Section 6 Local strength

6.1 General

6.1.1 Plating and supporting stiffeners are to have adequate scantlings to meet the requirements of Pt 2, Ch 2, 4 Global strength assessment, and these scantlings are not to be less than the requirements given in this Section. Alternatively, the local scantlings may be determined in accordance with a recognised national or international standard subject to prior agreement being obtained from LR.

6.2 Symbols

6.2.1 The symbols used in this Section are as follows:

s	=	spacing of secondary stiffeners, in mm
S	=	spacing or mean spacing of primary members, in metres
ρ	=	relative density (specific gravity), see Pt 2, Ch 2, 1.2 Basis of design 1.2.2
k	=	local scantling higher tensile steel factor, see Pt 2, Ch 2, 1.4 Materials

6.3 Plating

6.3.1 The minimum thickness, in mm, of side shell plating, bottom plating and transverse bulkhead plating is to be not less than the greater of:

10 mm

where

f	=	but not to be taken greater than 1,0
h	=	load head, in metres, measured as follows: For bulkhead plating, the distance from one third above the bottom of the strake of plating to half the height of the overflow above the tank top. For bottom plating, the distance from the baseline to the maximum operating water level, see Pt 2, Ch 2, 2.3 Hydrostatic loads, or the distance from the baseline to half the height of the overflow above the tank top, whichever is the greater. For side shell plating, the distance from one third above the bottom of the strake of plating to the maximum operating water level, see Pt 2, Ch 2, 2.3 Hydrostatic loads, or the distance from the baseline to half the height of the overflow above the tank top, whichever is the greater.

s, S, k and ρ are defined in Pt 2, Ch 2, 6.2 Symbols 6.2.1

6.4 Stiffening

6.4.1 The minimum section modulus of rolled or built stiffeners and double plate bulkheads is given by:

where

h	=	load head, in metres, measured as follows: For bulkhead stiffeners, the distance from the middle of the effective length to half the height of the overflow above the tank top. For bottom and side shell stiffeners, the distance from the middle of the effective length to the maximum operating water level, see Pt 2, Ch 2, 2.3 Hydrostatic loads, or the distance from the baseline to half the height of the overflow above the tank top, whichever is the greater.
γ	=	1,4 for rolled or built sections and double plate bulkheads
γ	=	1,6 for flat bars

s, S, k and ρ are defined in Pt 2, Ch 2, 6.2 Symbols 6.2.1.

l_e is defined in Pt 2, Ch 2, 3.3 Structural idealisation 3.3.7

6.4.2 The minimum inertia of rolled and built stiffeners and double plate bulkheads is given by:

where Z, k and l_e are defined in Pt 2, Ch 2, 6.4 Stiffening 6.4.1.

6.5 Stringers and webs

6.5.1 The minimum section modulus of stringers or webs supporting vertical or horizontal stiffening is given by:

where ρ, k, h and l_e are defined in Pt 2, Ch 2, 6.4 Stiffening 6.4.1.

6.5.2 The minimum inertia of stringers or webs supporting vertical or horizontal stiffening is given by:

where Z, k and l_e are defined in Pt 2, Ch 2, 6.5 Stringers and webs 6.5.1.

6.6 Double plate bulkheads

6.6.1 In addition to the requirements of Pt 2, Ch 2, 6.4 Stiffening, the following proportion checks are also to be complied with:

s/t_p is not to exceed at the top and at the bottom;
d/t_w is not to exceed at the top and at the bottom;
d is not to be less than 39l_e; and
A_w is to be less than at the top and at the bottom.

where

A_w is the shear area, in cm², of the webs of the double plate bulkhead

t_w, t_p and d are defined in Figure 2.3.1 Double plate bulkhead dimensions

s is defined in Pt 2, Ch 2, 6.2 Symbols 6.2.1

6.6.2 The plating thickness at the middle of span l_e of corrugated or double plate bulkheads is to extend not less than 0,2l_e m above mid span.

6.7 Pillars

6.7.1 Pillars are to comply with the requirements of Table 2.6.1 Pillars.

6.7.2 Pillars are to be fitted in the same vertical line wherever possible, and effective arrangements are to be made to distribute the load at the heads and heels of all pillars. Where pillars support eccentric loads, they are to be strengthened for the additional bending moment imposed upon them.

6.7.3 Tubular and hollow square pillars are to be attached at their heads to plates supported by efficient brackets, in order to transmit the load effectively. Doubling or insert plates are to be fitted under the heels of tubular or hollow square pillars, and to decks under large pillars. The pillars are to have a bearing fit and are to be attached to the head and heel plates by continuous welding. At the heads and heels of pillars built of rolled sections, the load is to be well distributed by means of longitudinal and transverse brackets.

