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Terminology: Mass Force and Weight

Terms used in both commerce and container handling often appear to be inconsistant, the units also may be confusing, in particular mass and weight, often given in the same units kg and lbs. For legal reasons an effort was made to define both terms and their associated units as long ago as 1972 given in the "Amendments to the International Convention for Safe Containers." This was 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, summarised here:
Mass first and foremost 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, or resistance to change. When we need to move the mass of a container from one place to another, sufficient force will have to be applied to overcome the resistance to want to be moved. Newton called this resistance "inertia" and became his 1st Law. He next gave us the equation to evaluate the magnitude of the force needed to overcome this inertia, his 2nd Law, the Law of Motion and is Force= mass x acceleration. Now if the container is being transported, then, whatever it is mounted on must provide enough force to move it (if it's on an incline it may even move on its own accord). The main problem, however, arises when anything has to be lifted, again Newton's 2nd Law applies but this time the force required is that to overcome gravity.
This is where the anomaly with the terminology becomes a problem because the same units are STILL used to define force and mass. So to understand the difference we need to clearly define the units of force so restating Newton's 2nd Law, with its units.

Now acceleration is due to gravity and assumed to be constant around the world 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, termed weight and should be stated in force units N, but nobody does, nor does anyone ever ask for 4.9N of apples just half a kilo so we just carry on using kgs or lbs, as usual.

When lifting a container with a hook attached to slings, 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 the same units: kg, or lbs, the unit of mass, so to differentiate weight (force) due to gravity from mass, the suffix g is added. Hence a container of mass m, ( Payload P plus Tare T) needs a force of R_g to lift it, the g clearly differentiate that it is a force and not mass.

Contemporary Mass/Weight Units

Relevant information used over the past 40 plus years was in units, based on Series 1 Freight Container classification, dimensions ft/ins and ratings lbs/Ton, however units now in general use are given in table below. Also limiting lifting equipment capacities, were expressed as Safe Working Loads (SWL), rated in Imperial Ton or kg (mass) units, now supposedly deprecated and replaced with Working Load Limits (WLL)and should be in force units, is it?

However, SWL has now assumed secondary importance and replaced by the tighter defined WLL but such is the conservative nature of the lifting industry both definitions are still used.

Having precisely defined the units of mass, weight and force, in accordance with the 1993 ammendment, the units used on the CSC Safety Approval Plate, should be;  as shown on this extract from a container made in 2016. There are many containers that still display a mixture of units in upper and lower case, singular and plural and force units in kg and lb and NOT Newtons.

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 need evaluating before selecting appropriate Working Load Limits (WLL) for lug and avoid failure.

Working Load Limits

Lifting sling arrangements have evolved from the first ISO-668 Standard for Series 1 containers based on maximum Gross Weights (MGW) and high specification chain strength capacities of 16mm and 20mm link diameters and are respectively 8.5 Ton and 12 Ton. All ancilliary chain attachments must be equal to or greater than that of the chain. (Wire rope is also in use with the same loading restrictions. )
In cases where there is a high risk of overloading a Factor of Safety (F of S) is introduced to avoid failure

From ISO 3874, the mandatory minimum
Factor of Safety is 4

For simple stress states, as with a top lifting twistlock, the F of S is introduced at the design stage when all the necessary information is available, ie ultimate tensile strength, yield stress, stress concentration factors, fatigue endurance limits etc). For complex stress states, as with side lifting lugs, where it is difficult or prohibitive to evaluate, the fail load must be found from a test to destruction from which the F of S is given by dividing the fail load by the WLL. A test was conducted on our lug and terminated, for safety reasons, at 64.6 Tons. The lug sustaining deformation, but without any component parts failing, so even though the maximum Breaking Strength (M B S) was not attained the minimum F of S for Preston Shoe are at least: