Towers in the sky: How those cranes work

They jut above the skyline they are helping to create. They stand more than 300 feet high and can lift more than 40,000 pounds in a single load.

The tower cranes can be seen in almost every direction in downtown Sarasota, lifting concrete forms and other materials around construction sites and swinging them into place with the stately movements of a giraffe.

The massive machines have become fixtures that are easy to ignore. But anyone who takes a moment to consider these tall needles with arms thrust out horizontally has to wonder how they work — how they to got the worksites, how they are operated.

And why they don't tip over and crash to the ground, particularly in a high wind.

Tower cranes were invented in the early 1900s to help builders cope with narrow European streets. They have been developed and improved over the years, notably after World War II by Hans Liebherr, a prominent figure in Germany's reconstruction and founder of one of the biggest manufacturers of construction machinery.

Interactive: Tower cranes explained  

One of the main features of tower cranes is that they can be taken apart, the pieces trucked where needed and then put together again. 

“They are transported in pieces and are assembled on site using a helper crane,” said Matt Hayden, vice president of Maxim Crane Works National Tower Crane Division. "To make them taller or reach further out, the erection crew just adds additional tower sections or jib pieces.

The biggest parts, both with names borrowed from the age of sailing ships, are the vertical tower mast and the horizontal jib, or working jib.

“A 200-foot-tall tower crane with 200 feet of working jib will typically take about 10 tractor-trailer loads to haul to/from the job site and two days to assemble,” Hayden said.

The base is bolted into foundation anchors in a large concrete pad. The tower mast, consisting of a number of sections stacked one on top of the other and bolted together, also has tie-backs that secure the crane to the building it is helping to construct. Each tower mast section is typically 8 feet by 8 feet by 20 feet high.

To enable the crane to lift loads with its working jib without tipping over, it has counterweights on the opposite arm, called the machinery arm, to balance the weight of the load. Some cranes can move the counterweights in and out as needed to create more leverage and balance the load. The ability of the crane to lift varying loads at varying distances from the spindly tower mast is carefully calculated. The crane is equipped with electronic sensors to ensure that the loads it endures are within its safe limits.

The machinery arm also is the location of motors and the reel that winds up or lets out cable to lift the load. 

To move the load closer to or farther away from the tower mast, a trolley mechanism runs the length of the working jib. The closer the load is to the crane's tower mast, the more it can safely lift. 

To allow the entire working jib to rotate atop the tower mast, the crane has massive gears and a motor, together called the slewing unit, just below the working jib.

Also just below the working jib is the operator's cab, a small compartment where a single person uses a computerized system to direct the crane.

Despite the crane's ability to lift and move tons of weight, the crane operator reaches the cab by climbing a ladder built into the steel latticework of the tower mast.

Cranes can be leased with varying specifications to handle varying jobs.

They can weigh from 100,000 to 250,00 pounds and lift almost 40,000 pounds. A small “CITY” type crane can lift about 10,000 pounds at most.

“Height is theoretically unlimited,” Hayden said. “The Burj (Khalifa) tower in Dubai was built with tower cranes and it's roughly 2,800 feet tall. ... You just have to keep adding more towers and tie-backs to the building.”

Many of the cranes can be made taller as the building they are helping to build grows in height. The cranes have hydraulic rams that can lift the working jib and slewing unit high enough for another tower section to be inserted.

Though various manufacturers and models create tower cranes in a wide price range, most of them cost $750,000 to $1.5 million. The taller the crane, the higher the price.

While some can be bought outright, they usually are rented. The monthly fee is typically $15,000 for a 150-foot-tall tower crane, with additional charges for extra climbing and mast sections.