Section 9 Strength of container securing arrangements

9.1.1 The securing system is to be designed on the basis of the most severe combination of the forces specified in Pt 3, Ch 14, 8 Determination of forces for container securing arrangements in such a manner that the resultant forces on the containers and securing devices are within allowable limits., the forces are to be calculated for the most severe condition applicable to each arrangement.

9.1.2 The resultant forces in the containers are not to exceed the allowable values given in Pt 3, Ch 14, 9.4 Allowable forces on containers, fittings and lashing devices.

9.1.3 The resultant forces in the securing devices and supports are not to exceed the allowable working loads 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.

9.1.4 The container orientation is to follow the cargo securing manual, however, if unknown the container door end is assumed to face aft.

9.1.5 The forces in lashing devices, where fitted, are not to exceed the allowable safe working loads (SWLs) of the lashings as determined from Pt 3, Ch 14, 3 Loose container securing fittings, materials and testing.

9.1.6 For containers that are stowed in the fore and aft direction, it is sufficient to calculate the forces in the lashing rods based on assessment of the transverse and vertical loads acting on the containers. The vertical forces in the corner posts are to include the effects of transverse and longitudinal loads acting on the containers.

9.1.7 For containers that are stowed athwartships, the transverse, longitudinal and vertical loads acting on the containers will need to be taken into account in calculating the forces in the lashing rods. The vertical forces in the corner posts are to include the effects of transverse and longitudinal loads acting on the containers. The approach to be adopted should be agreed with Lloyd’s Register.

9.2.2 Table 14.9.1 Acceleration force application shows a method to apply the forces to a container, assuming centre of gravity of the container is to be taken as half way along its length, midway between the port and starboard wall, and c_VGGi off the container floor.

Table 14.9.1 Acceleration force application

Acceleration Force	Figure
(a) Longitudinal acceleration force
	where:
	FTop =
	FBottom =
(b) Transverse acceleration force
	where:
	FTop =
	FBottom =
(c) Vertical acceleration force
	Where:
Note The force application assumes the centre of gravity of the container is to be taken as half way along its length, midway between the port and starboard wall and cVCGi off the container floor.

9.3.1 LR’s Guidance Notes for Calculation Procedure of Container Stack Analysis describes a methodology that complies with these requirements.

All these aspects need to be considered in the stack analysis.

9.3.3 The forces in the containers, twistlocks and lashing devices are calculated from the stack distortion. Using the forces applied to the stack in 9.2 Forces applied to each container and the stiffness of the stack, the overall stack deformation is calculated using the standard force-displacement equations. Non-linearity of the system as a consequence of twistlock separation and lashing rods is to be considered in evaluating the deformation. The inclusion of twistlock separation and corner post stiffness results in an increase in the loads in the lashing devices. The inclusion of a lashing bridge structure significantly affects the support a lashing device can provide for the stack and hence must be considered.

9.3.4 The overall distortion behaviour of a container is a combination of its shear stiffness and corner post stiffness. A container is to be modelled as a combination of shear only panels representing the walls, and rod elements representing the corner posts and horizontal frame structures. The shear stiffness of the container walls is to be taken as specified in Table 14.9.2 Shear spring stiffness of container walls. The area and Young’s Modulus of the corner posts and horizontal frame members is to be taken as specified in Table 14.9.3 Container post properties.

9.3.5 Lashing devices are to be taken into account and their properties modelled to capture their behaviour. These include but are not limited to lashing rods and shore devices.

9.3.6 Lashing rods are to be modelled using tension only rod elements. Elongation may be determined by reference to an effective cross-sectional area and an effective modulus of elasticity of the lashing (allowance for straightening and stretching), which in the absence of actual test values,is to be taken as specified in Table 14.9.4 Effective modulus of elasticity of lashing devices.

9.3.7 Lashing rod stiffness, k_Lr, in kN/mm may be derived as follows:

where

9.3.8 For external and internal para-lash arrangements, each lashing rod is to be individually modelled and connected to the relevant container casting.

9.3.9 Other lashing rod arrangements, e.g. an internal para-lash with load equaliser arrangements, will need to be specially considered.

9.3.10 Any other element introducing flexibility into the structure between the lashing point and the base of the container stack is to be evaluated and taken into account. Examples of this could be flexibility of the lashing bridge, sliding of a hatch cover or torsional deformations of the hull.

9.3.11 Where lashing devices are attached to a lashing bridge, the lashing bridge transverse stiffness is to be taken into account and may be modelled as an additional rod element. For lashing bridge designs with multiple lashing platforms, a lashing bridge rod element is required for each platform. All the lashing bridge rod elements are to be connected in series. The bottom of the lashing bridge can be assumed to be rigidly fixed.

