervane”.
Planning of Lifts
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, if they are the same height as the crane they will mostly provide shelter, although local wind loads can be increased in some situations. Where surrounding buildings are significantly taller they will often generate increased wind loading on nearby lower cranes.
NOTE: It is essential that site specific weather forecasts for the period of the lift are obtained in plenty of time to allow for effective planning and redeployment of resources if the lift has to be postponed due to high winds.
