Terms used in both commerce and container handling often appear to be inconsistant, the units also may be confusing, in particular mass and weight, both usually given in the same units kg or lbs. For legal reasons an effort was made to define both terms and their associated units as long ago as 1972 and are given in the
Admiralty and Maritime Law Guide
amendments to the International Convention for Safe Containers. This is necessary because in commerce and the legal profession, weight means mass whilst in lifting and engineering it means force. To clearly define weight and mass, their basic units are those adopted from the SI system and are summarised here:
First and formost is mass, this is the amount of matter an object contains and at last we now know what keeps it together: the Higgs Field, but we are not absolutely certain how this works but it does. The units used to quantify mass are kilograms (kg or kgs) and in some parts of the world pounds (lb or lbs) are still used mainly because of tradition, familiarity, and resistance to change.
Now in this equation acceleration is that due to gravity and is assumed to be constant around the world and is taken as 9.81m/s2. (The standard value for general use is 9.8m/s2 and for approximate use 10m/s2) so the force required to lift 1kg is 9.8N and this is what is termed WEIGHT and should be stated in Force units N but nobody does, nor does anyone ever ask for 9.8N of apples just a kilo so we just carry on using kgs or lbs, as usual.
The resulting force due to gravity is known as a Body Force and acts throughout the mass towards the centre of the earth and in this form is difficult to manage, however, to simplify the problem the total resulting body force is considered to act at the mass centre, generally known as the Centre of Gravity (C of G) acting in the direction of acceleration, thus simplyfing the evaluation of all other forces involved when lifting freight units. For example when top lifting a container with a hook attached to a sling assembly, the force at the hook must equal the weight of the container expressed in N's but as already stated it isn't. Instead we use kg, or lbs, the unit of mass, so to differentiate weight (force) due to gravity from mass, the suffix g is added. Hence a containers having total mass (payload plus tare) rated at R needs a force of Rg to lift it, the units being the same, the difference between them is just a g.
If still confused, an elegant explanation is given at Weight or Mass
Having precisely defined mass, weight and force and their units in the 1993 ammendment to be used on CSC Safety Approval Plate these terms should be stated as:
Maximum Operating Gross Mass (GM) kg,kgs and lb,lbs (Upper or lowercase)
When it comes to lifting where forces are expected GM becomes GW same units.
Allowable stacking load for 1.8g, same as GM above.
Side/ End wall strength newtons or N (Transverse Racking Test Force)
Slowly these units have been adopted as shown on this CSC plate for a container made in 2016. But many containers still bear the a mixture of units (in upper and lower case, singular and plural )
Finally Newton's 3rd Law states that To every action there is an equal but opposite reaction. Lifting forces are the reaction forces susceptible to failure and to prevent this they have to be predicted and evaluated in order to select the appropriate gear and avoid failure. Having evaluated a WLL based on the all the known information there is the famous "unknown unknowns" scenario where the lifting forces induced may be well in excess of those predicted and here is where the Factor of Safety (F of S) comes into play. The mandatory minimum F of S given in ISO 3874 is 4. It is straight-forward when applied to a top lifting twistlock subjected to simple tension only whearas bottom side lifting lugs are subjected to a complex stress state induced by offset loading whose F of S can only be ascertained by a test to destruction. A test to destruction conducted on a lug prior to general release was terminated at 64.6 Tons and did not fracture.