Wind on cranes and suspended loads: pressure, sail area and limits

Wind acts on the boom, the hoist ropes and the suspended load. The crane maker allows for a stated wind speed with a stated amount of load surface area. A load with more surface area than that (a panel, a vessel, a roof truss, a blade) can be unsafe at wind speeds well inside the chart limit.

1. From wind speed to force

Dynamic wind pressure in standard air is:

q = 0.613 × v²    (q in Pa or N/m², v in m/s)     Wind force F = q × A × C_f

A is the projected area facing the wind and C_f the force (shape) coefficient: around 1.2 for many compact shapes, up to 2 for flat plates square to the wind.

Pressure rises with the square of the speed: double the wind speed means four times the force. Wind speed also increases with height above ground, and gusts exceed the average. Measure at boom-tip height (most cranes have an anemometer there) and use the 3-second gust value when the manufacturer’s limit is a gust limit.

2. Loads with a large surface area

Under EN 13000, manufacturers commonly base the permissible wind speed on a standard load surface area per tonne of load. Many use about 1.2 m² per tonne with a drag coefficient of 1.2; check your crane’s documentation. If the load’s own wind area per tonne is larger, the permissible wind speed falls. To keep the same wind force per tonne:

v_allow = v_chart × √[(A × C_f)_reference ÷ (A × C_f)_actual]   (both per the same load weight). Use the manufacturer’s own formula where one is given.

Example. A 10 t load with a projected area of 20 m² and C_f = 1.2, so A × C_f = 24 m². The reference for 10 t is 10 × 1.2 × 1.2 = 14.4 m². If the chart allows 9.8 m/s: v_allow = 9.8 × √(14.4 ÷ 24) = 7.6 m/s.

3. How far the wind pushes the load

A suspended load swings out of plumb until the wind force and the weight balance: tan θ = F ÷ W. The horizontal offset at the load is roughly the hoist length × F ÷ W.

Example. Same 10 t load (98.1 kN) at 10 m/s: q = 61.3 Pa, F = 61.3 × 20 × 1.2 = 1.47 kN. tan θ = 1.47 ÷ 98.1 = 0.015 (0.9°). With 30 m of hoist rope that is about 0.45 m of drift, before any gusting. Gusts make the load swing and add dynamic load. On a large, light load the offset can be several metres, enough to strike the boom or nearby structure.

4. Setting wind limits for a lift

• Take the lower of: the crane maker’s limit for the configuration, the limit adjusted for the load’s wind area, and any limit set by the load (for example blades or glazing).
• Write the limit into the lift plan as a stop criterion, with a lower hold value at which the lift is not started.
• Check the forecast for the whole lift duration, including setting down and releasing the rigging.
• Use tag lines to control rotation, never to resist the wind’s full push.
• When out of service, park the crane as the maker specifies for high winds (boom lowered or weathervaning, as applicable).

Wind limits on charts apply to the crane in service with the stated load area. They are not a guarantee that the load itself will be controllable. Judge the load as well as the crane.

5. Wind speed, pressure and the Beaufort scale

Wind pressure rises with the square of wind speed: q = 0.613 × v² (q in Pa, v in m/s). Doubling the wind speed quadruples the force. Typical crane limits fall in a narrow band where small speed changes make large force changes:

The Beaufort scale is defined for 10 m above open ground. It is useful for describing conditions, but a lift decision must use a measured speed at the right height, with the right averaging.

Gusts and averaging

Weather forecasts usually give a mean speed (for example a 10-minute average) and a separate gust speed. Crane manufacturers working to EN 13000 normally state limits as a 3-second gust speed at the highest point of the crane or load. Gusts are commonly 1.4 to 1.6 times the mean in open country, more in built-up areas. Compare like with like: a forecast mean of 7 m/s can easily mean gusts of 10–11 m/s, over a 9.8 m/s limit.

6. Wind speed and height

Ground friction slows the wind near the surface, so speed increases with height. A common model is the power law v_h = v₁₀ × (h ÷ 10)^α, with α about 0.14 for open country (more for rough or built-up terrain, in which case the speed near the ground is lower but rises more quickly).

