Weathervane that Crane!
In year 2010 there were an alarming number of worldwide tower crane collapses due to WIND!
The numbers speak for themselves, 27 out of 157 (17%) of the reported / uncovered tower crane accidents in year 2010 have been linked to high winds. However, in most cases the wind may have only been a contributing factor. Although this view is about “out of service” conditions, it would be well advised to take into consideration the additional stresses that come into play BIG TIME when operating a tower crane in high winds! Even though the manufacturer sets maximum “in-service” wind speed around 44mph / 72km – – prudent judgment by operators and management is vital.
Tower cranes are less prone to blowing over than what one would think by looking at them – – as long as one abides by the manufacturer’s instructions and uses common sense. In general, tower crane are design to withstand a maximum “out of service” wind speed of at least 100mph / 160km. However, in order to do so – a crane must be properly weathervane, i.e. slew brakes released, trolley left in the inner position, and hook raised with no load.
Because the surface area of the front jib is much larger than the rear jib – or “counter weight” jib, the front jib will follow the wind direction, and the cw jib will point against the wind. This orientation minimizes the surface area thereby reducing the wind “pressure” exerted on the structure. Furthermore, tower cranes are substantially counter weighted. A typical tower crane will with zero hook load have a reverse moment which is equal to the forward moment when the crane has its full design load at max 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 max radius.
This combination of jib direction, along with the substantial reverse moment – when out of operation and free to weathervane – comes into helping a tower crane endures high winds. Additionally, the larger the crane the more counterweight 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 Weathervane?
It is possible to tell from the ground whether or not a tower crane is in weathervane simply by looking for tell-tale signs such as the direction of roof-top flags or other tower cranes near by. There are several reasons for a crane not to weathervane; first and foremost is the operator simply not initiating it, next would be electrical or mechanical issues, and most serious – a problem with the slew bearing.
Depending on the crane model, the weathervane can be engaged either manually, electrically, or both. BEWARE, just because you have set the weathervane, it does not ensure that the brakes are in fact released. Therefore, it would be prudent to perform a “motion test” – upon erection and thereafter- periodically and or especially prior to an incoming storm. Additionally, a good habit to adopt is at the end of each work day; take the time to leave your crane with the direction of the prevailing winds. This will help minimize the odds (Murphy’s Law) for instance; let’s say you leave a crane pointed directly into a mild wind assuming that the weathervane will naturally turn the crane accordingly – – overnight a severe storm moves in with wind direction unchanged, ?? Another one bites the dust!
There are several ways of verifying a crane swings freely in “weathervane” such as:
1). On a windy day before climbing down, park the crane perpendicular to the wind – – set the weathervane – – the jib should turn with the direction of the wind.
2). On a day without wind, swing the crane gently – – while in motion, weathervane the crane – – and then turn off the power – – the crane should continue to swing freely. (This may not be possible with some crane models)
Unfortunately, few ever think of carrying out an actual motion test to verify if a tower crane does in fact – swing freely – when put into weathervane. This often forgotten safety feature is more important than most people realize – which is evident from the many accidents associated with it.