Section 3 Assessment of the stack

3.1.1 The deformation of the stack is solved using matrix methods. The resulting deformation can be used to derive the forces in the container end wall, corner posts, twistlocks and lashing devices using Hooke’s law.

F = K ·δ + C

δ = K^-1 · (F - C)

where

F = the applied nodal force matrix, in kN

K = the stiffness matrix of the stack and securing devices, in kN/mm

C = the twistlock force matrix of the stack, in kN, which is non zero when a twistlock is open

δ = the nodal displacement matrix, in mm

3.1.2 The stack stiffness matrix and twistlock force matrix are dependent on the state of the twistlock and lashing devices. This implies that the solution has a non-linear component requiring an iteration technique to solve.

3.2.1 To invert the stiffness matrix adequate constraints are to be imposed. It is sufficient to fix the base of the stack and the base of the lashing bridge. The boundary constraint can be included by imposing a large value along the diagonal of the stiffness matrix at the degrees of freedoms constrained.

3.3.1 From the calculated displacements of the container nodes, the racking force and corner post forces of the container end can be calculated. The locations of the forces recorded are shown in Figure 1.3.1 Container end wall showing locations for evaluating corner post forces.

3.3.2 The racking force of the container panels is calculated from the transverse shear deformation and the shear stiffness.

3.3.3 The vertical force in the corner post is to be calculated from the compression of the post plus the vertical force due to shear in the end wall acting on the post.

3.3.4 The absolute racking force on the container end must not exceed the allowable racking force on container end specified in Pt 3, Ch 14 Cargo Securing Arrangements, Pt 3, Ch 14, 9.4 Allowable forces on containers, fittings and lashing devices 9.4.12.

3.3.5 The maximum compressive force in each corner post of the container must not exceed the vertical forces at each top corner casting and in each corner post, compression, specified in Pt 3, Ch 14 Cargo Securing Arrangements, Pt 3, Ch 14, 9.4 Allowable forces on containers, fittings and lashing devices 9.4.12. The maximum tensile force in each corner post of the container must not exceed the vertical forces at each top corner casting, tension, specified in Pt 3, Ch 14 Cargo Securing Arrangements, Pt 3, Ch 14, 9.4 Allowable forces on containers, fittings and lashing devices 9.4.12.

3.3.6 The allowable forces for containers of other dimensions, e.g. 24 ft, 48 ft and 53 ft, will be determined on the basis of the values in Pt 3, Ch 14 Cargo Securing Arrangements, Pt 3, Ch 14, 9.4 Allowable forces on containers, fittings and lashing devices 9.4.12 and of the forces for which the container has been certified.

3.3.7 For mixed stack arrangements the allowable forces are to be taken from Pt 3, Ch 14 Cargo Securing Arrangements, Pt 3, Ch 14, 9.4 Allowable forces on containers, fittings and lashing devices 9.4.12.

3.3.8 The allowable forces for container end walls are shown in Table 1.3.1 Allowable forces for containers.

3.4.1 From the calculated displacements of the twistlock nodes, the twistlock vertical forces and twistlock shear forces can be calculated. Figure 1.3.2 Twistlock forces shows the location points where the forces are recorded.

3.4.2 The vertical force in the twistlock is to include the vertical forces from the triangular shear elements for both port and starboard twistlocks.

3.4.3 The calculated twistlock shear force is to be increased by 10 per cent due to misalignment, corrosion and design tolerances before comparing to the Safe Working Load (SWL).

3.4.4 The maximum vertical compressive force in the twistlock is not to be greater than the allowable vertical forces at each bottom corner casting, compression and the maximum vertical tensile force in the twistlock is not to be greater than the allowable vertical corner pull-out force at each bottom corner casting.

3.4.5 The twistlock tensile load is to be less than the SWL.

3.4.6  Pt 3, Ch 14 Cargo Securing Arrangements, Pt 3, Ch 14, 3.3 Prototype testing 3.3.1 shows the allowable shear force of the twistlocks. The calculated twistlock shear force is not to exceed the allowable shear force of the twistlocks.

3.4.7 The allowable forces for twistlocks are shown in Table 1.3.2 Allowable forces for twistlocks.

3.5.1 From the calculated displacements of the lashing rod nodes, the lashing rod force and applied casting forces can be calculated.

3.5.2 The forces and force components in the lashing devices must not be greater than the allowable lashing forces on the corner castings and the SWL of the lashing device.

3.5.3 The resultant forces in the securing devices are not to exceed the allowable SWL for which the device has been approved, see Pt 3, Ch 14, 2 Fixed cargo securing fittings, materials and testing and Pt 3, Ch 14, 3 Loose container securing fittings, materials and testing.

3.5.4 The forces in lashing devices, where fitted, are not to exceed the allowable SWL of the lashings as determined from Pt 3, Ch 14, 3 Loose container securing fittings, materials and testing.

3.5.5 The allowable lashing forces on the corner castings for containers of other dimensions, e.g. 24 ft, 48 ft and 53 ft, will be determined on the basis of the values in Pt 3, Ch 14 Cargo Securing Arrangements, Pt 3, Ch 14, 9.4 Allowable forces on containers, fittings and lashing devices 9.4.12 and of the forces for which the container has been certified.

3.5.6 The allowable forces for lashing devices are shown in Table 1.3.3 Allowable forces for lashing devices.