Dominating the skyline of any build in which they are used, tower cranes are permanent fixtures constructed on site to aid in lifting heavy loads to great heights. Skeletal in appearance and delicate in nature, these towering structures inspire the imagination and wonder of many who see them.
Viewed from afar however, most people are unaware of the massive bases constructed for tower cranes and most do not understand how they these machines work.
When we don’t know exactly how something works, we are often fascinated when technology goes awry. Though safety tests are always performed before tower cranes begin working on construction sites, they are not without error.
Just a short time ago, Vertikal.net, a crane news site, reported on an overturned tower crane. At an apartment construction site in Germany, a tower crane toppled while lifting concrete.
Although no was injured, the error caused damage to materials on the site, a nearby apartment home, and the crane itself. For a sobering look at why safety and proper engineering is important, check out the 10 Famous Crane Collapses of All Time.
During construction and operation of a tower crane, a variety of parts must interact safely and properly in order to enable the structure to wield its tonnage. A quick glance at the extensive OSHA regulations for tower cranes is nearly as sobering as images of toppled cranes. Still, with the careful work of expert engineers and operators, the majority of tower cranes safely perform their jobs daily, free of costly accidents or errors.
Construction of the Crane
Based on the simple cranes of our ancient ancestors, tower cranes are an extension of this technology that allow for up to 19 tons up to 230 feet, or about 20 stories.
On a square, concrete pad, an x-frame is laid out as the tower crane’s base. Then the section of frame is erected. Around the metal frame, large blocks of concrete are placed to anchor the frame for the hundreds of tons that will eventually be swung by the crane’s arm.
Craneblogger.com explains that, once the foundation is laid, the tower itself is constructed and “mast sections are added until the needed height is achieved.”.
Atop the tower, a slewing unit is added. According to an article on How Stuff Works, this slewing unit consists of “the gear and motor that allow the crane to rotate,” the horizontal jib which carries the load, a trolley along the jib to move the load, a machinery arm (complete with motor, electronics, and counter weights), and finally the operator’s cab.
All of these parts are all constructed separately at ground level and carefully and precisely hoisted into position by a mobile crane and sometimes, in the case of self-erecting cranes, by the crane itself.
If the tower crane is not self-erecting, the construction process of the crane can require “up to 13 trucks, can include seven crane erectors, three crane operators, two mobile cranes,” reports Craneblogger.com.
During the construction process, engineers and operators must work closely together. If errors or miscalculations occur – whether in the construction of the base, the tower’s mast, or in the slewing unit – costly accidents may occur on the build site.
With the concrete base poured and cured ahead of time, the construction of most tower cranes takes only a day followed by a second day installing the ropes and performing operating tests to ensure safety.
In an industry where deadlines are often strict and delays in construction might cost a client thousands of dollars, it is tempting to supersede safety with speed but expert crane crews are able to do both.
Operating the Tower Crane
Embedded within the structure of the crane are electrical and hydraulic systems of engines and brakes. These allow an operator to safely power the crane’s lifting ability and to control the crane’s hoist and swing from a cab.
One of the simplest safety measures taken on tower cranes is through limits on the hoist rope. Braking systems serve as safety measures to human error and only allow for a certain amount of hoist rope to be released at a time through a system of limits.
The greatest area of concern in the operation of tower cranes however is that of properly calculating countermeasures. According to the How Stuff Works article:
“The closer the load is positioned to the mast, the more weight the crane can lift safely. The 300 tonne-meter rating tells you the relationship. For example, if the operator positions the load 30 meters (100 feet) from the mast, the crane can lift a maximum of 10.1 tonnes.”
If these numbers are miscalculated or if the weight of the object being raised is unknown or guessed at, major errors could occur which might lead to the toppling of the crane.
From the laying of the initial concrete, to the construction of the crane, to the crane’s construction of a building or bridge, each step must be carefully engineered and executed to ensure the safety of the site, equipment, and personnel.