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Clasification Society Provisional Rules - Provisional Rules for Existing Ships, July 2015 - Chapter 7 Appendix 4 - Section 1 Shell envelope framing

Section 1 Shell envelope framing

Parent topic: Chapter 7 Appendix 4

1.1 Transverse stiffening

1.1.1 The modulus and inertia of main and topside tank frames in the midship region are to comply with the requirements given in Table 7.1.1 Shell framing. Arrangements of main frames in holds in association with web frames are not recommended in view of the vulnerability to cargo handling damage. Where such web frames are proposed the arrangements and scantlings will be specially considered.

1.1.2 Main frames in the cargo and ballast holds are to have a web thickness not less than either:

tmin = 7 + 0,03L mm in general, or 13mm whichever is the lesser, or
tmin = 1,15 (7 + 0,03L) mm, in the foremost hold where L is the Rule length, in metres, or 15 mm whichever is the lesser.

1.1.3 The web depth to thickness ratio of the frames is not to be greater than:

In the case of asymmetric sections, the breadth to thickness ratio of the flange is not to be greater than:

1.1.4 The upper and lower end brackets of the main frames in the cargo and ballast holds are to satisfy the requirements of Ch 7, 1.1 Transverse stiffening 1.1.5 to Ch 7, 1.1 Transverse stiffening 1.1.14 inclusive, based on the mild steel section modulus Z in cm3, derived from Table 7.1.1 Shell framing, or the equivalent mild steel section modulus for higher tensile steel frames.

Table 7.1.1 Shell framing

Location Modulus, in cm3 Inertia, in cm4
(1) Main frames in dry cargo holds
Z = 3,50skhT1 H2x 10–3
I =
(2) Main frames in cargo holds used for water ballast The greater of the following:
I =
   
(a) =
Z = 1,15 x modulus given in (1)
   
(b) =
Z = 6,7skh 4 H 2 x 10–3
(3) Transverse frames in topside wing tanks The greater of the following:
I =
   

(a) 1,15 x Z as given in location (1) of Table 1.6.2 in Chapter 1

   

(b) As required by 7.3.1 for the sloped bulkhead stiffeners

Symbols
D, T, s, k as defined in 1.5.1
h T1 = head, in metres, at middle of H
= in metres, for frames where the mid-length of frame is above the summer load waterline,
= is not to be taken less than 0,7 C W
= in metres, where the mid-length of frame is below the summer load waterline
h 4 = head, in metres, measured from the middle of H to the deck at side, or half the distance from the middle of H to the top of the overflow, whichever is greater.
h 6 = vertical distance in metres, from the summer load waterline at draught T to the mid-length of H
C W = a wave head, in metres
= 7,71 x 10–2 Le –0,0044L
where e = base of natural logarithms 2,7183
F λ = 1,0 for L ≤ 200 m
= (1,0 + 0,0023(L – 200)) for L > 200 m
H = length overall of frame, in metres, but is to be taken not less than 2,5 m

1.1.5 The lengths of the arms of the brackets, measured as shown in Figure 7.1.1 Diagrammatic arrangement of end brackets, are not to be less than:

  1. Frame connection to hopper tank.

    Athwartship arm:  
    Dry cargo hold
    Ballast hold
    Vertical arm:  
    Dry cargo hold
    Ballast hold

  2. Frame connection to topside tank

    Athwartship arm:  
    Dry cargo hold
    Ballast hold
    Vertical arm:  
    Dry cargo hold
    Ballast hold

In no case are the bracket arm lengths to be taken less than the following:

  1. athwartship arm

    la = 2,4d

  2. vertical arm

    lv = 2,4d or 0,125 H
where
= d is as defined in Figure 7.1.1 Diagrammatic arrangement of end brackets, and
= H is as defined in Table 7.1.1 Shell framing.

1.1.6 The section modulus of the frame and bracket or integral bracket, and associated shell plating at the location marked Za in Figure 7.1.1 Diagrammatic arrangement of end brackets is to be not less than 2,0Z

In addition, the minimum depth of the frame and bracket or integral bracket at the location indicated in Figure 7.1.1 Diagrammatic arrangement of end brackets is to be not less than 1,5d.

1.1.7 The upper and lower integral or separate brackets are to have a web thickness not less than the following:

  1. Lower bracket

    t = tmin + 2 mm, where tmin is derived from Ch 7, 1.1 Transverse stiffening 1.1.2

  2. Upper bracket thickness not to be less than frame web thickness

The toes of the brackets are to be designed to avoid notch effects by making the upper and lower toes concave or otherwise tapering them off (see also Pt 3, Ch 10,5.1.7).

1.1.8 In general, except as indicated in Ch 7, 1.1 Transverse stiffening 1.1.9, frames are to be fabricated symmetrical sections with integral upper and lower brackets. The side frame face plate is to be curved (not knuckled) at the connection with the end brackets. The radius of curvature, r, is to be not less than:

r =
where
b f = breadth of the bracket face plate, in mm
tf = thickness of the bracket face plate, in mm

The brackets are to be arranged with soft toes and the frame section face bar tapered symmetrically to the toes with a taper rate not exceeding 1 in 3. Where the free edge of the bracket is hollowed out, it is to be stiffened or increased in size to ensure that the section modulus of the bracket through the throat is not less than that of the required straight edged bracket.

1.1.9 In ships of length, L, less than 190 m, mild steel fabricated frames may be asymmetric and fitted with separate brackets. Brackets are to be arranged with soft toes. The free edges of the brackets are to be stiffened as follows:

  1. Where a flange is fitted, its breadth, bf, is to be not less than:

    bf =

    or 50 mm, whichever is the greater.

    The flange is to be tapered at the ends with a taper rate not exceeding 1 in 3.

  2. Where the edge is stiffened by a welded face flat, the cross-sectional area of the face flat is to be not less than:

    1. 0,009bft cm2 for offset edge stiffening

    2. 0,014bft cm2 for symmetrically placed stiffening

where
t = web thickness of bracket, in mm
= The face plate is to be tapered at the ends with a taper rate not exceeding 1 in 3.

1.1.10 For mild steel construction with separate brackets where the frames are lapped on to the bracket, the length of the overlap is to be adequate to provide for the required area of welding to achieve equivalent strength.

1.1.11 Double continuous welding is to be adopted for the connections of frames and brackets to side shell, hopper and topside tank plating and web to face plates. For this purpose, the following weld factors are to be adopted:

0,44 in Zone ‘a’ and

0,40 in Zone ‘b’, see Figure 7.1.1 Diagrammatic arrangement of end brackets.

Where the hull form is such that an effective fillet weld cannot be made, edge preparation of the web of the frame and bracket may be required, in order to ensure the required efficiency of the weld connection.

Figure 7.1.1 Diagrammatic arrangement of end brackets

1.1.12 The design of end connections and their supporting structure is to be such as to provide adequate resistance to rotation and displacement of the joint. For this purpose, in the hopper and topside tanks, the thickness of the supporting brackets (which must align with the hold main frame brackets) is to be not less than the following:

  1. Lower brackets (In hopper tank):

    t = t min + 0,5 mm, where t min is de