The transverse distance of the centres of the corner fittings is 2.26 m. When loaded homogeneously, the bottom of the container has to bear shear forces and bending moments as shown in Figure 6.
Figure 6: Shear forces and bending moments
in the container bottom from a homogeneous load
The maximum bending moment from the load P1 (for one metre) is calculated as:
For a longer or shorter distance t in longitudinal direction the moment is obtained as:
This expression serves as limiting value for inhomogeneous loads, which may appear when loading steel coils.
If a coil of the mass mC placed on two longitudinal square timbers in the container with the transverse distance s (Figure 7), a bending moment results in the middle of the bottom:
Figure 7: Load on two longitudinal beams in the container
This bending moment must not exceed the limiting value presented by equation (1a). The mass mC must therefore be restricted to:
This equation is of restricted validity for values of s greater than 1.7 m. If the spread s comes close to the width of the container, the equation yields very large mC-figures, which may exceed the longitudinal strength of the container. Then the limiting value for mC is the coil mass determined by assuring the longitudinal strength. But concerns regarding the vertical shear loads in the bottom girders are unfounded.
Notice: The two dashed beams in Figure 7 between the two outer longitudinal beams are often found in skids supplied with the coil. They do not support the coil in the shown load case, because the container bottom deflects elastically, the coil, however, does not follow this deflection due to its greater stiffness. The inner beams are therefore not necessary, but also not harmful. If they would be bearing however, the bending moment in the container bottom would become greater than shown by equation (2).
A favourable option for shipping small coils on longitudinal beams is shown in Figure 8. Also for this case the equations (2) and (3) are valid with the corresponding value of s.
Figure 8: Two loads transverse in the container