3 Structure Subjected to Impact Loads

3.1 General

3.1.1 Application

The requirements of this sub-section cover the strengthening requirements for local impact loads that may occur in the forward structure. The impact loads to be applied in [3.2] and [3.3] are described in Ch 4, Sec 5, [3].

3.1.2 General scantling requirements

The requirements of [3.2] and [3.3] are to be applied in addition to applicable scantling requirements in Ch 6. Local scantling increases due to impact loads are to be made with due consideration given to details and avoidance of hard spots, notches and other harmful stress concentrations.

3.2 Bottom slamming

3.2.1 Application

Where the minimum draughts forward, T_F-e or T_F-f, as specified in Ch 4, Sec 5, [3.2.1], are less than 0.045 L, the bottom forward is to be additionally strengthened to resist bottom slamming pressures.

The draughts for which the bottom has been strengthened are to be indicated on the shell expansion plan and loading guidance information, as required in Ch 1, Sec 5.

The load calculation point of the primary supporting members is specified in Ch 3, Sec 7, [4].

3.2.2 Extent of strengthening

The strengthening is to extend forward of 0.3 L from the FP over the flat of bottom and adjacent plating with attached stiffeners up to a height of 500 mm above the baseline, see Figure 2.

Figure 2 : Extent of strengthening against bottom slamming

Outside the region strengthened to resist bottom slamming the scantlings are to be tapered to maintain continuity of longitudinal and/or transverse strength.

3.2.3 Design to resist bottom slamming loads

The design of end connections of stiffeners in the bottom slamming region is to provide end fixity, either by making the stiffeners continuous through supports or by providing end brackets complying with Ch 3, Sec 6, [3.2]. Where it is not practical to comply with this requirement, the net plastic section modulus, Z_pl-alt, in cm³, for alternative end fixity arrangements is not to be less than:

where:

Z_pl : Net plastic section modulus, in cm³, as required by [3.2.5]. Scantlings and arrangements of primary supporting members, including bulkheads in way of stiffeners, are to comply with [3.2.7].

3.2.4 Shell plating

The net thickness of the hull envelope plating, t, in mm, except for the transversely stiffened bilge plating within the cylindrical part of the ship, is not to be less than:

where:

C_d : Plate capacity correction coefficient taken as:

C_d = 1.3.

The transversely stiffened bilge plating within the cylindrical part of the ship is to comply with the requirement given in Pt 1, Ch 6, Sec 4, [2.2].

C_a : Permissible bending stress coefficient taken as:

C_a = 1.0 for acceptance criteria set AC-I.

3.2.5 Shell stiffeners

The shell stiffeners within the strengthening area defined in [3.2.2] are to comply with the following criteria:

The net plastic section modulus, Z_pl, in cm³, is not to be less than:
where:

C_s : Permissible bending stress coefficient taken as:
- C_s = 0.9 for acceptance criteria set AC-I.
The net web thickness, t_w, in mm, is not to be less than:
where:

C_t : Permissible shear stress coefficient taken as:
- C_t = 1.0 for acceptance criteria set AC-I.
The slenderness ratio is to comply with Ch 8, Sec 2.

3.2.6 Bottom slamming load area for primary supporting members

The scantlings of primary supporting members according to [3.2.7] are based on the application of the slamming pressure defined in Ch 4, Sec 5, [3.2] to an idealised slamming load area of hull envelope plating, A_SL, in m², given by:

3.2.7 Primary supporting members

The size and number of openings in web plating of the floors and girders is to be minimised considering the required shear area as given in a):

Net shear area
The net shear area, A_shr-n50, in cm², of each primary supporting member web at any position along its span is not to be less than:

Q_SL : The greatest shear force due to slamming for the position being considered, in kN, based on the application of a patch load, F_SL to the most onerous location, as determined in accordance with b) or c).
C_t : Permissible shear stress coefficient taken as:
- C_t = 0.9 for acceptance criteria set AC-I.
Simplified calculation of slamming shear force
For simple arrangements of primary supporting members, where the grillage effect may be ignored, the shear force, Q_SL, in kN, is given by:

where:
f_pt : Correction factor for the proportion of patch load acting on a single primary supporting member, taken as
f_SL : Patch load modification factor taken as:
f_dist : Factor for the greatest shear force distribution along the span, according to Figure 3.
F_SL : Patch load, in kN, taken as:
ℓ_SL : Extent of slamming load area along the span, in m, taken as:
- but not to be greater than 0.5 ℓ_shr.
b_SL : Breadth of impact area supported by primary supporting member, in m, taken as:
- but not to be greater than S
A_SL : Surface defined in [3.2.6].

Figure 3 : Distribution of f_dist along the span of simple primary supporting members
Direct calculation method for slamming shear force
For complex arrangements of primary supporting members, the greatest shear force, Q_SL, at any location along the span of each primary supporting member is to be derived by direct calculation in accordance with Table 1.
Web thickness of primary supporting member
The net web thickness, t_w, in mm, of primary supporting members adjacent to the shell is not to be less than:
where:

s_W : Plate breadth, in mm, taken as the spacing between the web stiffening.

Table 1 : Direct calculation methods for derivation of Q_SL

Type of analysis

Model extent

Assumed end fixity of floors

Beam theory

Overall span of member between effective bending supports.

Fixed at ends

Double bottom grillage

Longitudinal extent to be one cargo tank length.

Transverse extent to be between inner hopper knuckle and centreline.

Floors and girders to be fixed at boundaries of the model.

Note 1: The envelope of greatest shear force along each primary supporting member is to be derived by applying the load patch on a square area as defined in [3.2.6], to a number of locations along the span.