A default Rule stiffness for each platform of the lashing bridge is to be modelled with a rod with a transverse spring stiffness (K_lbyi) calculated from the following:

where

H_lbi is the height of lashing bridge tier i, taken in m, see Figure 14.9.1 Lashing bridge transverse spring stiffness

C_lb is the lashing bridge stiffness coefficient, taken as 50 for single elevated lashing bridge tiers at the outer stack, see Figure 14.9.1 Lashing bridge transverse spring stiffness, and 70 otherwise.

Figure 14.9.1 Lashing bridge transverse spring stiffness

9.3.12 For lashing bridge designs having a higher transverse stiffness than the default stiffness value of 70/H_lbi, in kN/mm, the stack analysis can be based on the actual stiffness value determined in accordance with the ShipRight ADP Procedure for the Assessment of Container Ship Lashing Bridge Structures.

9.3.13 Alternative proposals for the transverse stiffness of the lashing bridge will be individually considered.

9.3.14 Inherent in the twistlock design is a need for play in order to be able to fit the device. This play allows easy installation of the twistlocks, but this also means that vertical lifting of the top container occurs before the twistlock comes in to tension. Twistlocks therefore have three states, see Table 14.9.5 Twistlock separation and states. This vertical lifting distance is referred to as twistlock separation. If this separation occurs then it can significantly increase the transverse stack deformation and the forces in the lashing devices. The assessment of the effects of twistlock separation is particularly critical for external lashing arrangements.

Table 14.9.5 Twistlock separation and states

State	Figure	Description
Twistlock closed		Twistlock closed and in compression. The distance between the container castings is equal to the twistlock height.
Twistlock open		Twistlock open and in tension. The distance between the container castings is equal to the twistlock height plus twistlock separation.
Twistlock float		Float condition with zero axial force. The distance between the container castings is in between closed and open state.

9.3.15 Twistlock separation and height is to be provided by the lashing manufacturer. If this information is not available, the twistlock flange height is to be taken as 28 mm and the twistlock separation as 18 mm for FATs, SATs and manual twistlocks.

9.3.16 In the analysis of container stacks, it may be assumed that shear deformation of twistlocks can be ignored.

9.3.17 Initial displacement of containers due to tolerances in container fittings will be considered in conjunction with the stowage arrangement proposed. Generally, initial displacement may be neglected in calculation procedures for conventional stowages.

9.4.1 The racking force of the container panels is calculated from the transverse shear deformation and the shear stiffness. The absolute racking force on the container end and side panels must not exceed the allowable racking force specified in Table 14.9.6 Allowable forces in containers in stacks with the same base size

9.4.2 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. 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 specified in Table 14.9.6 Allowable forces in containers in stacks with the same base size.

9.4.3 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.

9.4.4 The twistlock tensile load is to be less than the safe working load.

9.4.5 The forces and force components in the lashing devices must not be greater than the allowable lashing forces on the corner castings and the safe working load of the lashing device.

9.4.6 Table 14.9.6 Allowable forces in containers in stacks with the same base size and Pt 3, Ch 14, 9.4 Allowable forces on containers, fittings and lashing devices 9.4.12 show the values for allowable forces for ISO containers according to ISO 1496-1:1990 including Amendment Nos. 1, 2, 3 and 4. Other configurations will be specially considered.

9.4.7 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 Table 14.9.6 Allowable forces in containers in stacks with the same base size and of the forces for which the container has been certified.

9.4.8 The resultant forces in the securing devices and supports are not to exceed the allowable safe working loads 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.

9.4.9 The forces in lashing devices, where fitted, are not to exceed the allowable safe working loads (SWLs) of the lashings as determined from Pt 3, Ch 14, 3 Loose container securing fittings, materials and testing.

9.4.10  Table 14.2.2 Test loads and test modes for fixed container securing fittings and Table 14.3.2 Test loads and test modes for loose container securing fittings show the allowable safe working loads for loose fittings, including twistlocks, and securing devices. The forces in lashing devices are not to exceed the safe working loads (SWLs).

9.4.11 Table 14.3.2 Test loads and test modes for loose container securing fittings show the allowable shear force of the twistlocks. 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.

9.4.12 When Fully Automatic Twistlocks (FATs) are used, it is to be ensured that at least one of the port and starboard FATs at each end is in compression. This is to ensure that the FATs would not become disengaged.

Table 14.9.7 Allowable compression forces for posts and corner castings of 40 ft and 45 ft containers in mixed stacks

Allowable force in kN

Note 1. See Notes 1 and 2 of Table 14.9.6 Allowable forces in containers in stacks with the same base size Note 2. The allowable compression force for the bottom container casting may be increased by (1,8 Rg)/4

Figure 14.9.2 Allowable forces for 20 ft or 40 ft containers constructed to ISO 1496-1:1990 including Amendment Nos. 1, 2 and 3