An anemometer on the boom tip measures the speed where it matters. Without one, a ground reading of 7 m/s at 10 m implies about 8.8 m/s at 50 m. Local effects can be stronger than the profile: buildings shelter the load near the ground and then release it to the full wind as it rises above roof level, and gaps between buildings or round cooling towers can funnel the wind into a jet.

7. Worked examples

Allowable wind for a large module

A 120 t module with a projected area of 300 m² and C_f = 1.3. The chart allows 9.8 m/s for a standard load area of 1.2 m² per tonne with C_f = 1.2 (A × C_f = 1.44 m² per tonne). The module has 300 × 1.3 ÷ 120 = 3.25 m² per tonne.

v_allow = 9.8 × √(1.44 ÷ 3.25) = 9.8 × 0.666 = 6.5 m/s

A light, bulky load is worse still: a 4 t load with 25 m² and C_f = 1.5 has 9.4 m² per tonne, so v_allow = 9.8 × √(1.44 ÷ 9.38) = 3.8 m/s, a gentle breeze. Lifts of roof sheets, cladding panels, formwork and wind turbine blades are often limited this way. Always use the manufacturer’s own method if it differs.

How far it drifts

The same 4 t load (39.2 kN) at 7 m/s: q = 30.0 Pa, F = 30.0 × 25 × 1.5 = 1.13 kN. tan θ = 1.13 ÷ 39.2 = 0.029. On 40 m of hoist rope that is an offset of about 1.15 m in a steady wind, with gusts making it swing further and back again.

Rotation and tag lines

A flat panel 6 m × 3 m (18 m²) with C_f = 2.0 feels F = 0.613 × v² × 18 × 2.0 = 22.1 × v² newtons: about 550 N at 5 m/s, 1,790 N at 9 m/s and 3,180 N at 12 m/s. If the centre of pressure is 0.5 m off the suspension axis, the turning moment at 9 m/s is 1,790 × 0.5 ≈ 894 N·m. A person on a tag line can only hold a modest steady pull, in the order of a couple of hundred newtons. Fixed to the end of the 6 m panel (3 m from the axis), a tag line needs about 300 N just to stop the rotation, before any gust. That is the limit of what two people can manage safely, and far below what is needed to resist the 1,790 N push itself.

• Tag lines control rotation and steer a load into position. They are not for holding a load against the wind.
• Attach them as far from the suspension axis as practical, and use two where the load must not turn either way.
• Never wrap a tag line round a hand or body, and never stand where a swinging load could trap you against structure.
• For heavy or large loads, use mechanical tag-line systems or rotation control devices instead of people.

8. In service and out of service

A crane has two kinds of wind limit. The in-service limit applies while it is lifting: above it, lifting stops and the load is landed. The out-of-service limit applies to the crane left standing: above the in-service limit but below the out-of-service condition, the crane must be parked as the maker specifies. Tower cranes are left free to weathervane; mobile cranes usually lower or retract the boom, or park it at a set angle into the wind, according to the boom length and the expected wind. Check the forecast before leaving a long boom erected overnight.

9. Common mistakes

• Comparing a forecast mean wind speed with the crane’s gust limit.
• Using the ground-level reading for a lift at 60 m.
• Using the chart wind limit for a load with a much larger area per tonne than the chart assumes.
• Starting a long lift close to the limit, with no hold point and no plan for landing the load if the wind rises mid-lift.
• Relying on people on tag lines to hold a large panel against the wind.

10. Self-check questions

General guidance only. It does not replace the manufacturer's data, the applicable standards or the judgement of a competent person.

References

Technical references consulted for this chapter. The text and worked examples are original to LiftPlan Pro.

• J. Keith Anderson, Rigging Engineering Calculations (Industrial Training International).
• ASME B30.5 Mobile and Locomotive Cranes; BS EN 13000 Mobile cranes; BS 7121 Code of practice for safe use of cranes.
• Rigging Handbook, 2nd edition (Bechtel Equipment Operations, 2002).

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