11 Feb 2019

A Quick Guide to Tower Cranes

Tower Crane at building construction site
 

Tower cranes are a staple of any urban skyline. Used to hoist and move heavy items and materials like steel, concrete, generators and other large tools, they’re fascinating things to look at. But how do they fit together? How have they changed since that famous “Lunch atop a Skyscraper” photo in 1932? And why on earth don’t they tip over?

How Do Tower Cranes Work and What Are They Used For?

Tower cranes are essential for moving heavy goods, materials or tools around a site. They’re great for speeding up construction, keeping you on schedule and cutting costs of time and manpower in the process.

Tower cranes are impressive feats of engineering, soaring up to 265 ft tall and lifting up to 19.8 tons. No matter their size, they’re all made using the same key parts: 

  • A base that’s bolted down to a concrete pad and connects to a mast or tower.
  • A “slewing unit” attached to the top that’s made up of a motor and ring gear, which the crane uses to rotate. 
  • A jib, also called a “working arm”, which is the bit that actually carries the load. A trolley runs along the jib to move the weight, while the motor contained in the machinery arm lifts the load with the help of counterweights. Here you’ll also find the operator’s cab.

Putting these up is a complex job. First, a mobile crane is used to move the horizontal sections - the jib and machinery parts - onto a 40ft mast and to add the counterweights. The crane then grows by one mast section at a time, all the way up to its maximum height. The crew does this by attaching a top climber between the slewing unit and the top of the mast, using a hydraulic ram to push the slewing unit another 20ft further up, and then lifting another 20ft mast into the gap and bolting it firmly in place.  They repeat this until the crane has grown to its maximum height. It’s a slow, careful, delicate process.

Why Don’t Tower Cranes Fall Over?

This is mostly down to the concrete base, which is massive and needs to be poured weeks before the crane arrives. The triangulated cross-member structure of the mast gives it more stability and prevents bending. Plus, it’s anchored and bolted to the ground.

In addition, massive concrete ballast blocks hang from the jib arm as counterweights. These have to be perfectly calculated. Any mistake and the crane will fall over. 

How Have They Changed Over the Years?

Tower cranes originated in Europe and most are still manufactured across Europe. The first were “derrick” designs, named after the Elizabethan-era hangman Thomas Derrick, and these involved a boom hinged to a rotating base. 

In the increasingly densely populated cities of Europe, these cranes became harder to use, leading to the development of the gantry design in the early twentieth century. These cranes used suspended beams with trolleys moving across them, which made them better for urban environments. Unfortunately, they were very heavy and took ages to construct, working better at permanent sites like shipyards. 

The modern tower crane wasn’t created until 1949, when Hans Liebherr unveiled the design to support post-war rebuilding efforts in Germany. This design combined a vertical, rotating mast connected to a horizontal jib that could move 360 degrees, picking up material and shifting it anywhere within reach. Not only that, the design was easily transportable and quick to assemble. The design inspired many new versions and adaptations. Today, they come in all kinds of sizes, heights and reach, for various functions and site types.

Historically, tower cranes were hydraulically powered, but most manufacturers made the switch to electric in the 1970s. This gave them more advanced mechanics, variable speeds and better hoisting winches while reducing the required power and making them more energy efficient. Even so, they consume a lot of energy, with the average electrical tower crane using 800 amps (244 kW, 208V).  

Importance of Tower Cranes on Your Jobsite

Tower cranes allow you to tackle the more difficult tasks on a jobsite: lifting and setting steel at every floor level, pouring concrete or setting precast panels in place, and placing the permanent electrical and mechanical systems for final installation.

They’re incredibly important to construction sites and to the industry as a whole, allowing companies to create larger buildings in more sustainable ways, often in very tight urban spaces, while moving away from thick wall brick and mortar construction. It also allows them to work faster, more efficiently and at lower costs. 

With the amount of electricity used by tower cranes, a reliable supply (and backup) is needed on your jobsite. If the power is unavailable due to utility delays or is shut off unexpectedly, schedules will be delayed, budgets may be impacted, and the overall safety of the site can quickly be put into jeopardy. Power and distribution play a key role throughout a construction project, and not having a proper plan in place for one of the key components of a project is a disaster waiting to happen.

When you notice the ever-changing skylines of our cities today, you’re bound to see the equipment making it happen every day and night. Construction has come a long way from that iconic Rockefeller Building, and much of it is thanks to tower cranes.