Tower Crane: How It Climbs Its Own Mast

Anatomy: the mast, the top and the climbing cage

A top-climbing tower crane has two parts that matter here. The mast is a stack of identical square lattice sections, each with four corner chords joined by bracing and bolted or pinned to the section below at all four corners. Liebherr's HC masts use standard 5.8 m sections; other makers use roughly 3–6 m. Above it sit the upper works: the slewing unit (slewing ring, supports and cab), the jib, and the counter-jib carrying the hoist winch and concrete ballast.

The climbing tool is the climbing cage: a steel frame around the top of the mast, pinned beneath the slewing unit's fixed base so that it does not turn with the jib. One face is open, with a roll-in beam projecting from it to receive new sections. Its working parts are:

• Guide rollers on all four corners at two levels, typically mid-height and bottom of the cage, running on the mast chords. They centre the cage and take sideways load while the top is unbolted, and the cage is tall enough that both levels still bear on the mast after the top has risen a full section.
• A hydraulic ram, fed by a power pack on the cage, whose support shoe seats on horizontal supports built into each mast section: K-frame bracing on some masts, climbing lugs on others.
• Holding pawls on most designs, which rest the cage on the mast between ram strokes, and a load-holding valve on the cylinder so that a burst hose cannot let the top sink.

Above the freestanding height given in the manufacturer's tables, the mast must also be tied in to the building with steel collars and struts, so new collars go in as the crane rises.

The climbing sequence, step by step

A climb, called a jump on site, follows the manufacturer's written procedure, with a trained crew and the operator in the cab:

• 1. Line up and load. The operator slews the jib over the cage's open face and uses the hook to set the new section on the roll-in beam.
• 2. Balance. The operator lifts a specified load, typically another mast section, and trolleys it to the balancing radius marked in the manual or on the jib. This puts the centre of gravity of everything about to be lifted on the ram's line of action. In balance, no guide roller is pressed hard against its chord.
• 3. Transfer the weight. The slewing brake is locked, and nobody slews, hoists or trolleys from here on. The ram's shoe is seated and the ram pressurised until it carries the top's weight. Only then are the bolts or pins joining the slewing unit to the top section removed.
• 4. Jack. The ram lifts the upper works and cage. Its stroke is usually shorter than a section, so this typically takes two or more strokes: the cage rests on its pawls, the ram retracts, the shoe re-seats on the next support up, and the ram extends again, until the gap is slightly taller than one section.
• 5. Insert. The new section rolls into the gap and is joined to the mast below. The ram lowers the slewing unit onto it, and all four corner chords are bolted to the specified torque, or pinned, at both joints.
• 6. Release and resume. The ram is relieved and the balance load set down. The crew repeats the cycle, or the crane is inspected and returned to service.

Each cycle adds one section, 5.8 m on a Liebherr HC mast, so five climbs add 29 m, roughly eight storeys.

The physics: balance, roller couples and ram force

With the bolts out, the top is held by only two things: the ram, pushing up along one line, and the guide rollers, which can push only sideways.

Balance. If the centre of gravity of the lifted weight W sits a horizontal distance e from the ram's line of action, the weight and the ram force form a couple M = W·e that tries to tip the top. Nothing vertical can resist it, so the rollers take it as a horizontal couple, the two levels pushing on the mast in opposite directions. With the levels a height h apart, each carries H = M / h. A well-balanced top (e ≈ 0) makes H nearly zero: the ram carries almost pure axial load and the rollers merely guide. A badly balanced one presses the rollers into chords designed mainly for axial force, which can jam the cage or bend a chord locally. Slewing, trolleying or dropping the balance load while the top is loose changes M instantly.

Ram force. A hydraulic cylinder pushes with F = p·A. A 200 mm bore has A = π × (0.1 m)² ≈ 0.0314 m², so at 300 bar (30 MPa) it pushes 30 × 10⁶ × 0.0314 ≈ 942,000 N: about 940 kN, or ~96 tonnes-force.

Wind. Wind force scales with dynamic pressure q = ½ρv². With ρ ≈ 1.25 kg/m³, 12 m/s gives q = 90 Pa and 20 m/s gives 250 Pa, so the climbing limit allows only about 36% of the wind pressure the crane works in. During a climb that force passes through rollers and a ram rather than preloaded bolted joints, and it upsets a balance set in still air.

Worked example: jacking a 55-tonne top

Take an illustrative mid-size crane whose lifted mass during the climb is 55 t, counting the upper works, the cage, the new section and a 4 t balance load on the hook. The ram has a 200 mm bore and the roller levels are 5 m apart.

• Weight: W = 55,000 kg × 9.81 m/s² ≈ 540 kN.
• Pressure needed: p = W / A = 540,000 N / 0.0314 m² ≈ 17.2 MPa, about 172 bar. At 300 bar the ram could push ≈ 940 kN, a margin of about 1.7×.
• Trolley stopped 5 m past the mark: the moment shifts by 4 t × 9.81 × 5 m ≈ 196 kN·m, moving the centre of gravity by e = 196 / 540 ≈ 0.36 m. Each roller level then carries H = 196 / 5 ≈ 39 kN, about 4 tonnes-force pushing sideways on the mast.
• Balance load lost mid-climb: if that 4 t load, out at a balancing radius of, say, 25 m, were set down or dropped, the moment would swing by roughly 4 × 9.81 × 25 ≈ 980 kN·m towards the counter-jib. Each roller level would then see H ≈ 196 kN, about 20 tonnes-force, before any shock from the sudden release.

