Photo Credit: two stout monks
Cranes are one of those enigmas in life we just seem to take for granted. These giant machines occupy the landscapes of most major cities around the world often being much taller than the building they are constructing. They vary in size and some are so tall that you cannot see the tops as they are often covered by clouds. The skyscrapers in iconic cities like New York and Chicago would not be anything like they are now without the existence of cranes.
So how exactly do theses technological marvels actually work? Here we attempt to find out. When we look at a crane, we might think that they shouldn’t be able to stay up right, let alone have the ability of lifting extremely heavy loads. They look top- heavy and in some ways rather fragile. Of course this couldn’t be more wrong as cranes are extremely strong, capable of handling a whole range of different forces.
It all starts at the base
All cranes have the same basic component parts. These consist of the base which is attached to a very large durable concrete platform on the ground and the mast (which is the bit that gives it its height), the slewing unit (the part in the middle which rotates and holds the gears). There is then the jib which is the horizontal arm and is the part of the crane that allows the loads to be carried by a trolley attached to the jib. On the other side of the jib, we have the machinery arm which is where you will find a large concrete counter weight. This is the basic physics which allows the crane to lift the heavy loads. In fact a crane can lift a maximum weight of 18 metric tonnes and the heavy weights are achieved the nearer the load is to the mast. If for example a load is positioned 100 metres from the mast, then the load ratio possible will be 10:1. The counterweight will be around 20 tonnes to allow for these significant loads. There are safe guards in place called limit switches which are installed in the cab and ensure that the loads do not exceed these maximum weights.
The huge concrete pad which the crane is bolted to has to be poured weeks in advance before a crane arrives on the building site. The pad itself will typically have the dimensions of 30ft x 30ft x 4ft and weigh approximately 200 tonnes. The structure of the crane is held in place with anchor bolts fixed deep into the pad. This is the reason they don’t generally fall over.
Getting it up
Cranes can of course reach some considerable heights with the average free standing height being around 265 ft. Of course they can reach much higher as we can observe from the exceptional heights of the tallest skyscrapers.
This often raises the question of how they build these tall buildings. The answer is surprisingly quite simple. The massive cranes that you see which are often taller than the buildings themselves are constructed step by step with the help of smaller mobile cranes. This is made possible as the crane effectively grows via a climbing frame between the slew and the top of the mast.
The horizontal jibs are attached to the mast at a lower level of around 30ft initially and then gradually the mast grows as it is increased in height much like a piece of Lego. The mobile crane lifts every piece into place at intervals of 20ft until the crane has surpassed the height required to build the structure. Once the skyscraper has been built, the whole task has to be reversed to bring the crane back down again!
This insight into the mechanics of cranes was produced by UK home insurance service, Policy Expert.
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