Weather vane your tower crane for Hurricane Irene
By Alex Dahm26 August 2011
Tower crane owners on the US Eastern seaboard are urged to ensure that their cranes are properly set to weather vane safely in light of Hurricane Irene forecast for this weekend (27 and 28 August). Wind is implicated in around a quarter of tower crane accidents, according to crane industry consultant Terry McGettigan.
On Friday 26 August winds at the centre of Hurricane Irene were reported at 110 miles per hour (180 km/h), making it a category 2 storm. Irene is forecast to hit parts of the coast not usually affected, including the Jersey Shore and Long Island. Areas expected to be at the centre of the storm include Wilmington, North Carolina, Virginia Beach, Atlantic City and New York City.
At the time of writing on Friday morning (US time) the hurricane watch extended from north of Sandy Hook in New Jersey to the northern edge of Massachusetts, including Long Island, Block Island, Martha's Vineyard and Nantucket.
For more information on weather vaning your tower crane, see IC June 2010, page 51 or see an abridged version of the text of the article below.
Tower cranes are less prone to blowing over than one might think by looking at them but it is crucial that one abides by the manufacturer's instructions and uses common sense. Tower cranes are typically designed to withstand a maximum out of service wind speed of at least 100 mph (160 km/h). To do so, however, a crane must be properly "weather vaned", i.e. slew brakes released, trolley left in the inner position and hook raised with no load.
Due to the surface area of the front jib being much larger than that of the rear, or counter, jib, the front jib will follow the wind direction, and the counter jib will point against the wind. This orientation minimises the surface area exposed to the force of the wind, which reduces the wind "pressure" exerted on the crane structure.
In addition, tower cranes carry substantial counter weight. A typical tower crane will, with zero hook load, have a reverse moment equal to the forward moment when the crane has its full design load at maximum radius. As a consequence, a tower crane will be in balance, i.e. there is zero moment, when the crane has half its full design hook load at its maximum radius.
This combination of jib direction, with the substantial reverse moment - when out of operation and free to weather vane - is a factor in helping a tower crane endure high winds. In addition, the larger the crane the more counterweight there is likely to be opposing the wind load, i.e. the higher wind speed it will tolerate. Simply put, it takes some wind to overcome the reverse moment, it takes more wind to equal full load moment, and it takes yet more wind to overcome the strength of the tower.
Is your crane in weather vane?
It is possible to tell from the ground whether or not a tower crane is in weather vane simply by looking for tell-tale signs, for example, the direction of roof-top flags or other tower cranes nearby. There are several reasons for a crane not to weather vane. First and foremost is the operator simply not initiating it. Next is electrical or mechanical issues and, most serious, a problem with the slew bearing.
Depending on the crane model, weather vaning can be engaged either manually, electrically, or both. Be aware that just because you have set the crane to weather vane, it does not ensure that the brakes are in fact released. In light of this it is prudent to perform a "motion test", on erection and periodically thereafter, especially prior to an incoming storm. In addition, a good habit to adopt is at the end of each work day; take the time to leave your crane in the direction of the prevailing wind. This will help minimise the likelihood of a problem.
There are several ways of verifying that a crane swings freely in "weather vane," including the following:
1). On a windy day before climbing down, park the crane perpendicular to the wind - set the weather vane and the jib should turn with the direction of the wind.
2). On a day without wind, swing the crane gently while in motion, weather vane the crane and then turn off the power. The crane should continue to swing freely. It should be noted that it might not be possible to do this on all crane models.