Tower Crane Types, Major Components, Structural Principles and Stability
Tower cranes are generally classified by how they are mounted and how they articulate. Mounting types include static/fixed foundations, ballasted base, rail-mounted travelling cranes, and climbing cranes that jack up inside a building core. Jib types include hammerhead (horizontal jib with a counter-jib and trolley), luffing jib (jib raises and lowers, useful on tight sites and where height clearance matters), and self-erecting cranes. Each has different radius, height and wind behaviour, and questions often test which type suits a constrained site or a low oversail limit.
Major components: foundation or base; mast sections with pins and bolts; slewing ring and slewing mechanism; operator cab; turntable; jib; counter-jib carrying counterweights; trolley and hoist rope; hook block; and the safety devices - moment limiter (rated capacity limiter), hoist and trolley limit switches, slew limits, anemometer, and often zone-limiting or anti-collision systems. Structural principles: the crane is a balance machine. The load moment (load × radius) must be resisted by the counterweight moment, and the mast carries compression plus bending. Stability depends on the foundation, the counterweight, the mast's plumb and integrity, and the load chart - never on the operator's judgement alone.
Traps: confusing the moment limiter with the hoist limit switch; assuming more counterweight increases capacity (it is designed-in, not adjustable); forgetting that wind acts on both the load and the crane's own structure; and mixing up luffing and hammerhead characteristics.
Worked example: load 4 t at 20 m radius = 80 t·m load moment. At 40 m radius the same crane can lift only 2 t for the same moment, before any chart derating.