A tower-crane counter-jib is the short rear arm that carries the machinery house, hoist winches, and stacked ballast slabs, with the counterweight typically positioned just short of the jib end to maximise the counter-moment lever arm [S1]. Selection of slab count, material, and rear-arm geometry is governed by the load-moment equation, where load weight times its radius must be matched by counterweight mass times its distance from the slewing centre [S5].
Design is configuration-specific: hammerhead, flat-top, luffing-jib, and self-erecting models carry different counter-jib lengths and ballast capacities, with hammerhead units historically delivering the highest lift capacities on open sites and luffing-jib units trading radius for compact counter-jib envelopes on congested sites [S2]. The Potain IGO T 130 self-erecting unit, for example, ships with 12 counterweight slabs and a 164 ft offsetable jib, illustrating how ballast stack sizing scales with jib length and maximum tip load [S4].
Load-Moment Balance and the Role of the Counter-Jib
The load moment on a tower crane is the product of the suspended load weight and its horizontal distance from the slewing axis; the counter-jib must generate an equal and opposite moment using stacked ballast mass times its rear-arm length [S5]. Operators adjust the effective counter-moment by adding or removing slabs, or by repositioning the stack along the counter-jib to change the lever arm, matching the configuration to the heaviest lift and longest reach called for by the load chart [S1][S2].
Because the slewing axis sits between the jib and counter-jib like a fulcrum, even modest changes in load radius shift the required counterweight mass non-linearly, so doubling the lift radius at constant load roughly doubles the required counter-moment [S5]. The machinery house (hoist, trolley drive, electrical cabinet) is mounted on the counter-jib close to the tower, contributing a fixed portion of the rear-arm moment and reducing the live ballast mass the configuration must carry [S2].
Counterweight Materials: Density, Cost, and Trade-Offs
Reinforced concrete is the most common counterweight material in tower and gantry cranes, with a density of about 2.5 g/cc, offering high strength, weather and fire resistance, and the ability to be cast into modular slabs sized for stacking and transport [S3]. Concrete slabs allow custom shapes and sizes, keep raw-material cost low, and are durable with low maintenance, making them the default for static, long-term installations [S5].
Steel, at 7.85 g/cc, is roughly three times denser than reinforced concrete and is used where a compact counterweight envelope is required, such as large mobile or marine cranes, or where stacking space on the counter-jib is limited [S3]. The trade-off is corrosion susceptibility and strength loss at high temperatures, which mandates protective coatings or galvanizing, particularly in marine service [S3]. Cast iron, lead, and liquid ballast are also used in niche configurations, with lead and liquid systems offering adjustable mass but introducing containment and environmental-handling constraints [S3].
Comparison of Common Counterweight Materials
The table below lines up the four most common counterweight materials against the decision criteria that drive tower-crane counter-jib specification: density, compactness, corrosion behaviour, and typical use case. Values for density are drawn from the reference data; qualitative ratings reflect the materials' general engineering behaviour as described in the source material [S3].
Counterweight Stacking, Slab Geometry, and Erection
Counterweight slabs are typically modular castings with integral lift points, stacked on the counter-jib platform and secured against shift during slewing and weather loading. The Potain IGO T 130 self-erecting example uses 12 counterweight slabs paired with a 164 ft jib and 122 ft under-hook height, demonstrating the direct relationship between slab count, jib length, and the maximum lift envelope the configuration can support [S4]. For heavier hammerhead and flat-top units, slab count rises with maximum lifting capacity; some mid-size flat-top units in the 4 to 6 tonne class ship with comparable slab stacks sized for transport and on-site handling [S4].
Erection sequencing requires the counter-jib and its ballast stack to be assembled before the jib is raised, because the counter-jib is the first major component pinned to the slewing table once the turntable is set on the mast [S1]. The crane is then jacked up inside the structure (a process called "climbing" or "jumping") as the building rises, with the counter-jib and ballast configuration remaining in place and the mast extended from inside the building [S1][S2]. Where the surrounding structure cannot host tie-ins, the base may be ballasted with concrete or anchored to a heavy foundation block sized to keep the assembly within stability limits without building ties [S2].
Adjustability, Load-Change Response, and Stability Limits
Counterweight configuration must be re-evaluated for each lift, with three canonical cases: lighter loads need less ballast, heavier loads require additional slabs on the counter-jib, and longer radii may need the stack moved outward to increase balancing torque [S5]. For self-erecting units, where the counter-jib is short and the stack is compact, ballast is often pre-engineered into fixed positions because the lift envelope is narrower and the configuration set is smaller [S2].
Stability limits are governed by both structural capacity of the jib and the moment envelope defined in the crane's load chart, the same framework that governs mobile crane stability, as covered in detail in the related analysis of structural versus stability limits in truck crane load charts. Wind adds an out-of-plane moment that the counterweight must resist together with the central ballast at the base, so counter-jib design also considers aerodynamic drag on the stack itself and on the machinery house [S2][S5].
Failure Modes, Inspection Points, and Maintenance Signals
The dominant failure modes in counter-jib and counterweight assemblies are slab cracking, lift-point wear, fastener loosening, and shifting of the stack under slewing acceleration, each of which can shift the centre of mass rearward and reduce the effective counter-moment. Concrete slabs can spall under freeze-thaw or impact during handling; steel stacks can corrode at lift-point bores and lose section at the contact faces between slabs [S3].
Periodic inspection should verify slab count against the active load-chart configuration, check lift-point integrity, confirm hold-down fasteners are torqued, and measure stack position against the as-installed drawing; any deviation outside the as-built position must be corrected before the crane returns to service. Field practice for similar mast-based lifting equipment, including seismic anchoring of stacker-crane masts, is summarised in the field reference on stacker-crane mast seismic design and rail anchoring in 2026, where rail-anchoring and ballast discipline follow the same load-path logic that governs tower-crane counter-jib design.
Specification Checklist for Engineers
A defensible counter-jib and counterweight specification should lock down: required maximum lift and radius from the project lift plan, the matching slab count and individual slab mass, the stack position along the counter-jib (rear-arm length to slab centre of mass), the slab material with density confirmed, and the central base ballast or building tie required to keep the assembly within stability limits [S1][S2][S3][S5]. The specification should also name the governing standard family (for example, the relevant FEM/EN or ASME B30 series for tower-crane structural and stability design) and require the supplier to deliver a configuration matrix that maps slab count to permissible load-radius pairs [S1].
Trackable signals for the next planning cycle include: revisions to manufacturer load charts that change slab count for a given jib length, the rollout of higher-density composite ballast blocks aimed at reducing counter-jib stack height, and updates to European and North American standards for tower-crane stability that tighten the safety factor on minimum counter-moment. For a broader view of how counter-jib and counterweight decisions connect to overall construction-machinery selection, the construction machinery and equipment reference page collects the related categories.
Component reference pages worth checking: tower crane, and counter meter.