01
What a dynamic amplification factor is, and is not
It is a load, expressed as a multiplier. It exists because a static calculation of a hanging weight is not a calculation of a lift.
Hang a mass on a rope and the rope carries its weight. Start hoisting it and, for a moment, the rope carries more: the load has to be accelerated upward, and the extra force is the mass times the acceleration. Stop hoisting sharply and the load's momentum keeps it going, stretching the rope further. Snatch a load that is stuck and the arrival of the force can be violent.
The dynamic amplification factor is an allowance for all of that, wrapped into one multiplier on the static weight. It is a load-side quantity, and three consequences follow.
It is not a safety factor. A safety factor is a margin against uncertainty in resistance. This is an estimate of a force that genuinely occurs. Adding one to the other because both are numbers greater than one is how a design ends up twice as heavy as it needs to be with no more margin than before.
It belongs with the other load factors, not instead of them. Weight contingency covers not knowing the weight. Skew load covers the load not sharing as assumed. Consequence factors cover what happens if a particular component fails. Each answers a different question and each is applied.
It is a property of the operation, not of the load. The same vessel lifted by a tower crane in a yard, by a mobile on a windy site, and by a floating crane in a seaway carries three different factors. Nothing about the vessel changed.
02
What actually drives the number
Weight and motion. The first surprises people, because heavier loads carry smaller factors, and that is not a concession - it is physics.
Weight. A dynamic amplification factor falls as the item gets heavier. The reason is inertia: a given crane motion accelerates a light item much more than a heavy one, because the crane's rope stiffness and the operator's hoist speed are roughly fixed while the mass is not. A one-tonne item on the end of a long fall is lively. A five-hundred-tonne item is not.
Motion of the crane and its support. A crane on firm ground moves only as the operator drives it. A crane on a barge in a seaway moves whether the operator wants it to or not, and the load moves with it. This is the difference between the onshore and offshore columns of any factor table, and on a light item it is a factor of more than two.
- Onshore, standard
- Offshore, sheltered
- Offshore, open sea
Two readings of that chart change how you use it.
The onshore column is nearly flat. It holds 1.10 from one tonne to sixty and only then steps down. That is why 1.10 has become a habit onshore, and the habit is mostly harmless as long as nobody forgets what it assumes.
The offshore column never comes down to the onshore one. A light item lifted in open water can carry more than twice its static weight, and even at five hundred tonnes the factor is still above the onshore value. An onshore engineer who ports a familiar 1.10 into a marine job is understating the load by more than a factor of two on small items.
03
The other two factors on the same route
Dynamic amplification, skew load and a consequence factor for lift points. Together they multiply to more than any of them separately, which is the number people are surprised by.
The marine warranty route this article works applies three factors, and each answers a different question.
Dynamic amplification covers the extra force from motion, as above.
Skew load covers the load not sharing between its lift points as the geometry says it should. Sling length tolerance, fabrication tolerance in the attachment positions, and the indeterminacy of a four-point lift all mean one point takes more than its calculated share.
A consequence factor covers what a failure of that particular component would mean. Lift points and their attachments carry a higher one than the structure between them, because a lift point failure drops the load and a member failure usually does not.
None of the three replaces the others, and none of them is optional on this route.
Design demand
- is the characteristic load
- is consequence factor for the component being checked
On the worked onshore case they compound to 1.573.
1.573, not 1.10. That is the number an onshore lift point carries on this route, and the gap between it and the familiar 1.10 is entirely the two factors that are not the dynamic one. Anybody who has ever wondered why offshore lift points look so heavy has most of the answer there.
04
The same lift point, onshore and in open water
Identical steel, identical characteristic load. One passes with room and one fails, and every factor in between was selected by the engine from a duty decision rather than typed in.
A 300 kN lift point: 35 mm plate in S355, 280 mm wide, 64 mm hole on a 60 mm pin, 100 mm edge distance, welded all round with a 12 mm fillet 260 mm long.
Lifting Lug Calculator · computed at page render
Onshore, standard lift
The marine warranty route applied to an ordinary land lift. The factors are derived from the lift category and the weight, not entered.
| Characteristic load | 300.0kN |
|---|---|
| Dynamic amplification selectedonshore column | 1.10 |
| Skew load factor | 1.10 |
| Consequence factorlift points and their attachments | 1.30 |
| Design demanda total of 1.573 on the characteristic load | 471.9kN |
| Double-plane shear-out | 80.6% |
| Fillet weld throat resultant | 74.8% |
Passing with a sensible margin. Note that the lift point is already substantial for a 30 t load, and the reason is the 1.573 rather than anything about the steel.
Open this example in the calculatorLifting Lug Calculator · computed at page render
The same lift point, open sea
One field changed: the lift category. Everything downstream follows from it.
| Characteristic loadunchanged | 300.0kN |
|---|---|
| Dynamic amplification selectedagainst 1.10 onshore | 1.45 |
| Design demanda total of 2.076 | 622.9kN |
| Double-plane shear-outagainst 80.6% onshore - failed | 106.4% |
| Fillet weld throat resultant | 98.7% |
The load never got heavier and the lug never got thinner. The lift moved to open water, and the factor selected for that column at this weight took the demand up by 32 percent.
Open this example in the calculator05
Choosing a factor when nothing imposes one
Most onshore lifts have no standard that gives a number. That does not make the choice arbitrary; it makes it yours, and it has to be written down.
