Section 4 Movable decks

4.1.1 Movable vehicle decks hoisted by means of a hydraulic or electrical winch, or lifted by a mechanical lift on board the ship, are to comply with Ch 6 Ro-Ro Access Equipment of LR's Code for Lifting Appliances in a Marine Environment, July 2022 .

4.1.2 Movable decks other than described in Pt 3, Ch 9, 4.1 Classification 4.1.1, and which are subjected to general cargo loading, are to comply with the requirements of this Section.

4.2.1 The symbols used in this Section are defined in Pt 3, Ch 9, 1.2 Symbols and in the appropriate sub-Section.

4.3.1 Movable decks are generally to be constructed as pontoons comprising a web structure with top decking. Other forms of construction will be individually considered.

4.3.2 These requirements assume that the pontoons are to be constructed of steel. Other materials will be considered on the basis of equivalent strength.

4.3.3 Positive means of control are to be provided to secure decks in the lowered position.

4.3.4 The decks are to be efficiently supported, and hinges, pillars, chains or other means (or a combination of these) are to be designed on the basis of the imposed loads. Where supporting chains and fittings are required, they are to have a factor of safety of at least two on the proof load.

4.3.5 Plans showing the proposed scantlings and arrangements of the system are to be submitted.

4.3.6 Where it is proposed to stow the pontoons on deck, when not in use, details of the proposals for racks, fittings, etc. are to be submitted for consideration.

4.4.1 Design loading for general cargo is to be specified on the plans, together with details of fork lift trucks where appropriate, and submitted for consideration.

4.5.1 Where the pontoon is constructed of steel decking with stiffening webs, the deck plate thickness, t, is to be not less than that required by Pt 3, Ch 9, 3.4 Deck plating.

4.5.2 The plate thickness, t, for aluminium pontoons is to be not less than:

where t ₁ is the mild steel thickness as determined from Pt 3, Ch 9, 3.4 Deck plating.

For aluminium pontoons designed for the exclusive carriage of unladen wheeled vehicles:

4.5.3 Where it is proposed to use plywood decking, the arrangement and thickness will be considered. Plywood alone, is not, generally, to be used for axle loads in excess of 7,8 kN.

4.5.4 Attention is drawn to National fire regulations which in certain cases may ban the use of plywood and certain other materials in `special category spaces' on passenger ships.

4.6.1 The minimum section modulus of webs and stiffening of steel pontoon secondary stiffeners is to be calculated in accordance with Pt 3, Ch 9, 3 Decks loaded by wheeled vehicles, Table 9.3.4 Secondary stiffener requirements using the Scenario 1 structural design factor given in Pt 3, Ch 9, 3 Decks loaded by wheeled vehicles, Pt 3, Ch 9, 3.5 Deck longitudinals and beams 3.5.4 and the Scenario 1 loads defined in Pt 3, Ch 9, 3 Decks loaded by wheeled vehicles, Table 9.3.7 Design load cases.

4.6.2 The section modulus of webs and stiffening of aluminium pontoons is to be not less than that defined in Pt 3, Ch 9, 4.6 Pontoon webs and stiffeners 4.6.1, replacing σ_o by σ_a, where σ_a is defined in Pt 3, Ch 2, 1.3 Aluminium.

4.6.3 Where plywood decking is proposed, or in other arrangements where the decking is not integral with the stiffening webs, the arrangement of the grillage of webs is to be such as to provide the required strength.

4.7.1 Where wheeled vehicles are to be used, the supporting arrangements are to be such that the movement at the edge of one pontoon relative to the next does not exceed 50 mm during loading or unloading operations.

4.8.1 As an alternative to Pt 3, Ch 9, 4.3 Arrangements and design to Pt 3, Ch 9, 4.7 Deflection, the structure may be designed on the basis of a direct calculation using a grillage idealisation. The method adopted and the stress levels proposed for the material of construction are to be submitted for consideration.