Load and crane
Wind force on a suspended load
- where
- wind (dynamic) pressure (Pa)
- air density, 1.25 kg/m³
- gust wind speed at the load (m/s)
- wind force on the load (N)
- drag coefficient
- largest projected area facing the wind (m²)
- wind area per tonne (m²/t)
- mass of the load (t)
Explain this equation
Estimates the sideways wind force on a load hanging from a crane, and flags loads that catch more wind than standard crane charts assume. Use it for panels, formwork, sheeted items and other light loads with large faces.
q is the wind pressure, from air density ρ (1.25 kg/m³) and the gust wind speed v at the load. C_d is the drag coefficient for the shape. A is the area facing the wind. F_w is the resulting force in newtons. A_w is the wind area, including drag, per tonne of load.
Moving air carries kinetic energy of ½ρv² per cubic metre, which becomes a pressure when it stops against a surface. The force is that pressure times the area, adjusted by C_d for shape. Because v is squared, doubling the wind speed gives four times the force.
Sense check: 9 m/s gives about 50 Pa. On a 1.2 m² face with C_d 1.2 that is about 73 N. Standard charts assume about 1.2 m² per tonne, so a 1 t panel of 10 m² is far outside that.
Watch out: use gust speed at load height, not the ground-level forecast. If A_w exceeds 1.2 m²/t, the crane's permitted wind speed must be reduced, using the manufacturer's data.
Works out the sideways wind force on a load hanging from a crane and flags loads with more sail area than standard crane charts assume.
Enter your values to see the result.
Good to know
q = ½ρv² is the dynamic pressure; ρ = 1.25 kg/m³ is the usual design air density. Drag coefficient: about 1.2 for an average crane load, 1.1 or more for flat-sided items such as panels and containers, 0.2–0.3 for smooth curved shapes. Standard (EN 13000-type) charts assume about 1.2 m² of wind area per tonne; Liebherr states this figure already includes the drag coefficient. Panels, formwork, cladding and scaffold sheeting are typical high-area, low-weight loads. Use the gust speed at the height of the load, which is higher than at ground level.
Common mistake
Using the forecast mean wind speed at ground level instead of the gust speed at the boom head, or forgetting that a large panel turning broadside can double its wind area in a moment.
Limits
Steady wind, no gust factor, shielding or dynamic swing. The crane’s permissible wind speed comes only from the manufacturer’s data. For planning only: this does not replace a lift plan prepared by a competent person under LOLER 1998 (reg. 8) and BS 7121, and the manufacturer’s WLL markings, load charts and data always govern over this result.
Sources
- Liebherr, Safe to use in high winds (standard load charts assume a wind-exposed area of 1.2 m² per tonne including the cw value; 9 m/s gives about 50 N/m²)
- Cranes & Access (Vertikal), wind and crane loads (EN 13000 charts assume 1.2 m² sail area per tonne; drag coefficient about 1.2 for an average load, 1.1+ flat-sided, 0.2–0.3 curved, 1.6 rotor)
- BS 7121-1:2016 Code of practice for safe use of cranes. General (lift planning, appointed person, multiple-crane lifts, ground conditions, wind)
- The Lifting Operations and Lifting Equipment Regulations 1998 (SI 1998/2307), reg. 8: every lifting operation properly planned by a competent person, appropriately supervised and carried out safely
- HSE L113 Safe use of lifting equipment. Lifting Operations and Lifting Equipment Regulations 1998. Approved Code of Practice and guidance
Results are calculations from the published methods and figures shown, and depend on the data you enter. For planning and checking only. Scaffolds must be erected to NASC TG20 or a bespoke design by competent persons, and lifting operations planned under LOLER by a competent person. Never rely on these results instead of a design, a lift plan or manufacturer ratings.