Section 4 Decks

4.1.1 The design deck loadings for all unit types are not to be less than those defined in Pt 4, Ch 6, 1 General requirements and Pt 4, Ch 6, 2 Design heads.

4.1.2 The scantlings of deck structures are to comply with:

• Pt 10 SHIP UNITS for ship units; and
• Pt 4, Ch 4, 4 Surface type units for other surface type units.

The requirements of Pt 4, Ch 6, 4.1 General 4.1.5 and Pt 4, Ch 6, 4.1 General 4.1.6 are also to be complied with as applicable.

4.1.3 The minimum scantlings of deck structures on column-stabilised units, self-elevating units, tension-leg units, buoys and deep draught caissons are to comply with this Section.

4.1.4 The scantlings of deck structures are also to satisfy the overall strength requirements in Pt 4, Ch 4 Structural Unit Types and be sufficient to withstand the actual local loadings plus any additional loadings superimposed due to overall frame action. The permissible stress levels are to comply with Pt 4, Ch 5 Primary Hull Strength.

4.1.5 Where decks form watertight boundaries in damage stability conditions, the minimum scantlings are not to be less than required for watertight bulkheads given in Pt 4, Ch 6, 7 Bulkheads.

4.1.6 For units fitted with a process plant facility and/or drilling equipment, the support stools and integrated hull support structure to the process plant and other equipment supporting structures to drilling derricks and flare structures, etc., are considered to be classification items regardless of whether or not the process/drilling plant facility is classed and the loadings are to be determined in accordance with Pt 3, Ch 8, 2 Structure. Permissible stress levels are to comply with Pt 4, Ch 5 Primary Hull Strength.

4.2.1 The requirements are in general applicable to strength/weather deck plating with stiffeners fitted parallel to the hull bending compressive stress. When other stiffening arrangements are proposed, the scantlings will be specially considered, but the minimum requirements of Pt 4, Ch 6, 4.3 Deck stiffening 4.3.3 are to be complied with.

4.2.2 The minimum thickness of deck plating is to comply with the requirements of Pt 4, Ch 6, 4.3 Deck stiffening 4.3.3, except for decks in way of erections above the upper deck. For erection decks, see Pt 4, Ch 6, 6 Decks loaded by wheeled vehicles.

4.2.4 The deck plating thickness and supporting structure in way of towing brackets, winches, masts, crane pedestals, davits and machinery items, etc., is to be suitably reinforced, see also Pt 4, Ch 6, 1 General requirements.

4.2.5 Where plated decks are sheathed with wood or approved compositions, consideration will be given to allowing a reduction in the minimum plating thickness given in Pt 4, Ch 6, 4.3 Deck stiffening 4.3.3.

4.3.1 The scantlings of deck stiffeners are to comply with the requirements of Pt 4, Ch 6, 4.4 Deck supporting structure 4.4.2. Stiffeners fitted in way of concentrated loads and heavy machinery items, etc., will be specially considered.

4.3.2 The lateral and torsional stability of stiffeners together with web and flange buckling criteria are to be verified in accordance with Pt 4, Ch 5, 3 Buckling strength of plates and stiffeners.

4.4.1 The minimum scantlings of girders and transverses supporting deck stiffeners are to comply with the requirements of Pt 4, Ch 6, 4.4 Deck supporting structure 4.4.2.

4.4.2 Transverses supporting deck longitudinals are, in general, to be spaced not more than 3,8 m apart when the length, L, is 100 m or less, and (0,006L + 3,2) m apart where L is greater than 100 m.

Table 6.4.2 Deck Stiffeners

Symbols	Location	Modulus, in cm3	Inertia, in cm4
= depth of stiffener, in mm, see Note 2	(1) Weather decks	Z = 5,5s k 2 x 10–3	––
= weather head, in metres
= work area head, in metres	(2) Work areas	Z = 5,5s k 2 x 10–3	––
= storage head, in metres
= tank head, in metres, as defined in Pt 4, Ch 6, 7.3 Watertight and deep tank bulkheads 7.3.4	(3) Storage areas	Z = 5,0s k 2 x 10–3	––
= accommodation head, in metres
k = steel factor defined in Pt 4, Ch 2, 1.2 Steel	(4) Accommodation decks and crew spaces	Z = 4,5s k 2 x 10–3	––
= span point, in metres as defined in Pt 4, Ch 6, 3.3 Self-elevating units but not less than 1,5 m
s = spacing of stiffeners, in mm	(5) In way of the crown or bottom of tanks	As (1), (2), (3) or (4) as applicable, or whichever is the greater
γ = 1,4 for rolled or built sections
= 1,6 for flat bars
ρ as defined in Pt 4, Ch 6, 7.3 Watertight and deep tank bulkheads 7.3.4
NOTES
1. The load heads , , and are to be determined from the maximum design uniform loadings and are not to be less than the minimum design load heads given in Pt 4, Ch 6, 2.3 Stowage rate and design heads 2.3.2.
2. The web depth, , of stiffeners is to be not less than 60 mm.