A 0.36 m centre-of-gravity error in a 55 t top is invisible from the ground, yet it puts tonnes of side load on the mast. That is why the balance is fixed before a single bolt comes out.

Wind limits, standards and inspection

• ISO 12480-3 (safe use of tower cranes) advises no climbing in winds above 12 m/s (~43 km/h). That compares with the ~20 m/s (72 km/h) in-service limit, which matches the 250 Pa in-service design wind pressure used in ISO 4302 and EN 14439. A lower figure in the manufacturer's manual governs. Wind is read from an anemometer at the top of the crane.
• In the United States, ASME B30.3 and OSHA 29 CFR 1926.1435 apply. OSHA requires compliance with every manufacturer prohibition during climbing, a registered professional engineer's verification that the host structure can carry the loads from braces, anchorages and supporting floors, and a default mast plumb tolerance of 1:500 where the manufacturer gives none.
• Before a climb, crews check the power pack and relief-valve setting, roller adjustment, shoes and pawls, the new section and its fasteners, the tie-ins for the new height and the wind forecast.
• After a climb, joint bolts are tightened to their specified preload or pins secured, plumb is checked with a theodolite or total station, limit switches and the load-moment limiter are checked, and a competent person inspects the crane before it lifts again.

303 East 51st Street and how climbs go wrong

The best-known climbing-related accident happened on 15 March 2008 at 303 East 51st Street in Manhattan. An hour after the crane was jumped four sections, the crew had raised an 11,000 lb (about 5 t) steel tie-in collar to the 18th floor and hung it from the mast on four polyester slings while tie beams to the building were fitted, when the slings failed. The collar fell, struck the tie-in collar at the 9th floor and knocked it loose, and the mast, left with no tie above the 3rd floor, toppled onto neighbouring buildings. Seven people were killed: six construction workers and a woman in a nearby building.

Dismantling carries the same risks. On 27 April 2019 in Seattle, part of a tower crane being taken down toppled onto a street, killing four people. Pins joining mast sections had been removed early, against the manufacturer's procedure, and a wind gust struck. Both disasters happened while ties or pins were being changed, not during routine lifting. The recurring failure modes are:

• Imbalance: the wrong balance load, the wrong trolley position or slewing mid-climb, overloading rollers and chords through H = M/h.
• Load-path shortcuts: bolts out before the ram carries the weight, a shoe or pawl not fully seated, or pins pulled early.
• Incomplete joints: missing, wrong-grade or under-torqued bolts, or unsecured pins. Mast joints see load reversals whenever the crane slews and rely on preload to resist fatigue.
• Rigging: damaged synthetic slings, or slings without edge protection cut on sharp steel.
• Ties: climbing beyond the permitted height above the last tie-in, or onto anchorages the building was never checked to carry.

Top climbing versus internal climbing and self-erecting cranes

An internal climbing crane stands inside the building, usually in a lift shaft or the core. Its fixed-length mast is held by climbing collars or beams set into two or more floor levels, and hydraulic jacks lift the whole crane, mast included, to new supports every few floors. No sections are added, and the building carries the crane's weight, so its slabs must be checked for crane loads.

A self-erecting crane unfolds a folding or telescoping mast and jib with its own drives. It serves low-rise work and has no climbing cage.

Common misconceptions:

• The crane lifts itself with its hook. The hook only delivers sections and holds the balance load; the ram does the lifting.
• The mast telescopes. Nothing slides inside the mast. The cage slides up its outside and the height comes from a new section, though some manuals still call climbing telescoping.
• Any heavy load will balance it. The load and radius are specified for that crane's jib length and ballast, and changing either moves the balancing radius.
• A climb can be paused if the wind rises. The top cannot be left unbolted on the ram. The crew must finish, or lower the top back and bolt it, before the wind reaches the limit, so climbs start only with a settled forecast.

Top-climbing tower cranes and the look-alikes they are confused with: how each gains height and what carries it.

Crane type	How it gains height	What carries the crane	Typical use
Top-climbing (external) tower crane	A climbing cage jacks the upper works up and a new mast section is inserted beneath the slewing unit; the mast grows	Its own foundation, with tie-in collars to the building above its freestanding height	Mid-rise and high-rise buildings whose height grows over months
Internal climbing crane	A fixed-length mast is jacked up as a whole through the building every few floors; no sections are added	Climbing collars or beams on the building's floor slabs, usually in the core or a lift shaft	Very tall towers and tight sites with no room for a base
Self-erecting crane	Unfolds a folding or telescoping mast and jib with its own drives; height is fixed by the design	A ballasted base or chassis on the ground	Low-rise housing and short jobs
Tower crane erected to full height	A mobile crane stacks every mast section at the start; height is fixed unless a climbing cage is fitted later	Its own foundation, with ties if needed	Jobs whose final height is modest and known from day one