Dynamic loads: hoisting factors, shock loading, snatch lifts and load swing

Every calculation so far has used the static weight. Real lifts move. The hoist accelerates and decelerates, slings take up slack, the crane slews and stops, and the load swings. These dynamic effects are partly covered by the allowances built into crane charts and rated loads, but only for smooth, normal operation. Snatching, dropping or swinging a load can produce forces several times the weight. This chapter shows where those forces come from and how large they are.

1. Normal hoisting: the dynamic factor

When a load is lifted off the ground, the crane structure and ropes stretch and the load oscillates briefly, so the peak force is a little higher than the weight. Crane design standards express this as a dynamic factor applied to the hoist load. In BS EN 1991-3 (and the crane standard BS EN 13001-2) it is:

φ₂ = φ₂,min + β₂ × v_h   v_h = steady hoisting speed (m/s); φ₂,min and β₂ depend on the hoisting class HC1 (flexible, smooth) to HC4 (stiff, harsh)

Two lessons follow. Faster hoisting gives a larger factor, so slow down for the first lift off the ground. And stiffer systems (short ropes, rigid structures such as a runway beam or a lifting frame) see more dynamic load than flexible ones. Rated capacities of cranes and hoists already include the designer’s factor for normal use. Temporary lifting points, beams and lugs that you design or check yourself must include one (Chapters 5 and 13).

Offshore lifts from or onto moving vessels have much larger dynamic factors, set by marine standards such as DNV-ST-N001 according to the hook load, crane type and sea state. They are outside the scope of this guide.

2. Shock loading: a drop into a slack sling

If a load falls even a short distance before the rigging takes its weight (a load slips off a support, a sling slips on a hook, a choke slides, or slack is released suddenly), the rigging must absorb the energy of the fall by stretching. Treating the sling as a spring of stiffness k:

Peak force F = W × [1 + √(1 + 2 × h × k ÷ W)]   h = free fall distance; k = EA ÷ L for a sling of length L

Even with no drop at all (h = 0), a load applied suddenly gives F = 2W.

Example. A 2 t load (19.6 kN) drops 0.1 m before a 3 m wire rope sling goes tight.

• A 20 mm steel-core rope has a metallic area of about 180 mm² and an effective modulus of about 100 GPa: k = 100,000 × 182 ÷ 3,000 ≈ 6.1 kN/mm (6.1 MN/m).
• 2hk ÷ W = 2 × 0.1 × 6.07 × 10⁶ ÷ 19,620 = 61.9; F = 19.6 × (1 + √62.9) = 19.6 × 8.93 = 175 kN (17.9 t).
• The same drop into a 3 m polyester roundsling that stretches about 2.5% at 2 t (k ≈ 0.26 MN/m): F ≈ 2.9 W (5.8 t).

In a real lift the crane boom and hoist ropes also stretch, which reduces the peak, but the order of magnitude is right: a short drop into stiff rigging can multiply the load many times and break slings, shackles or lugs that were correctly sized for the static weight. Synthetic slings absorb shock better, but they are not a licence to drop loads.

3. Snatch lifts

If the hoist is moving at speed v when the rigging goes tight on a load sitting on the ground, the load is suddenly accelerated. Ignoring crane flexibility, the peak force is about F ≈ W + v × √(k × m).

Example. The 2 t load on the 3 m wire rope sling, hoist at 0.2 m/s: v × √(k × m) = 0.2 × √(6.07 × 10⁶ × 2,000) = 22.0 kN. F ≈ 19.6 + 22.0 = 41.7 kN, about 2.1 W. The crane’s own flexibility reduces this in practice, but the rule is clear: take up slack slowly, pause, then lift. Never let the hook “bounce” to free a load that is stuck; the force is unknown and may be enormous.

4. Load swing: the pendulum

A suspended load is a pendulum with a period T = 2π × √(L ÷ g), where L is the length from the boom tip sheave to the load’s CoG. With 30 m of rope T = 11 s; with 10 m, 6.3 s. A swing started by a jerky movement takes several periods to die away, so a long hoist rope gives a slow swing that is easy to underestimate.

Slewing: outward swing

While the crane slews at a steady angular speed ω, the load swings outwards until tan θ = ω² × R ÷ g.

Example. 1 rpm (ω = 0.105 rad/s) at 20 m radius: ω²R = 0.22 m/s², θ = 1.3°. With 30 m of rope the load moves 0.67 m further out, increasing the radius and reducing the capacity margin. On a lift near the chart limit, that matters.

Starting and stopping a slew

At 1 rpm and 20 m radius the boom tip moves at 2.1 m/s. Stopping in 3 seconds decelerates the load at 0.70 m/s², about 7% of g. The load swings forward about 4° (2.1 m sideways on 30 m of rope) and pulls sideways on the boom tip with about 7% of its weight, several times the side load a boom is typically designed for (Chapter 7). Start and stop slewing smoothly, and let the load settle before the next movement.

