Weights and lifting
Crane radius from plan offsets and load chart utilisation
- where
- working radius from centre of slew (m)
- plan offsets of the hook position (m)
- load chart utilisation (%)
- total lifted weight (t)
- chart capacity at the radius (t)
- spare capacity (t)
Explain this equation
Finds the radius the crane will work at from plan offsets taken off the drawing, and how much of the load chart capacity the lift uses. Use it when choosing a crane position and checking each pick.
d_x and d_y are the distances, along two directions at right angles, from the crane’s centre of slew to where the hook must be. R is the straight-line radius. W is the total lifted weight and C is the chart capacity at that radius. U is the percentage used and S_p is what is left over.
The radius is the hypotenuse of a right-angled triangle, so it comes from Pythagoras. Utilisation is just weight divided by capacity, times 100 to make a percentage.
Sense check: offsets of 12 m and 9 m give a 15 m radius. A 3.2 t lift on a 4 t duty uses 80% of the chart.
Watch out: read the chart at the next radius step above R, never below, and check both the pick-up and landing positions. Booms deflect under load, which pushes the radius out further.
Works out the hook radius from plan offsets taken off the drawing, then compares the total lifted weight with the capacity you read from the crane’s load chart at that radius.
Enter your values to see the result.
Good to know
Measure from the centre of slew (not the outrigger or track edge) to the furthest position the hook will reach for this piece, both at pick-up and at landing. Read the chart at the next radius step above the calculated radius, for the actual boom or jib length, counterweight and outrigger set-up. Check the chart notes for whether the hook block is deducted.
Common mistake
Checking only the landing radius when the pick-up point is further out, or reading capacity at a radius smaller than the true one.
Limits
Plan geometry only: no allowance for boom deflection, which increases the radius under load, wind, or dynamic effects. The lift plan by the appointed person governs.
Sources
- BS 7121-1:2016 Code of practice for safe use of cranes – General (lift planning, appointed person, total load on the hook)
- The Lifting Operations and Lifting Equipment Regulations 1998 (SI 1998/2307), reg. 8: lifting operations planned by a competent person, supervised and carried out safely
- HSE L113 Safe use of lifting equipment (LOLER Approved Code of Practice and guidance)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Always work to the engineer’s drawings and bending schedules, BS 8666, BS EN 1992, BS EN 1090-2, BS 5531/the BCSA National Structural Steelwork Specification and manufacturers’ data. Lifting must be planned under LOLER by a competent person.