Section 4 Global strength assessment

4.1.1 The verification of the stress level, buckling capability and deflection of the dock gate’s primary members is to be assessed by direct calculation.

4.1.2 In general, the direct calculation is to be based on a three-dimensional (3-D) FEA carried out in accordance with the procedures contained in this Section. Where alternative procedures are proposed, these are to be agreed with LR before commencement.

4.1.3 If the computer programs employed are not recognised by LR, full particulars of the programs will also be required to be submitted, see Pt 3, Ch 1, 3.1 Alternative arrangements and scantlings of the Rules and Regulations for the Classification of Ships, July 2022.

4.1.4 A detailed report of the calculations is to be submitted and is to include the following information. The report must show compliance with the specified structural design criteria given in Pt 2, Ch 2, 4.8 Structural design criteria.

• list of plans used, including dates and versions;
• detailed description of structural model, including all modelling assumptions;
• plots to demonstrate correct structural modelling and assigned properties;
• details of material properties used for all components;
• details of boundary conditions including seal stiffness properties;
• details of applied loading and confirmation that individual and total applied loads are correct;
• details of boundary support forces and moments;
• plots and results that demonstrate the correct behaviour of the structural model to the applied loads;
• summaries and plots of global and local deflections;
• summaries and sufficient plots of von Mises, directional and shear stresses to demonstrate that the design criteria are not exceeded in any manner;
• plate buckling analysis and results;
• tabulated results showing compliance, or otherwise, with the design criteria; and
• proposed amendments to structure, where necessary, including revised assessment of stresses and buckling properties.

4.1.5 Where required, a seismic assessment is to be carried out in accordance with Pt 2, Ch 2, 5.1 Seismic assessment.

4.1.6 A fatigue assessment is to be carried out in accordance with Pt 2, Ch 2, 5.2 Fatigue assessment.

4.1.7 Where required, an impact assessment is to be carried out in accordance with Pt 2, Ch 2, 5.3 Impact assessment.

4.1.8 Where required, a temperature assessment is to be carried out in accordance with Pt 2, Ch 2, 5.4 Temperature assessment.

4.2.1 Linear elastic methods capable of accounting for the following are to be used:

1. global shear lag effects;
2. asymmetry between the supports and the dock gate centreline;
3. global deformation of the dock gate; and
4. rotation/deformation at the boundaries.

4.2.2 Non-linear methods need not normally be employed; however, if they are used then the following aspects are to be considered:

1. Loss of stiffness/strength in the event of severe buckling; and
2. Loss of contact at the boundaries.

Non-linear methods are to be agreed with LR prior to the commencement of any investigations.

4.3.1 The global FEM strength assessment is to be carried out based on net scantlings, i.e. an appropriate corrosion addition, t_c, see Pt 2, Ch 2, 4.4 Corrosion additions, is to be deducted from the gross offered thickness.

4.3.2 The gross offered thickness, t, is the gross thickness provided at the newbuilding stage, which is obtained by deducting any thickness for voluntary addition from the as-built thickness, i.e. any additional thickness specified by the Owner or builder is not to be included when considering compliance with the Rules.

4.3.4 The net sectional properties of stiffeners are obtained by deducting half the applied corrosion addition from each surface of the profile cross-section.

4.4.1 The corrosion addition for each of the two sides of a structural member, t_c1 or t_c2, is specified by Table 2.4.2 Corrosion addition for one side of a structural member.

4.4.2 A reserve thickness, t_res, of 0,5 mm is also to be included.

4.5.1 A 3-D FE model of the complete dock gate is to be used to assess the primary structure.

4.5.2 The proposed scantlings, excluding any Owner’s extras, are to be used throughout the model. The selected size and type of elements are to provide a satisfactory representation of the deflection and stress distributions within the model.

4.5.3 In general, the plate element mesh is to follow the primary stiffening arrangement. The minimum mesh size requirements are:

• transversely, one element between every longitudinal stiffener;
• longitudinally, three or more elements between web frames;
• vertically, one element between every stiffener; and
• three or more elements over the depth of floors, side transverses, vertical webs and horizontal stringers of bulkheads.

The mesh density of the side shell plating in way of the side frames is to be similar to those adjacent to the side shell plating.

4.5.4 Secondary stiffening members are to be modelled using line elements positioned in the plane of the plating having axial and bending properties (bars). The bar elements are to have:

• a cross-sectional area representing the stiffener area, excluding the area of attached plating; and
• bending properties representing the combined plating and stiffener inertia.

4.5.5 Face plates and plate panel stiffeners of primary members are to be presented by line elements (rods or bars) with the cross-sectional area modified, where appropriate, in accordance with Table 2.4.3 Line element effective cross-section area area and Figure 2.4.1 Effective area of face bars.