6.7.4 Where pillars are not directly above the intersection of plate floors and girders, partial floors and intercostals are to be fitted as necessary to support the pillars. Manholes are not to be cut in the floors and girders below the heels of pillars. Where longitudinal framing is adopted, equivalent stiffening under the heels of pillars is to be provided, and where the heels of pillars are carried on a tunnel, suitable arrangements are to be made to support the load.

6.7.5 Where pillars are fitted inside tanks, the tensile stress in the pillar and its end connections is not to exceed 108 N/mm² at the test heads. In general, such pillars should be of built sections, and end brackets may be required.

6.7.6 Pillars are to be fitted below windlasses, winches, capstans and elsewhere where considered necessary.

Table 2.6.1 Pillars

Symbols

Parameter

Requirement

b = breadth of side of a hollow rectangular pillar or breadth of flange or web of a built or rolled section, in mm

(1) Cross-sectional area of all types of pillar

See Note

d_p = mean diameter of tubular pillars, in mm

k = local scantling higher tensile steel factor, see Pt 2, Ch 2, 1.4 Materials, but not less than 0,72

(2) Minimum wall thickness of tubular pillars

The greater of the following

but not to be less than
t = 5,5 mm where L < 90 m
or

t = 7,5 mm where L ≥ 90 m

l = overall length of pillar, in metres

l_e = effective length of pillar, in metres, and is taken as:

for hold pillars 0,65l

for 'tween deck pillars 0,80l

l_p = distance, in metres, between centres of two adjacent spans of girders or transverses supported by the pillar

r = least radius of gyration of pillar cross-section, in mm, and may be taken as:

(3) Minimum wall thickness of hollow rectangular pillars or web plate thickness of I or channel sections

The lesser of (b) and (c) and not to be less than (a):

but not to be less than

5,5 mm where L < 90 m

7,5 mm where L ≥ 90 m

A_p = cross-sectional area of pillar, in cm²

C = stowage rate, in m³/t

to be taken as 1,39

S = spacing or mean spacing of primary members, in metres

H_g = deck loading, in kN, see Pt 2, Ch 2, 2.4 Deck loading

I = least moment of inertia of cross-section, in cm⁴

(4) Minimum thickness of flanges of angle or channel sections

The lesser of the following:

P = load, in kN, supported by the pillar and is to be taken as

but not less than 19,62 kN

P_a = load, in kN, from pillar or pillars above (zero if no pillars over)

(5) Minimum thickness of flanges of built or rolled I sections

The lesser of the following:

Note As a first approximation A_p can be taken as

and the radius of gyration estimated for a suitable section having this area.

Note If the area calculated using this radius of gyration differs by more than 10 per cent from the first approximation, a further calculation using the radius of gyration corresponding to the mean area of the first and second approximation is to be made.

6.7.7 Where truss arrangements, comprising top and bottom girders in association with pillars and diagonal bracing, are used in the support of the deck loads, the diagonal members are generally to have angles of inclination with the horizontal of about 45° and cross-sectional area of approximately 50 per cent of the adjacent pillar in accordance with Table 2.6.1 Pillars.

6.8 Roadways

6.8.1 The scantlings and supporting arrangements of roadways are to be in accordance with the requirements of Pt 3, Ch 5 Bridge/Vehicle Ramp Strength of the Rules and Regulations for the Classification of Linkspans, July 2022, see also Pt 2, Ch 2, 2.4 Deck loading.

6.9 Design of supporting devices

6.9.1 Mitre and flap gates are to be provided with adequate means of closing and supporting which are to be commensurate with the strength and stiffness of the surrounding structure. The strength of the dock bottom/sides and connection with the closing and supporting devices (i.e. interface works) are not included in the scope of classification.

6.9.2 Closing and supporting devices are to be designed to withstand the forces the dock gate is subjected to in the closed position, see Pt 2, Ch 2, 2 Loading, using the following permissible stresses:

6.9.3 The arrangement of securing and supporting devices is to be such that threaded bolts are not to carry support forces. The maximum tensile stress in way of threads of bolts, not carrying support forces, is not to exceed:

6.9.4 For steel to steel bearings in securing and supporting devices, the nominal bearing pressure is not to exceed 0,8σ_o. For other bearing materials, the permissible bearing pressure is to be determined according to the manufacturer’s specification. The nominal bearing pressure is to be calculated by dividing the design force by the projected bearing area.

6.9.5 The distribution of the reaction forces acting on the supporting devices is to be supported by direct calculations taking into account the flexibility of the dock gate structure and the actual position and stiffness of the supports.