Tower cranes are a modern form of balance crane that consist of the same basic parts. Fixed to the ground on a concrete slab (and sometimes attached to the sides of structures), 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. Further 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 which are: the long horizontal jib (working arm), shorter counter-jib, and the operator's 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 a signaller (known as a 'dogger', 'rigger' or 'swamper'). They are most often in radio contact, and always use hand signals. The rigger or dogger directs the schedule of lifts for the crane, and is responsible for the safety of the rigging and loads.ComponentsTower cranes are used extensively in construction and other industry to hoist and move materials. There are many types of tower cranes. Although they are different in type, the main parts are the same, as follows:Mast:the main supporting tower of the crane. It is made of steel trussed sections that are connected together during installation.Slewing unit: the slewing unit sits at the top of the mast. This is the engine that enables the crane to rotate.Operating cabin: the operating cabin sits just above the slewing unit. It contains the operating controls.Jib: the jib, or operating arm, extends horizontally from the crane. A"luffing" jib is able to move up and down; a fixed jib has a rolling trolley that runs along the underside to move goods horizontally.Counter jib: holds counterweights, hoist motor, hoist drum and the electronics.Hoist unit: the hoist unit houses the hoist drum, hoist cable, gear box, gear shift, brake, and supporting components.Hook: the hook (or hooks) is used to connect the material to the crane. It hangs at the end of thick steel cables that run along the jib to the motor.Weights: Large concrete counterweights are mounted toward the rear of the mast, to compensate for the weight of the goods lifted.This crane's main beam was broken due to an overloadA tower crane is usually assembled by a telescopic jib (mobile) crane of greater reach (also see"self-erecting crane" below) and in the case of tower cranes that have risen while constructing very tall skyscrapers, a smaller crane (or derrick) will often be lifted to the roof of the completed tower to dismantle the tower crane afterwards, which may be more difficult than the installation. Tower Cranes can often be Pedestrian Operated by a remote control, removing the need for a cab for the Crane Operator to work from.