4.5.6 In general, the use of triangular plate elements is to be kept to a minimum. Where possible, they are to be avoided in areas where there are likely to be high stresses or a high stress gradient. These include areas:

• in way of lightening/access holes; and
• adjacent to brackets, knuckles or structural discontinuities.

4.5.7 Access openings, lightening holes, etc. in primary structures are to be represented in areas of interest, e.g. where they are of sufficient size to influence member stiffness. Additional mesh refinement could be necessary to model these openings, but it could be sufficient to represent the effects of the opening by deleting the appropriate elements.

4.5.8 The lightweight of the dock gate is to be represented in the model.

4.5.9 Solid ballast shall be included in the model in its correct position and can be modelled as lumped masses attached to appropriate model nodes provided that the load is appropriately distributed to local nodes. Ballast which is moveable such as pig iron is to be assumed to be located in its most unfavourable position.

4.5.10 The effects of added mass (i.e. entrained water located outside the dock gate, but which is considered to move with the dock gate) are to be considered using a method which is to be agreed with LR.

4.6.1 In general, the dock gate will be supported vertically at the sill base and laterally on two sides. The support will be provided via a soft seal and the stiffness of the seals is to be included in the analysis.

4.6.2 The stiffness and damping characteristics of the seals are to be provided by the gate designer or seal manufacturer together with how these properties can vary around the dock gate perimeter when different seal stiffnesses are specified by the designer. The stiffnesses of seals employed in the structural models are to be consistent with the direction of displacement of the seals, i.e. for translations perpendicular to the compression axis of the seal, shear stiffness of the seal is to be modelled.

4.6.3 The lateral sway of the dock walls should not impose in-plane forces on the dock gate other than those transferred via seal shear stiffness and friction. If there is insufficient edge clearance between the dock walls and the dock gate to ensure that this condition is met, then the interaction between the walls and the dock gate is to be addressed.

4.7.1 The load scenarios given in Pt 2, Ch 2, 4.7 Application of loads 4.7.2 to Pt 2, Ch 2, 4.7 Application of loads 4.7.5 are to be considered where the applied loads are to be taken as the loads given in Pt 2, Ch 2, 2.1 General (as appropriate) are to be multiplied by the load factors given in Pt 2, Ch 2, 4.7 Application of loads 4.7.6 and Pt 2, Ch 2, 4.7 Application of loads 4.7.7.

4.7.2 The following load scenarios are to be considered:

1. normal operating; and
2. extreme.

4.7.3 The normal operating scenarios comprise the following:

1. lightweight only;
2. lightweight + hydrostatic (operating);
3. lightweight + hydrostatic (operating) + wind;
4. lightweight + deck loads + hydrostatic (operating);
5. lightweight + deck loads + hydrostatic (operating) + wind;
6. lightweight + internal loads;
7. lightweight + hydrostatic (operating) + internal loads + deck loads + wind; and
8. lightweight + air pressure test load.

4.7.4 The extreme scenarios comprise the following:

1. lightweight + deck loads + hydrostatic (maximum credible).

4.7.5 Other load scenarios can be considered at the request of the Owner.

4.7.6 The partial load factors are given in Table 2.4.4 Partial load factors.

4.7.7 Consideration can be given to reducing the partial load factors for secondary loads in accordance with BS 6349 Maritime Works. Note that hydrostatic loading is considered to be a primary load.

4.8.1 The stresses resulting from the application of all load scenarios are not to exceed the following:

• combined stress – 1,0 σ_o;
• direct stress – 0,9 σ_o;
• shear stress – 0,5 σ_o.

σ_o is defined in Pt 2, Ch 2, 1.4 Materials.

4.8.2 Stress criteria are based on the coarse mesh described in Pt 2, Ch 2, 4.5 Modelling 4.5.4. If a finer mesh is used, then stress can be averaged over an area equal to the size of the coarse mesh element in way of the structure being considered. The averaging is to be based only on elements with their boundary located within the desired area. Stress averaging is not to be carried out across structural discontinuity or abutting structure.

4.8.3 For all load scenarios, except for the extreme load scenarios, plate buckling is to be investigated for all areas of primary structure. The factor against buckling is to be taken as 1,1.

4.8.4 Panel buckling calculations are to be based on the net thickness of the plating.

4.8.5 In general, the applied stresses for buckling assessment are to be increased by a factor equal to the original thickness divided by the thickness after corrosion.

σ_o is defined in Pt 2, Ch 2, 1.4 Materials

4.8.7 Deflections of individual panels under lateral hydrostatic loading shall not exceed 1/200 of the principal panel dimension.

4.8.8 Deflections of any stiffener supporting plate subjected to hydrostatic loading shall not exceed 1/200 of the stiffener span.

4.9.1 For floating dock gates with an L > 6D, longitudinal strength calculations are to be carried out in accordance with Pt 3, Ch 4 Pontoons of the Rules and Regulations for the Classification of Ships, July 2022, where the service factor is to be taken as 0,5.