#### Limiting Gross Weights for both Uniform and Eccentric container loading

The forces in the lifting gear to which the top of chain is connected, may well be in excess of those for the lugs.

Bottom Corner Lifting

Bottom Corner Lifting

Lifting freight units of any form, is a highly skilled operation and should only ever be undertaken an HSE publication

- Container Lifting
- Uniform Loading
- Eccentric Loading
- Tilt and Swing

However, to get the load moving in the first place it must be accelerated to overcome gravity, this is the dynamic or live load and is an inertia force, eventually the load becomes stationary before being lowered. The total lifting force, therefore, is that due to gravity plus the live load , the later being rarely quantifiable and must always be kept to an absolute minimum, consequently the VERTICAL LIFTING FORCE is defined as Rg, the g denoting that it is a force and not mass see UNITS.

Plated containers are certified to withstand lifting forces twice their Maximum Gross Weight (MGW) or Gross Rating (GR) and must satisfy the lifting simulation Test No 3.

The Working Load Limits of lifting lugs are intended to cover the MGW range of Series 1 containers and are generally 8.5 Ton and 12 Ton giving maximum capacities of 34 Ton and 48 Ton respectively for four lugs when loaded vertically. When loaded at an angle, within a specific range, their MGW capacity is reduced and charts are given to cover their loading ranges for Uniform and Eccentric loading. All lugs are proof loaded to twice their WLL, this normally covers any excessive overloading resulting from unforseen circumstances during handling and should function without failure. All lifting lugs, therefore, must have a minimum Factor of Safety of 4 for them to be used in the container lifting industry.

This is one of the two ISO Standards that applies to bottom side lifting Lugs,
the other being the offset distance of 38mm between the aperture face and line of action of lug force
as stated in BS ISO 3874 1997 6.4.1.

When lifting, the projecting retaining head in the lug engages within the corner fitting's cavity and sustains the forces necessary to lift the unit. The four vertical corner forces add up to Rg and the proportion at each corner depends upon payload distribution and, in some cases, kinematic forces due to motion whilst being handled with say, a luffing jib crane.

Corner Fitting Forces
V verical force

H horizontal force

The vertical corner lifting forces are applied by the sling assembly and the resulting force systems for the two loading methods: H horizontal force

The sling angle has two components α and θ. The principal angle α is determined by sling assembly whilst θ is the tilt angle due to eccentric loading and swing. Swing is transient and virtually imposible to quantify whilst tilt is and found when offset C of G is at maximum limiting 5% of container length and at 50% of container height.

The basic expression giving the lug force F in terms of total sling angle (α+θ)
and vertical corner force is given by equation (1).
The Load Factor depend on both Loading and
Lifting Mode
The sling angles and load factors are covered separately in detail for the two loading cases

For uniform loading the vertical forces are a quarter of the Gross Weight giving a load factor of 0.25.

F =

R_{g} x Load Factor

sin(α+θ)

Basic Lug force equation

**R _{g}** defines lifting force due to gravity

Sling angle α range for ISO containers is given in Table 2.

Outboard tensile forces on botton side rails

In addition to the vertical corner forces the angled sling force F induces horizontal inboard acting ** H ** forces
that apply compressive forces to the container's bottom side rails. When handling flats these forces could cause the unit to buckle hence the need to use vertical slings to eliminate the ** H ** forces.

When acting outboard, these forces induce more problematic tensile forces
that plated containers should be capable of reacting providing they comply with
Restraint Simulation Test.

Rather than present the limiting lifting sling forces over the angle range in terms of the Gross Mass, an alternative method would be to give the Gross Weight capacity over the sling angle range dictated by the WLL of the lugs.

**GR** is the Gross Rating or total mass **m** of freight unit,
Tare **T** plus Payload **P**

The force/mass/weight units are described in detail at
Terminology.

From equation(1) the Gross Weight **GW **becomes the lifting force **R**_{g} and in terms of sling angle and the WLL of the lug is

GW =

WLL sin(α+θ)

Load Factor

Gross Weight equation

For slings of equal lengths, their forces at the two ends are different, those at the 'heavy end' are used to determine the limiting gross weight (GW) capacity when sling forces are at the WLL of lugs. The effect of vertical position of C of G, considers them at container heights of 25%, 50% and 75% . Results for both uniform and eccentric loading are shown below for lugs having WLL of 8.5 Ton and 12 Ton both loading cases are covered in detail via

All Gross Weights given here are for the lifting lugs and ancilliary gear that when loaded, appart from the lug should not make contact with the freight unit sides.

The forces in the lifting gear to which the top of chain is connected, may well be in excess of those for the lugs.

The forces in the lifting gear to which the top of chain is connected, may well be in excess of those for the lugs.