5. Other dynamic effects

• Travelling with a suspended load (pick and carry, crawlers, forklifts with jibs): bumps and braking make the load swing and bounce. Use the maker’s pick-and-carry chart, travel slowly on prepared ground, and keep the load low and restrained.
• Lifting out of water: suction on the seabed, water trapped in the load and added mass as the load breaks the surface can increase the load considerably. Lift slowly at breakout and allow for drained weight.
• Load release: setting a load down and releasing a stiff rigging system suddenly can make the boom whip back. Lower until the rigging is slack before unhooking.
• Wind gusts (Chapter 10) add dynamic swinging to the steady wind force.

6. Controls

• Lift slowly off supports, pause at a few centimetres, then continue.
• Keep slack out of the rigging; make sure slings are seated and chokes bedded before lifting.
• Do not drop loads onto slings, and do not let loads fall onto supports.
• Slew, luff and travel smoothly; use variable-speed controls and let the load settle between motions.
• Use tag lines to control swing; never try to stop a large swinging load by hand.
• Include a dynamic factor in every check of temporary lifting points, beams and lugs, and state its value in the lift plan.

6a. Worked example: a temporary lifting lug

A lug is to be welded to a skid for a single lift of a 10 t load with a mobile crane at moderate hoisting speed. How should dynamics enter the design load?

• Static load on the lug: 10 t (including the share of rigging below it).
• Dynamic allowance for normal hoisting: if the plan uses φ₂ = 1.17 (HC2 at 0.2 m/s), the design load is 11.7 t. Many procedures for lifting points instead specify a fixed minimum factor; use whichever your procedure requires.
• The design standard for the lug (for example BTH-1, Chapter 5) then applies its own design factor on top of that load. The dynamic factor and the design factor do different jobs: one estimates the real peak load, the other covers uncertainty in strength.
• No reasonable factor covers a drop into slack rigging (Section 2), which could load the lug several times over. The control for that is the lifting method, not the lug design.

6b. Swing and clearance

A swinging load needs room. If a load hung on 20 m of rope swings with an amplitude of 0.5 m, it passes through the plumb position at its highest speed, about 0.5 × √(g ÷ L) = 0.5 × 0.70 = 0.35 m/s, enough to injure someone or damage equipment. Allow for swing in clearance distances, keep people out of the swing zone, and let the load settle before landing it or guiding it by hand.

6c. When a stuck load breaks free

A load that is stuck (frozen to the ground, wedged between structure, held by a forgotten bolt or by suction) resists until the pull is large enough to free it. At that moment all the extra pull is suddenly free to accelerate the load. If the hoist is pulling 1.5 times the weight when the load frees, the net upward force is 0.5 W, so the load accelerates upwards at about 0.5 g, overshoots, and bounces on the rope. Meanwhile the boom, which had been deflected by the extra load, springs back and can whip. The crane’s load indicator may show a brief overload or an alarm, and the rigging may have been loaded well above its rating before the load came free.

Controls: find out why the load is stuck and free it by other means (unbolt it, break the suction with water or air, jack it, cut the obstruction) before lifting. Set a stop value on the load indicator or load cell a small margin above the expected weight, and stop the lift if it is reached. Never use the crane to “test” whether a load is free.

6d. Using load cells and load indicators

• A crane’s rated capacity indicator measures the load at the hook continuously. During the first lift off, watch it for a reading higher than the calculated gross load: it is the most direct check of weight and of any snagging.
• A load cell in the rigging shows the real share in one leg or one crane, which is valuable in tandem lifts (Chapter 9) and multi-leg lifts (Chapter 3).
• Readings during movement include the dynamic part. Read the static value with the load hanging still; use the peak value, where the instrument records it, to see how much dynamic load the lift generated.

7. Common mistakes

• Sizing rigging for the static weight with no margin for dynamic effects, then lifting fast.
• “Jerking” a stuck load free with the hoist.
• Slewing quickly and stopping abruptly with a long hoist rope.
• Leaving slack chains or slings in rigging that will snatch tight.
• Assuming that a crane’s chart allowance covers abnormal operation such as pick and carry, offshore lifts or snatching.

8. 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.

• BS EN 1991-3 Actions induced by cranes and machinery; BS EN 13001-2 Crane safety: load actions; DNV-ST-N001 Marine operations and marine warranty.
• ASME B30.5 Mobile and Locomotive Cranes; BS EN 13000 Mobile cranes; BS 7121 Code of practice for safe use of cranes.
• J. Keith Anderson, Rigging Engineering Calculations (Industrial Training International).

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