Eccentrically Loaded Containers

Container Lifting Supplement

It must be almost impossible to guarantee that all containers are that well packed such that they are symmetrically loaded and many cannot even be guaranteed to be within the statutary load limits as was born out by the enquiry into the grounding of

MV Riverdance

at Blackpool UK in Jan 2008. It found that a contributary factor was the under declaration of trailer and container weights (limited to a maximum of 36mt) and another issue, often overlooked, was that of shifting payload when improperly secured within containers. More information on this issue is given at

overweight-containers

To ensure that limiting weight restrictions are enforced for international sea transportation, the International Maritime Organization (IMO) amended the Safety of Life at sea regulations (SOLAS) by introducing the Verified Gross Mass (VGM) regulations in 2016. Although intended for International container shipments they may vary for internal port to port shipments dedending on local country regulations. A comprehensive account of VGM is presented in

Shipping and Freight Resource

Freight units with offset C of G presents the most problematic method of lifting with Bottom Lift Slings because the C of G must aligns itself with top lifting point causing the freight units to respond in an unpredictable manner that, in extreme circumstances, could cause serious damage, this section deals with some of these problems both kinematic and lifting lug forces.
End lifting is covered separately in

Side Loaders

Where applicable, further information is given at

Tilt and Swing

Eccentric Payload Distribution

The 60% and 40% distributions each extend over 50% of container length giving maximum longitudinal offset for

C of G equation(5)

as 5% of container length

Vertical End Lifting Forces

The resulting lifting

Forces (2.2Table)

at the two ends for eccentric loading gives a maximum vertical corner force of 27.6% Rg.

Eccentric Payload and Vertical End Lifting Forces

The 60%, 40% distributions each extend over 50% of container giving maximum longitudinal offset for

C of G equation(5)

as 5% of container length. Resulting

Forces (2.2Table)

at the two ends for constrained level lifting gives a maximum vertical corner force of 27.6% Rg.

Tilt angles for Unconstrained lifting

Rotation due to offset Loading.

Lifting commences only when the centre of gravity aligns itself with the top lifting point P when the C of G and P are vertically aligned. For slings of equal length the container will rotate and could cause serious problems.
The tilt angle is determined by both limiting 5%L longitudinal offset and the vertical position of the C of G. These are considered at positions of 25% 50% and 75% of container height, results given at

tilt angle evaluation

Sling forces are further complicated by the rotation of the container since the lifting sling angles now includes the tilt angle θ, shown below.

Tilt angle ranges for 20ft and 40ft sling angles.

Single point lifting scenarios for eccentric loading

Free lifting point, tilt only results.

Free lifting point.

Initially, lifting force line of action is not coincident with C of G and as lifting commences the container will pivot about the heavy end still in contact with support. When top lifting point is self-aligning it will position itself over the C of G and after rotating through the tilt angle it should lift clear from the support with very little swing.

Constrained lifting point results with tilt and swing

Constrained Lifting Point.

If the top lifting point is fully constrained, the C of G aligns itself with lifting point and heavy end slides accordingly. Friction constrains container at the point still in contact with the ground. When free, from the contact point,it will swing and possibly pendulate causing damage to contents or anything in it's way. This is "worst case scenario."

Aligned lifting point, unit remains horizontal.

Aligned Lifting Point

The ideal solution is to use slings whose lengths are such that lifting point is directly above C of G thereby holding the container level during a lift. Although not always possible it is good practice to adopt especially when handling expensive process units. (In this case vertical position of C of G is irrelevant)

Corner Fitting Forces for Eccentric Loading

Corner Fitting Forces when lifting tilted containers.

Unlike uniformly loaded containers where all 4 slings are equally loaded, eccentric loading results with one end being heavier than the other.
When Unconstrained the container will rotate through θ as it attains equilibrium and the final lifting sling angle becomes (α + θ) and is used to quantify the effect of the tilt angle on the limiting gross weight Rg, given by

equations (9)

and

(10)

However, from the tilt angle chart above, the angle values for the upper sling angle range beyond say 85° to 90° are insignificant compared to those at the lower range starting at 30°(40ft units) and 45°(20ft units) also, it is not possible to have two leg slings angled at 90°, therefore vertical corner lifting forces are taken as those for

level lifting equn(6)

Limiting lifting forces for eccentric loading

The object is to evaluate the effect on limiting lifting forces, ie Gross Weights GW. These are determined by the position of the C of G both longitudinally and vertically. The longitudinal eccentricity is limited to 5% of container length whilst the limiting vertical position, not defined in current regulations,other than that it should be as low as possible, were considered at three positions of 25%, 50% and 75% of container height H.
The GW values at the "heavy end" were found at (50% Height). and were approximately 10% lower that those for uniformly loaded containers. The rounded results from lifting 40ft 9.5ft high units, are shown below over the sling angle range 30° to 90° for both of 12 Ton and 8.5 Ton WLL lugs.

Limiting Gross Weights for eccentric loading.

All Gross Weights given here are for the lifting lugs and ancilliary gear attached to them ie chain, shackles/ chain connectors etc, that when loaded, appart from the lug do not make contact with the freight unit.
The forces in the top lifting gear, that connected to the slings may be in excess of those for the lugs.