Section 2 Loading and design criteria

2.1.1 This Section applies to Ro-Ro equipment operated whilst the ship is in a harbour or sheltered water environment, and where cargo or vehicles may be stowed on it in its seagoing condition whilst the ship is at sea, i.e. Standard Service Category.

2.1.2 Where the equipment is designed to operate in conditions other than those defined in Ch 6, 2.1 General 2.1.1, the design is to be subject to special consideration, i.e. Specified Service Category.

2.1.3 The operating and stowed loading conditions are to be clearly specified in all submissions together with hoisting speeds and braking times.

2.1.4 For the operating condition, the equipment is to be considered with respect to the following forces and loads:

1. Self-weight.

2. Applied loading.

3. Dynamic forces due to hoisting/lowering.

4. Forces due to static inclination of the ship.

2.1.5 The structure, its support and locking mechanism are also to be examined with respect to the sea-going condition for the following criteria, appropriate to the ship’s characteristics:

1. Self-weight.

2. Applied load due to vehicle or cargo loading.

3. Forces due to ship motion and static inclination.

4. Weather loading, where appropriate.

2.2.1 The self-weight load, L _w, is the load imposed on the hoisting mechanism by the weight of the structure and machinery.

2.2.2 The applied load, L _c, is the loading imposed on the structure by the cargo or vehicles.

2.2.3 The safe working load (SWL) is the maximum load for which the equipment is certified and is equal to the maximum value of L _c. For equipment that is manoeuvred unloaded, the SWL shall be taken as the self-weight of the structure for the determination of the required safety factors, where applicable.

2.3.1 To take account of acceleration and shock loading, the self-weight and applied load are to be multiplied by 1,20.

2.4.1 In general, Ro-Ro equipment is to be designed to operate in a harbour or sheltered water environment where there is no significant motion of the ship due to wave action.

2.4.2 For the operational condition, the Ro-Ro equipment is to be designed to operate safely and efficiently at an angle of heel of the ship of 5° and an angle of trim of 2°, acting simultaneously. If it is the intention to operate the Ro-Ro equipment at angles differing from the foregoing, it is to be designed for these angles and the certificate marked accordingly.

2.4.3 In addition to the loading/unloading conditions, the Ro-Ro equipment and its locking mechanisms are also to be designed to withstand the following forces in the sea-going condition:

1. Acceleration normal to deck of ±1,0 g
   Acceleration parallel to deck in fore and aft directions of ±0,5 g
   Static heel of 30°.

2. Acceleration normal to deck of ±1,0 g
   Accelerations parallel to deck in transverse directions of ±0,5 g
   Static heel of 30°.

2.4.4 Alternatively, where the Ro-Ro equipment is to be fitted to a conventional ship and the ship’s characteristics are known, the forces may be calculated for the combination of static and dynamic forces of Ch 4, 2.11 Forces due to ship motion for the ship’s motions and accelerations obtained from Table 4.2.2 Ship motions and Table 4.2.3 Forces due to ship motions in Chapter 4.

2.5.1 The design loads are to be consistent with the ship’s loading manual and are to include the details of the number and spacing of vehicles the Ro-Ro equipment is designed to accommodate, the type of vehicles, their weight, axle loading, tyre print dimensions, and number and spacing of wheels and supports.

2.5.2 Due account is to be taken of asymmetric loading where applicable. Where it is intended to restrict the position of a load, such that the Ro-Ro equipment is equally loaded, physical barriers are to be used. Other proposals will be specially considered.

2.5.3 In addition to vehicle loading, the Ro-Ro equipment is to be considered with respect to minimum uniform deck loading (UDL) of 2,5 kN/m² appropriate to the deck or decks. In cases where the vehicle load is higher than the given minimum value, the UDL is to be increased accordingly.

2.5.4 The Ro-Ro equipment is to comply with the appropriate requirements of Pt 3, Ch 3 Structural Design and Pt 3, Ch 11 Closing Arrangements for Shell, Deck and Bulkheads of the Rules for Ships.

2.5.5 Where the Ro-Ro equipment forms part of the ship’s watertight structure, it is to comply with the requirements of the Rules for Ships as appropriate.

2.5.6 Where the external Ro-Ro equipment is affected by wind load during the manoeuvring or in stowed position, it is to be considered and load combinations have to include the additional load as appropriate.

