By Greg Sitek
Cranes are usually an important piece of equipment on building construction projects.
On a construction job site there is always something to lift and place. Innovations in the art of construction have fostered the extended use of lifting devices…cranes, telehandlers, excavators with lifting hooks and the traditional jobsite strongman. Picking and placing concrete panels, floors, girders, roof trusses, pillars, pilings, pipe, heating and air condition units—you name it. Simply put, there is always something that has to be moved from “here” to “there.”
Usually there’s a rush to get the move over with and out of the way. “Move that panel or whatever, you’re holding up the job” is a common superintendent’s war cry on many construction projects. Often the results can be less than satisfactory. When lifting, use a crane that has the capacity and capability to handle the job. And have a skilled operator behind the controls. Once a pick is in the air…
Following are some of the main crane categories:
Truck Mounted Cranes
Truck mounted cranes sit on a commercial truck chassis. The truck engine is used to power the crane operation. There are telescopic boom models available.
These cranes may come with fixed operator control stations and cost less than an all terrain crane or truck crane. There are models where the cab swings with the crane. Another variation is an articulating boom unit. Usually these are specialized horizontal boom units designed to load/unload the truck’s payload. They are normally not used as general-purpose cranes.
Truck cranes can travel safely at highway speeds. They use purpose built carriers with separate cabs for the carrier and crane operations. The hydraulic boom units are designed for quick set up. The smaller and mid-range models generally carry boom, jib and counterweight on board. Some of the larger units may require separate transport arrangements to carry any additional counterweights or boom extensions.
Lattice boom truck crane models offer high lifting capacities and hook heights and are designed to handle the big lifting jobs. By their nature, lattice boom cranes require more set up time than hydraulic boom models.
Rough Terrain Cranes
Rough Terrain cranes still king of the jobsite. They handle tough off-road conditions with four-wheel drive and with various types of steering for maneuverability. They are simple—two-axle configuration and have only one cab from which the operator controls all functions.
They are relatively inexpensive in comparison to other types of cranes. Since they don’t have to travel at highway speeds they don’t require the horsepower or drive train components. The two-axle configuration is another major cost saver.
Their biggest negative comes from the fact that they have to be transported between jobs. Once on the job, they excel at pick and carry operations.
All Terrain Cranes
All Terrain cranes have gained contractor acceptance in a relatively short time because they travel at highway speeds and navigate the rough terrain of a typical jobsite. Multiple axles – steer, drive and tag — distribute the load, provide traction to handle tough job site conditions and give them added maneuverability. Suspension options can give you additional off-road clearance and SUV-like driving characteristics.
Lattice Boom Cranes
Lattice boom cranes are truck-mounted and crawler-mounted. With truck-mounted cranes, the crane’s upper structure is mounted on a truck-style carrier, which can travel at highway speeds. Major sections of the crane may have to be removed and transported separately on some of the larger units. The advantage over crawler cranes, which must be disassembled, is that the carrier is mobile and erection time is usually faster.
Crawler-mounted cranes are mounted on carbodys and are propelled on tracks. This design yields superior on-site mobility, however, crawler cranes are not easily transported. All modular components of a crawler crane have to be moved by trucks. Crawler cranes do offer a great deal of versatility, particularly for heavy lifts or long term lifting projects. From “pick-and-carry” capabilities to heavy duty or severe duty applications, such as pile driving and dragline, crawler cranes offer a great deal of application versatility.
Choosing a specific crane is typically based on job requirements. A lattice crane is typically the best choice when the job requires long, vertical reaches, significantly large lifts, or long-term work. Both truck and crawler-mounted lattice boom cranes are well adapted for lifting and moving large quantities of steel, constructing large tilt-up concrete panels, and for making very high and far-reaching picks. The design of lattice boom is inherently stronger and more stable at greater distances than telescopic boom cranes, plus lattice boom cranes utilize larger-diameter wire rope, requiring fewer parts of line for faster line speeds. Typically, a lattice crane yields higher capacity picks at a nominal base capacity unit, making a 100-ton capacity lattice crane outperform a 200-ton capacity telescopic crane.
The Hydraulic-Crawler-Mounted Crane recently entered the market. It’s available as a telescopic or lattice boom crane mounted on a crawler excavator carrier and offers yet more versatility in your choice of lifting device.
Tower cranes
Tower cranes are fixed to the ground on a concrete slab — sometimes attached to the sides of structures as well as erecter on the inside of structure. Tower cranes often give the best combination of height and lifting capacity and are used in the construction of tall buildings. The base is then attached to the mast, which gives the crane its height. The mast is attached to the slewing unit — gear and motor — that allows the crane to rotate. On top of the slewing unit there are three main parts: the long horizontal jib (working arm), shorter counter-jib, and the operators cab.
The long horizontal jib is the part of the crane that carries the load. The counter-jib carries a counterweight, usually of concrete blocks, while the jib suspends the load to and from the center of the crane. The crane operator either sits in a cab at the top of the tower or controls the crane by radio remote control from the ground. In the first case the operator’s cab is most usually located at the top of the tower attached to the turntable, but can be mounted on the jib, or partway down the tower. The lifting hook is operated by the crane operator using electric motors to manipulate wire rope cables through a system of sheaves. The hook is located on the long horizontal arm to lift the load which also contains its motor.
In order to hook and unhook the loads, the operator usually works in conjunction with other personnel. They are most often in radio contact, and always use hand signals to communicate
A tower crane is usually assembled by a mobile crane of greater reach. Some tower cranes are erected along with the construction of the building. A smaller crane will often be lifted to the roof of the completed tower to dismantle the tower crane. In some building designs the tower crane structure is left as part of the building’s structure.
Thanks to the inventive genius of Hans Liebherr there are also self-assembling, jack-up, or “kangaroo” cranes that lift themselves from the ground or lift an upper, telescoping section using jacks, allowing the next section of the tower to be inserted at ground level or lifted into place by the partially erected crane itself. They can thus be assembled without outside help, and can grow together with the building or structure they are erecting.
Reminders
We are assuming the crane being used, no matter what type — All Terrain, Rough Terrain, Truck Mounted, Lattice Boom truck, or crawler mounted, etc. — is assembled, ready for operation and has been transported to the lift zone. Let’s face it: If the crane isn’t properly rigged and assembled, you definitely don’t want to be operating it. Many of the larger cranes require some on-site assembly due to transportation size and weight restrictions. This type of assembly needs to be executed with the greatest care and caution to insure not only proper assembly but also to maintain the integrity of the components. A carful inspection of the crane after transport is essential.
All cranes must be leveled according to the manufacturer’s specifications. If the machine is not level, the lifted load will cause side stresses on the boom and the stability and structural integrity of the machine will be adversely affected. Since outriggers provide greater stability than tires, machines with outriggers should have the outrigger beams extended and set for lifting operations; consult with the manufacturer’s instructions for on-rubber operation.
When using outriggers, set the outrigger’s beams to their fully extended position, always extending the beams equally. When using outriggers, be sure all tires are clear of the ground and level the machine in all directions, as specified by the manufacturer.
There is no substitute for experience, especially for a crane operator. You can develop the skill and knowledge that will enable you to handle a wide variety of situations that can and do happen when handling loads through actual in-the-seat experience. There are manuals and video media, and today there are simulators that can provide the information and practice that can help get you get ready to start learning how to be a crane operator. Although simulators, videos and manuals will let you gain knowledge and practice, you don’t start becoming an experienced operator until you make your first pick.
This article appeared in June 2012 issues of the Associated Construction Publication magazines national section.