Table 6.4.3 Deck girders, transversers and deep beams

Location and arrangements	Modulus, in cm3	Inertia, in cm4
(1) Girders and transverses in way of dry spaces:	Z to be determined from calculations using stress and assuming fixed ends Z = 4,75k S
(a) Supporting point loads
(b) Supporting a uniformly distributed load
(2) Deep beams supporting deck girders in way of dry spaces:	Z to be determined from calculations using stress and assuming fixed ends Z = 4,75k S
(a) Supporting point loads
(b) Supporting a uniformly distributed load
(3) Girders and transverses in way of the crown or bottom of tanks	Z = 11,7 ρk
Symbols
= tank head, in metres, as defined inPt 4, Ch 6, 7.3 Watertight and deep tank bulkheads 7.3.4
k = steel factor as defined in Pt 4, Ch 2, 1.2 Steel
= span point, in metres, defined in Pt 4, Ch 6, 3.3 Self-elevating units
= weather head or work area head or storage head or accommodation head , in metres, as defined in Pt 4, Ch 6, 2.3 Stowage rate and design heads 2.3.2 whichever is applicable
S = spacing of primary members, in metres
ρ as defined inPt 4, Ch 6, 7.3 Watertight and deep tank bulkheads 7.3.4

4.4.3 The web thickness, stiffening arrangements and end connections of primary supporting members are to be in accordance with Pt 4, Ch 8 Welding and Structural Details.

4.4.4 Where a girder is subject to concentrated loads, such as pillars out of line, the scantlings are to be suitably increased. Also, where concentrations of loading on one side of the girder may occur, the girder is to be adequately stiffened against torsion.

4.4.5 Pillars are to comply with the requirements of Pt 4, Ch 6, 4.4 Deck supporting structure 4.4.6.

4.4.6 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.

Table 6.4.4 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
= mean diameter of tubular pillars, in mm		See Note
k = local scantling higher tensile steel factor, see Pt 4, Ch 2, 1.2 Steel 1.2.1, but not less than 0,72	(2) Minimum wall thickness of tubular pillars	The greatest of the following:
l = overall length of pillar, in metres		(a)
= effective length of pillar, in metres, and is taken as 0,80l		(b) t = mm
		(c) t = 5,5 mm where L < 90 m, or = 7,5 mm where L ≥ 90 m
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 the following:
		(a) t = mm
= cross-sectional area of pillar, in cm2		(b) t = mm
		but to be not less than t = 5,5 mm where L < 90 m, or = 7,5 mm where L ≥ 90 m
as defined in Pt 4, Ch 6, 4.4 Deck supporting structure 4.4.2	(4) Minimum thickness of flanges of angle or channel sections	The lesser of the following:
I = least moment of inertia of cross-section, in cm4		(a) = mm
P = load, in kN (tonne-f), supported by the pillar and is to be taken as: but not less than 19,62 kN (2 tonne-f)		(b) = mm
= load, in kN (tonne-f), 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:
= load, in kN (tonne-f), supported by pillar based on		(a) = mm
		(b) = mm
NOTE
As a first approximation, may be taken as and the radius of gyration estimated for a suitable section having this area.
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.

4.4.7 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 to decks under the heels of tubular or hollow square 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.

4.4.8 Where pillars are not fitted directly above the intersection of bulkheads, equivalent arrangements are to be provided.

4.4.9 In double bottoms where pillars are not directly above the intersection of the 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 floors and girders below the heels of pillars.

4.4.10 Where pillars are fitted inside tanks or under watertight flats, the tensile stress in the pillar and its end connections is not to exceed 108 N/mm² (11,0 kgf/mm²) at the test heads. In general, such pillars should be of built sections, and end brackets may be required.

4.4.11 Pillars or equivalent structures are to be fitted below deckhouses, machinery items, winches, etc., and elsewhere where considered necessary.

4.4.12 The thickness of primary longitudinal and transverse bulkheads supporting decks is to satisfy the requirements for the overall strength of the unit in accordance with:

• Pt 10 SHIP UNITS for ship units; and
• Pt 4, Ch 4 Structural Unit Types and Pt 4, Ch 5 Primary Hull Strength for other unit types.

When the bulkheads are to be watertight the scantlings are also to comply with the requirements of Pt 4, Ch 6, 7 Bulkheads.

4.4.13 The lateral and torsional stability of primary bulkhead stiffeners together with web and flange buckling criteria are to be verified in accordance with Pt 4, Ch 5, 3 Buckling strength of plates and stiffeners.

4.4.14 When openings are cut in the primary longitudinal and transverse bulkheads the openings are to have well rounded corners and full compensation is to be provided. All openings are to be adequately framed.

4.4.15 The minimum scantlings of non-watertight pillar bulkheads are to comply with the requirements of Pt 4, Ch 6, 4.5 Deck openings 4.5.7.

4.5.1 The corners of all deck openings are to be elliptical, parabolic or well rounded and the free edges are to be smooth. Large openings are to comply with Pt 4, Ch 6, 4.5 Deck openings 4.5.4 and Pt 4, Ch 6, 4.5 Deck openings 4.5.5.

4.5.2 All openings are to be adequately framed. Attention is to be paid to structural continuity, and abrupt changes of shape, section or plate thickness are to be avoided.

4.5.3 Arrangements in way of corners and openings are to be such as to minimise the creation of stress concentrations. Openings in highly stressed areas of decks, having a stress concentration factor in excess of 2,4, will require edge reinforcements in the form of a spigot of adequate dimensions, but alternative arrangements will be considered. The area of any edge reinforcement which may be required is not to be taken into account in determining the required sectional area of compensation for the opening

4.5.4 When large openings are cut in highly stressed areas of decks, the corners of the openings are to be elliptical, parabolic or rounded, with a radius generally not less than 1/24 of the breadth of the opening. The minimum radius for large openings is to be 150 mm, provided the inner edge of the plating is stiffened by means of a coaming or spigot. Where the inner edge is unstiffened, the minimum radius is to be 300 mm.

4.5.5 Where the corners of large openings are rounded, the deck plating thickness is to be increased at the corners of the openings.

4.5.6 Compensation will be required for deck openings cut in highly stressed areas.