2.6.1 The allowable stress, σ_a is to be taken as the failure stress of the component concerned multiplied by a stress factor, F, which depends on the load case considered. The allowable stress is given by the general expression:

where

2.6.2 The stress factor, F, for steels in which σ_y/σ_u ≤ 0,85 are given in Table 6.2.1 Stress factor, F :

where

2.6.3 For steel with σ_y/σ_u > 0,85, the allowable stress is to be derived from the following expression:

where

2.6.4 Steels with σ_y/σ_u > 0,94 are not generally acceptable and need to be specially considered.

2.6.5 The failure stress for the elastic modes of failure are given in Table 6.2.2 Failure stress.

2.6.6 For components subjected to combined stresses, the following allowable stress criteria are to be used:

1. σ_xx ≤ σ_a

2. σ_yy ≤ σ_a

3. τ_ο≤ τ_a

4. 

where

2.6.7 The allowable bearing stress is to be calculated as follows for all load cases 1 to 3:

2.6.8 In case the structural analysis is carried out by means of detailed finite element models, higher allowable stresses can be applied as follows:

1. σ_1.FE ≤ 1,1σ_a

2. σ_2.FE ≤ 1,1σ_a

3. τ_o.FE ≤ 1,1τ_a

4. σ_e.FE ≤ 1,1²σ_a

Higher allowable stresses, as defined above, may only be applied if the actual stresses are localised. In case the actual stresses can also be calculated by means of analytical methods, the above higher allowable stresses are not applicable and Ch 6, 2.6 Allowable stress – Elastic failure 2.6.1 are to be applied.

2.7.1 The allowable stress is to be taken as the critical buckling stress of the component concerned multiplied by the stress factor, F, as defined in Table 6.2.1 Stress factor, F . The critical buckling stress is obtained by reference to Ch 4, 2.21 Allowable stress – Plate buckling failure.

2.8.1 The deck plating thickness is to be in compliance with Pt 3, Ch 9, 3 Decks loaded by wheeled vehicles of the Rules for Ships.

2.9.1 The deflection of the Ro-Ro equipment or of any individual member with respect to Case 1 and 2, see Table 6.2.1 Stress factor, F , is to be limited to:

• where

2.9.2 Where applicable, the deflection is to be further limited to ensure the watertight integrity of the ship is maintained.

2.10.1 Arrangements are to be provided to restrict horizontal movements of Ro-Ro equipment during operation, by guide rails or other means as applicable.

2.10.2 Where guide rails are fitted, they are to be such that the maximum deflection, resulting from horizontal components of load, is not greater than 6,0 mm. The working clearance between the Ro-Ro equipment and guide rail is to be such as to allow free vertical movement of the Ro-Ro equipment.

2.11.1 Stowage locks are to be provided to resist the vertical, forward/aft and lateral loads as defined in Ch 6, 2.5 Design loads 2.5.1. Arrangements are to be such that the locks do not loosen and impair the watertight integrity of the ship. Reference is made to the applicable requirements of the Rules for Ships.

2.12.1 Where chains are used as part of the hoisting arrangement, they are to have a minimum safety factor of 4,0.

2.12.2 Where wire ropes are used as part of the hoisting arrangement as well as items of loose gear used therein, they are to have a safety factor given by:

but not less than 4,0 nor greater than 5,0

where

For the calculation of the safety factor, SF, only static forces need to be considered, with a friction allowance of 2 per cent for roller bearings.

2.12.3 In cases where the dynamic factor in sea-going conditions is greater than 1,6, the safety factor may be derived as follows:

2.13.1 Ro-Ro access equipment is to be constructed of steel which complies with the requirements of Ch 1, 1.6 Materials and fabrication and Ch 11 Materials and Fabrication. Proposals to use materials other than steel will be specially considered. The fabrication is to be in compliance with Ch 11, 2 Fabrication of classed lifting appliances or Ch 11, 3 Fabrication of certified lifting appliances.

2.13.2 The selected steel grade is to provide adequate assurance against brittle fracture. The steel is to comply with the Charpy V-notch impact test requirements given in Ch 11, 1.2 General material requirements 1.2.2, with the operating temperature chosen as being the lesser of either that from an assigned winterisation notation or the lowest temperature of operation for the Ro-Ro access equipment (see Ch 11, 1.2 General material requirements 1.2.4).

2.13.3 The required documentation for materials used for the construction of classed and certified Ro-Ro access equipment is defined in Ch 11, 4 Material documentation for certified and classed lifting appliances.