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.
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.