If a marine warranty scope or a client specification imposes factors, use those. If nothing does, the factor is a project decision, and a defensible one answers four questions.
How well is the weight known? A weighed item and an item taken off a data sheet are different problems, but that difference belongs in the weight contingency, not here. Keep them separate so a reviewer can see both.
How is it being hoisted? A controlled hoist with a variable-frequency drive is not the same as an on-off contactor hoist. A snatch off a trailer, a lift that starts with the load partly buried, or a hoist that has to be stopped quickly all sit outside a routine factor.
What is the crane standing on? Firm ground, a designed working platform, a barge or a floating vessel. This is the single largest discriminator.
Is there anything that shocks the load? A load that must be broken free, a bolted item that releases suddenly, a load lifted through a splash zone. Each of those produces a transient a routine factor was never intended to cover.
Write the answer as a sentence, not a number: "1.15 dynamic amplification, for a controlled hoist on a designed working platform, item weighed, no shock release." That is a decision a reviewer can accept or challenge. "DAF = 1.15" is not.
06
Six ways this goes wrong
Four are double-counting and two are the wrong column.
1. Treating it as a safety factor. It is an estimate of a real force, and it sits on the load side alongside contingency and skew. A resistance-side margin is a different thing and both are needed.
2. Applying it twice. Once in the crane's own duty allowance and once in the lift point's demand, or once by the rigging engineer and once by the structural engineer. This is the commonest error and it is invisible unless the load path is written out with each factor's owner named.
3. Mixing schemes. A factor from one standard's system, applied inside another's, where the second's design factor already covers part of the same allowance.
4. Using an onshore factor for a marine lift. At small weights the offshore column is more than double the onshore one.
5. Forgetting that it falls with weight. A factor chosen for a heavy item and reused for a light one on the same project understates the light one substantially.
6. Choosing a number rather than a case. "1.15" is not a decision anyone can review. "1.15, for a controlled hoist on a designed platform, weighed item, no shock release" is.
Common questions
- What dynamic amplification factor should I use for a crane lift?
- Whatever your project's route imposes, and where nothing does, a value you can justify from four answers: how the item is hoisted, what the crane is standing on, whether anything shocks the load, and how heavy it is. Onshore practice commonly lands near 1.10 to 1.15 for a controlled hoist on firm ground. Write it as a sentence rather than a number, so a reviewer can challenge the case rather than the digit.
- Is the dynamic amplification factor a safety factor?
- No. A safety factor is a margin against uncertainty in resistance; a dynamic amplification factor is an estimate of a force that genuinely occurs when a real hoist accelerates a real load. It belongs on the load side alongside weight contingency and skew, and adding it to a resistance-side margin because both are numbers greater than one produces a design twice as heavy with no more real margin.
- Why does the dynamic amplification factor get smaller for heavier loads?
- Inertia. The crane's rope stiffness and hoisting speed are roughly fixed, so a given crane motion accelerates a light item far more than a heavy one. In the sweep derived for this article, the open-sea column runs from 2.44 at one tonne down to 1.10 at 2500 tonnes, while the onshore column barely moves at all between 1.10 and 1.03 across the whole range.
- Why are offshore lift factors so much higher?
- Because the crane and its support are moving whether the operator wants them to or not. A crane on firm ground moves only as it is driven; a crane on a vessel in a seaway moves with the sea, and the load moves with it. At small weights the open-sea factor in this article's derived curve is more than double the onshore one, and it is still above the onshore value at five hundred tonnes.
- Is the dynamic factor the whole demand side?
- No, and this is where the surprise usually is. On the marine warranty route worked here it travels with a skew load factor, covering the load not sharing between its lift points as the geometry says, and a consequence factor for lift points and their attachments, which are higher-consequence than the structure between them. On an ordinary onshore case those three compound to 1.573 on the characteristic load, which is a long way from the 1.10 people quote.
Sources
Every document below is linked at its publisher or regulator. Xarpis reproduces no standard text; where a clause is named, the identifier is given so you can find it in your own copy.
DNV-ST-N001Marine operations and marine warranty
DNV · paid document
The marine warranty standard behind most offshore lift factor sets: dynamic amplification, skew load and consequence factors, and the load cases a marine operation is planned against. Widely applied onshore by contract even though its scope is marine.
ASME BTH-1Design of Below-the-Hook Lifting Devices
ASME · paid document
Structural, mechanical and electrical design criteria for below-the-hook lifting devices, used alongside ASME B30.20 which carries the safety requirements. The current edition is BTH-1-2023; Xarpis implements the 2020 edition and says so on every result.
LOLER 1998Lifting Operations and Lifting Equipment Regulations
UK Health and Safety Executive · free to read
The UK duty framework for lifting operations: planning by a competent person, supervision, and thorough examination of lifting equipment and accessories. Like OSHA's rules it governs the process, not the arithmetic.
EN 13155Cranes. Safety. Non-fixed load lifting attachments
BSI (national adoption of the CEN standard) · paid document
The harmonised European standard for non-fixed load lifting attachments - the family a spreader beam or lifting beam belongs to. It carries the load basis and the proof requirements, not member resistances, which is why an EN route needs EN 1993 alongside it. Now published as EN 13155:2020+A1:2025.
Run the check properly
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Something here wrong, or thinner than it should be? Tell us which paragraph and it gets rewritten. Articles carry the date they were last revised for exactly this reason.