Bottom Corner Lifting
Bottom Corner Lifting
Left hand lifting lug Lifting freight units of any form, is a highly skilled operation and should only ever be undertaken BY COMPETENT PERSONS when all conceivable problems have been anticipated. If in doubt it may be helpful to refer to: BS ISO 3874:1997 Series 1 freight containers- Handling and securing. In particular: Section 5: Handling, and Section 6: Specified lifting methods This covers all aspects of lifting containers, flats and containerised tanks stating what is "Allowed" and what is "Not Allowed" for both loaded and unloaded freight units, The statutary limits concerning MGW and load distribution of containers are also covered in 4.2 Packing Loading and emptying in the same document. The implications of these are dealt with via the links below.
an HSE publication Guidance in the Port and Docks industry is useful for those working it the industry.

Lifting Cycle

A typical lifting cycle requires a unit to be raised from one place and lowered to another and at some point during the cycle the load may be freely suspended such that the lifting force will equal the Gross Weight of the unit. This is the Static or Dead Load and is the operational Gross Mass GM or GR (Gross Rating) and equals Payload mass plus Tare.
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.

Basic Lifting Modes

This defines whether freight units remain horizontal Level Lifting or otherwise during a lift. If it doesn't then it is considered unconstrained and is normally associated with lifting eccentrically loaded containers. Various possible effects result when the C of G is aligned with top lifting point when using Bottom Lift Sling methods. During alignment the unit will no longer remain level but rotate and swing whilst attaining equilibrium, this is Unconstrained lifting.

Corner fitting forces when lifting

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 Lifting lug forces for offset loading 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: Uniform and Eccentric are both covered in this section. The sling force F is derived from vertical corner force and angle to the horizontal and must not exceed the Working Load Limit of the lug, nor any other component in the sling assembly.

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 =
 Rg x Load Factor
sin(α+θ)
#1
Basic Lug force equation

Rg defines lifting force due to gravity
Sling angle α range for ISO containers is given in Table 2.

Corner Fitting Horizontal Forces

Outboard tensile  forces on botton side rails 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.

Gross Weights

This is the most significant term associated with lifting and the maximum GW should not exceed the MGW stated on the CSC plate whilst the WLL should not be exceeded in any of the components in the lifting assembly.
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 Rg and in terms of sling angle and the WLL of the lug is
  GW =
WLL sin(α+θ)
Load Factor
#2
Gross Weight equation

Uniform and Eccentric Container loading

When lifting uniformly loaded freight units all four slings are equally loaded resulting with the maximum gross weights capacity.

Cargo Platforms should only ever be uniformly loaded

Eccentrically loaded containers have an indeterminate C of G position that must be within a mandatory longitudinal limit of 5% of container length and vertically, "as low as possible," (BS ISO 3874 ).
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
Gross weights for WLL 8.5 Ton and 12 Ton for uniform and eccentric loading

Limiting Gross Weights for both Uniform and Eccentric container loading

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.