  ### Container Lifting Support Central point load Rg
Vertical corner forces 25%Rg
This is the simplest loading case where the position of C of G acts through the centroid of container, consequently all four slings will be equally loaded and sustain the maximum possible loading, eccentric loading gives lower GW capacities. The first step involves evaluating the vertical forces at the bottom corner fittings in terms of the lifting force Rg, From these vertical forces the lug loading is detmined by the lifting sling angles. When the sling force is equal to the WLL of lugs the limiting gross weight GW gives the maximum limitimg values. Lug Force F for Uniform Loading

The applied force on the sling, that shown acting downwards, results from a quarter of the container weight Rg. It follows that the vertical reaction force V, acting on the corner fitting, must equal the applied vertical downward force and from equation (1) the sling force F is given by sling angle α and the container weight Rg by the following equation. F =
Rg
4 sinα
Since force Rg results when lifting Gross Weight GW, the limiting GW capacity is when lugs are loaded at their Working Load Limit WLL and is from equation (3) above.
GW = 4 WLL sinα  Gross Weight distribution in Tons from Equation ( 4 ) over sling angle range for Uniform Loading.

A simple graphical illustration of the above GW equation is shown here over sling angle range 30° to 90° for uniform loading where Load Factor is 0.25 and all slings carry the same load over that portion parallel to the side of container. (When spreaders are used the force in the sling portion above the spreader is greater than that below it and depends on lifting gear arrangement attached to top lifting point.)

The limiting load envelope is bounded by the semi-circular arc that encompases O and P having a radius of Twice WLL centered midpoint between them, the lifting force envelope for 12Ton WLL is the red area, whilst the purple area is for 8.5Ton WLL.

The GW values are given by the length of the radial lines eminating from lug centre O and intersecting the semi-circular boundary. With uniform loading the tilt angle θ is zero. When the sling angle is vertical sin α= 1, gives the maximum GW, whilst at 30°, sin α= 0.5 giving the lowest GW (the more acute the angle the lower the GW) the sling tension never exceeding WLL of lug. All Gross Weights given here are for the lifting lugs and ancilliary gear attached to the lugs ie chain, shackles, chain connectors etc, 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.