New heights are possible now with updated tower cranes and their technology
You see tower cranes every time you drive around. These massive structures that help build other massive structures are impossible to miss. They inspire both awe as to how something that tall are constructed and fear that they might tip over at any moment.
When you take a moment to really look at them and how they operate, you are left with more questions than answers.
How can these structures be so thin yet able to pick up those weights? It’s the same type of amazement you feel watching a 100 lb woman lift a ridiculously heavy barbell in Olympic weightlifting.
And how do these things ‘grow’ like vines sprouting out of a block in Super Mario? If your curiosity is killing you, this article will shed light on these questions.
Breaking the Tower Crane into Parts
To understand how these monstrosities work, let’s look at the important parts.
- The construction crew prepares a concrete pad weeks in advance to serve as the foundation for the crane. The pad typically measures 30 feet by 30 feet and four feet in depth. The weight is about 400,000 pounds.
- The base is bolted to the concrete pad.
- The mast is the tower-like structure that gives the tower crane its height. It is secured above the base. It is made up of sections that are stacked on top of each other and bolted together. Each section usually measures 8 feet by 8 feet and twenty feet high.
- At the top of the mast is the slewing unit. This houses the gears and motor that allow the ‘arms’ of the crane to rotate.
- Tower cranes stand like a person extending both arms to the sides, with one arm longer than the other. Both arms are part of the slewing unit. The shorter horizontal arm is called the machinery armwhich contains the motor, gears, and electronics of the crane. It also carries the counterweights.
- The jib is the longer horizontal arm. It is also called the working arm because it is the one that bears the load. It has a trolley that can move the load from the crane’s center to the outer segment back and forth.
- The third part of the slewing unit is the operator’s cab. This is the ‘cockpit’ where the pilot controls the movements of the crane like a giant mecha robot. Tower crane operators require extensive training.
A tower crane looks a lot like a toddler’s building blocks. You can stack blocks on top of one another and watch the tower grow. But there will come a point where just breathing on it will cause the whole structure to crash.
How does the tower crane avoid the fate of its little counterpart?
There are several differences between a tower crane and a tower block. The first factor that gives the crane stability is the strong base of support. The whole thing is essentially fixed to the ground due to the base being anchored to the heavy concrete pad.
The mast of a tower crane has sections much like the building blocks of a block tower. The difference is that the construction crew attaches the sections together with heavy-duty steel bolts. The mast itself has a lattice structure that contributes to its strength and stability.
A Balancing Act
The two arms of the tower crane are like a balance scale or a seesaw. In essence, it is a lever simple machine with the fulcrum in the middle.
Because the jib or working arm is longer, it has a mechanical advantage over the shorter machinery arm. This is why the machinery arm holds the heavy motor, gears, and additional concrete counterweights.
A free-standing tower crane is actually slightly off-balanced in favor of the machinery arm. The strong base foundation handles the additional stress. Adding load to the working arm and adjusting the distance of the trolley from the center put the crane into a balanced state.
From this, you can surmise how important load testing these machines are. The safety and stability of the whole structure depend on how carefully the weights are balanced. This is also the reason why electronic sensors are present to prevent overloading.
Tower Cranes Soaring into the Sky
How do tower cranes grow to gargantuan heights? The method is actually an ingenious one that uses various applications of physics.
Mast sections are added or rather inserted into a tower crane. A climber or climbing frame surrounds the section just below the slewing unit.
Here are the steps that pull off this feat.
- The construction crew loads a weight on the jib. This is important because of the next step.
- The crew then detaches the slewing unit which consists of the two arms and the operator cab. Remember when we said that a free-standing tower crane slightly favors the machinery arm? If weight is not hanged from the jib as a counterbalance, the slewing unit will tip over to the side of the machinery arm.
- The climbing frame will then lift the slewing unit using hydraulic rams, pushing it upwards a distance slightly above twenty feet.
- The mast section to be inserted is picked up by the crane. The crew then attaches the twenty-foot section into the gap created by the climber.
- Congratulations, the tower crane is now twenty feet higher. Repeat as needed.
So what can these bad boys do? Here are some specifications of your average tower crane.
- The maximum unsupported height is about 265 feet. To achieve heights greater than 265 feet, the crane needs to be fastened to the building with steel collars.
- The maximum reach is 230 feet.
- How much weight can it handle? Tower cranes are typically rated for weights of 18 metric tons or 39690 pounds. This maximum weight can only be safely carried by positioning the trolley as close to the center as possible.
- The counterweights on the machinery arm weigh about 20 tons or 40000 pounds.
The Importance of Knowledge and Training
Tower cranes are wonders of engineering that require extensive training to operate. We discussed the importance of the load chart and this is part of the curriculum that we offer.
Contact us now. We are happy to provide answers to your questions and any additional information you may need.