Left Hand  Lifting Lug right Hand  Lifting Lug

Container Handling Support

There are two mandatory strength tests in ISO 1496 Part 4 that apply specifically to bottom corner fittings of containers that must be satisfactorily complied with for them to be issued with a CSC Plate. Together they guarantee integrity of containment of corner fittings particularly the welding used to secure them to load bearing frames of container freight units. These are static simulation tests since it would be prohibitive to conduct live dynamic tests in reality and are a Lifting and a Constraint test descibed in more detail below.

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 and the lifting force is often determined from this weight alone, this is the Static or Dead Load and is the operational gross mass R and equals Payload P plus container mass Tare T. However to get the load moving in the first place it must be accelerated, against gravity, before reaching constant velocity then becoming stationry, before being lowered, this is the dynamic or live Load and is an inertia force denoted by Rg. The total lifting force, therefore, is that due to gravity plus the live load rquired to overcome gravity, however this is rarely quantifyable, and in any case must always be kept to the absolute minimum, consequently the VERTICAL LIFTING FORCE is taken as Rg only, the g denoting that it is a force and not just the mass, SEE UNITS. Plated containers are certified to withstand lifting forces induced when lifting twice the Static Load that includes the additional forces induced by the Live loads. This test is described in Tests 3 and 4 below, consequently lifting lugs must be proof loaded at twice their WLL. Any further excessive overloading of lugs resulting from unforseen circumstances during handling, should be contained without failing, therefore, all lifting lugs, should have a minimum Factor of Safety of 4 and is a requirement stated in BS ISO 3874 to which they must conform with 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.

ISO1496-3:2019 Testing Bottom Corner Fittings.

Test No3    Lifting from 4 bottom corner fittings.


Bottom Corner Lifting Test.
This test simulates the Static plus Live Loads as covered in "Lifting Cycle" above by doubling the MGW . A uniformly distributed internal payload P having the value of ( 2R - T) is applied over the floor of container to give twice the rating of 2R and is lifted by slings attached to a central beam at the limiting angles shown in Table  below and simply suspending it to induce a force of 2Rg or 2R.This lifting mode also tests the frame strength induced by the resolved horizontal compressive forces that result from the accutely angled slings. The unit remains suspended for 5 minutes and when lowered any permanent deformations of the frame are recorded .
Series 1
E
A
B
C
D
Nominal Size
45ft
40ft
30ft
20ft
10ft
Angle α
30°
30°
37°
45°
60°

Limiting Lifting Angle α From ISO 3874

Test No 4    External Constraints Restraint Simulation Test

When mounted on a transport unit, and in motion, twistlocks transfer forces to the unit when accelerating and braking via the corner fittings, these forces in turn, are reacted by the bottom side rails of the container frame via direct horizontal forces. These horizontal forces forces are influenced by the rigidity of the platform on which the unit is mounted. (Another set of vertical forces are also induced by the inertia force acting at the C of G at a point above the corner fittings resulting with the familiar effect of the forward end tipping down when braking and rearing up during acceleration This inertia or body force is product of mass x acceleration acting at the C of G of freight unit hence the need to keep C of G always as low as possible.)
In reality it would be prohibitive to conduct live tests on CTU's to ascertain integrity of these products subject to the above actual loading cases so an alternative mandatory simulation test described here is used. This test subjects the container to horizontal corner forces only and neglects the stiffness of the support platform and inertia effects.
Restraint Simulation Test
In this test the CTU is supported by the corner fittings alone, one pair being fixed, whilst the opposite pair are on sliding supports. Horizontal forces are applied to these corner fittings, usually by hydraulic cylinders, to simulate dynamic forces induced by acceleration and braking. These forces simulate ± 2g acceleration and are evaluated from the product of MGW x 2g only or Rx2g. The loading is again uniformly distributed having a total value of R - T.
These simulated dynamic forces are replicated by applying half the total horizontal force simultaneously to each corner fitting attached to sliding supports, first pulling and then pushing, inducing tension and compression respectively in the bottom side rail that, in addition is under bending whilst supporting the distributed internal loading. Perhaps the most significant aspect of this test is that it subjects the corner fittings to horizontal direct tensile forces (applied in opposite direction to that shown here) that are more critical than the compressive forces induced by pushing, as shown here in Restraint Simulation Test.