Note 2: A more extensive model in length and breadth can be considered.

3.3 Bow impact

3.3.1 Application

The side structure in the ship forward area is to be strengthened against bow impact pressures. The strengthening is to extend forward of 0.1 L from the FP and vertically above the minimum design ballast draught, T_BAL, defined in Ch 1, Sec 4, [3.1.5] and forecastle deck if any. See Figure 4.

Figure 4 : Extent of strengthening against bow impact

Outside the strengthening area the scantlings are to be tapered to maintain continuity of longitudinal and/or transverse strength.

3.3.2 Design to resist bow impact loads

In the bow impact strengthening area, longitudinal framing is to be carried as far forward as practicable.
The design of end connections of stiffeners in the bow impact region are to ensure end fixity, either by making the stiffeners continuous through supports or by providing end brackets complying with Ch 3, Sec 6, [3.2]. Where it is not practical to comply with this requirement, the net plastic section modulus, Z_pl-alt, in cm³, for alternative end fixity arrangements is not to be less than:
where:

Z_pl : Effective net plastic section modulus, in cm³, required by [3.3.4].
Scantlings and arrangements of primary supporting members, including decks and bulkheads, in way of the stiffeners, are to comply with [3.3.6]. In areas of the greatest bow impact load, the web stiffeners arranged perpendicular to the hull envelope plating and the double sided lug connections are to be provided. The main stiffening direction of decks and bulkheads supporting shell framing is to be arranged parallel to the span direction of the supported shell frames, to protect against buckling.

3.3.3 Side shell plating

The net thickness of the side shell plating, t, in mm is not to be less than:

where:

C_a : Permissible bending stress coefficient taken as:

C_a = 1.0 for acceptance criteria set AC-I.

3.3.4 Side shell stiffeners

The side shell stiffeners within the strengthening area defined in [3.3.1] are to comply with the following criteria:

The effective net plastic section modulus, Z_pl, in cm³ in association with the effective plating to which it is attached, is not to be less than:

where:
C_s : Permissible bending stress coefficient taken as:
- C_s = 0.9 for acceptance criteria set AC-I.
The net web thickness, t_w, in mm, is not to be less than:

where:

d_shr : Effective web depth of stiffener, in mm, as defined in Ch 3, Sec 7, [1.4.3].
C_t : Permissible shear stress coefficient taken as:
- C_t = 1.0 for acceptance criteria set AC-I.
The slenderness ratio is to comply with Ch 8, Sec 2.

3.3.5 Bow impact load area for primary supporting members

The scantlings of primary supporting members according to [3.3.6] are based on the application of the bow impact pressure, as defined in Ch 4, Sec 5, [3.3.1], to an idealised bow impact load area of hull envelope plating, A_BI, in m², is given by:

3.3.6 Primary supporting members

The section modulus of the primary supporting member is to apply along the bending span clear of end brackets and cross sectional areas of the primary supporting member are to be applied at the ends/supports and may be gradually reduced along the span and clear of the ends/supports following the distribution of f_dist indicated in Figure 3.
Primary supporting members in the bow impact strengthening area are to be configured to provide effective continuity of strength and the avoidance of hard spots.
End brackets of primary supporting members are to be suitably stiffened along their edge. Consideration is to be given to the design of bracket toes to minimise abrupt changes of cross section.
Tripping arrangements are to comply with Ch 8, Sec 2, [5.1.1]. In addition, tripping brackets are to be fitted at the toes of end brackets and at locations where the primary supporting member flange is knuckled or curved.
The net section modulus of each primary supporting member, Z_n50, in cm³, is not to be less than:

where:
f_bdg-pt : Correction factor for the bending moment at the ends and considering the patch load taken as:
f_BI : Patch load modification factor taken as:
ℓ_BI : Extent of bow impact load area, in m, along the span:
- but not to be taken as greater than ℓ_bdg.
b_BI : Breadth of impact load area, in m, supported by the primary supporting member, to be taken as the spacing between primary supporting members, S, as defined in Ch 1, Sec 4, Table 5, but not to be taken as greater than ℓ_BI.

A_BI : Bow impact load area, in m², as defined in [3.3.5].
f_bdg : Bending moment factor taken as:
- f_bdg = 12 for primary supporting members with end fixed continuous flange or where brackets at both ends are fitted in accordance with Ch 3, Sec 6, [4.4].
C_s : Permissible bending stress coefficient taken as:
- C_s = 0.8 for acceptance criteria set AC-I.
The net shear area of the web, As_hr-n50, in cm², of each primary supporting member at the support/toe of end brackets is not to be less than:

where:
f_PL : Patch load modification factor taken as:
ℓ_BI : Extent of bow impact load area, in m, along the span taken as,
- but not greater than ℓ_shr.
C_t : Permissible shear stress coefficient taken as:
- C_t = 0.75 for acceptance criteria set AC-I.
The net web thickness of each primary supporting member, t_w, in mm including decks/bulkheads in way of the side shell is not to be less than:

where:

ϕ_w : Angle, in deg, between the primary supporting member web and the shell plate, see Figure 5.

σ_cr : Critical buckling stress in compression of the web of the primary supporting member or deck/bulkhead panel in way of the applied load given by Ch 8, Sec 5, [2.2.3], in N/mm². In the calculation, both σ_x and σ_y given in Ch 8, Sec 5, [2.2.3] are to be considered and UP-B is to be applied.

Figure 5 : Angle between shell primary member and shell plate

3 Structure Subjected to Impact Loads

Table 1 : Direct calculation methods for derivation of QSL

Table 1 : Direct calculation methods for derivation of Q_SL