5 Intersection of Stiffeners and Primary Supporting Members

5.1 Cut-outs

5.1.1 Cut-outs for the passage of stiffeners through the web of primary supporting members, and the related collaring arrangements, are to be designed to minimise stress concentrations around the perimeter of the opening and on the attached web stiffeners.

5.1.2 The total depth of cut-outs without collar plate is to be not greater than 50% of the depth of the primary supporting member.

5.1.3 Cut-outs in way of cross tie ends and floors under bulkhead stools or in high stress areas are to be fitted with full collar plates, see Figure 7.

Figure 7 : Full collar plates

R ≥ 0.2b but not less than 25 mm.

Figure 8 : Symmetric and asymmetric cut-outs

The details shown in this figure are only used to illustrate symbols and definitions and are not intended to represent design guidance.

Figure 9 : Primary supporting member web stiffener details

t_ws, t_ws1, t_ws2 : Net thickness of the primary supporting member web stiffener/backing bracket, in mm.

d_w, d_w1, d_w2 : Minimum depth of the primary supporting member web stiffener/backing bracket, in mm.

d_wc, d_wc1, d_wc2 : Length of connection between the primary supporting member web stiffener/backing bracket and the stiffener, in mm.

t_f: Net thickness of the flange in mm. For bulb profile, tf is to be obtained as defined in Pt.1 Ch.3 Sec.7 [1.4.1].

5.1.4 Lug type collar plates are to be fitted in cut-outs where required for compliance with the requirements of [5.2], and in areas of high stress concentrations, e.g. in way of primary supporting member toes. See Figure 8 for typical lug arrangements.

5.1.5 At connection to shell envelope longitudinals below the scantling draught, T_sc and at connection to inner bottom longitudinals, a soft heel is to be provided in way of the heel of the primary supporting member web stiffeners when the calculated direct stress, σ_w, in the primary supporting member web stiffener according to [5.2] exceeds 80% of the permissible values. The soft heel is to have a keyhole, similar to that shown in item (c) in Figure 9.

A soft heel is not required at the intersection with watertight bulkheads and primary supporting members, where a back bracket is fitted or where the primary supporting member web is welded to the stiffener face plate.

5.1.6 Cut-outs are to have rounded corners and the corner radii, R, are to be as large as practicable, with a minimum of 20% of the breadth, b, of the cut-out or 25 mm, whichever is greater. The corner radii, R, does not need to be greater than 50 mm, see Figure 7. Consideration is to be given to other shapes on the basis of maintaining equivalent strength and minimising stress concentration.

Note 1: Except where specific dimensions are noted for the details of the keyhole in way of the soft heel, the details shown in this figure are only used to illustrate symbols and definitions and are not intended to represent design guidance or recommendations.

5.2 Connection of stiffeners to PSM

5.2.1 General

For connection of stiffeners to PSM in case of lateral pressure other than bottom slamming and bow impact loads, [5.2.2] and [5.2.3] are to be applied. In case of bottom slamming or bow impact loads, [5.2.4] is to be applied.

The cross sectional areas of the connections are to be determined from the proportion of load transmitted through each component in association with its appropriate permissible stress.

5.2.2 The load, W₁, in kN, transmitted through the shear connection is to be taken as follows.

• If the web stiffener is connected to the intersecting stiffener:

  

• If the web stiffener is not connected to the intersecting stiffener:

  W₁ = W

where:

W : Total load, in kN, transmitted through the stiffener connection to the PSM taken equal to:

P₁, P₂ : Design pressure applied on the stiffener for the design load set being considered, in kN/m², on each side of the considered connection.

S₁, S₂ : Spacing between the considered and the adjacent PSM on each side of the considered connection, in m.

s₁, s₂ : Spacing of the stiffener, in mm, on each side of the considered connection.

α_a : Panel aspect ratio, not to be taken greater than 0.25.

A₁ : Effective net shear area, in cm², of the connection, to be taken equal to:

A₁ = A_1d + A_1c

In case of a slit type slot connections area, A₁, is given by:

A₁ = 2 A_1d

In case of a typical double lug or collar plate connection area, A₁, is given by:

A₁ = 2 A_1c

A_1d : Net shear connection area, in cm², excluding lug or collar plate, as given by:

A_1d = ℓ_d t_w 10^-2

ℓ_d : Length of direct connection between stiffener and PSM web, in mm.

t_w : Net web thickness of the primary supporting member, in mm.

A_1c : Net shear connection area, in cm², with lug or collar plate, given by:

A_1c = f₁ ℓ_c t_c 10^-2

ℓ_c : Length of connection between lug or collar plate and PSM, in mm.

t_c : Net thickness of lug or collar plate, not to be taken greater than the net thickness of the adjacent PSM web, in mm.

w : Width of the cut-out for an asymmetrical stiffener, measured from the cut-out side of the stiffener web, in mm, as indicated in Figure 8.

A_w : Effective net cross sectional area, in cm², of the PSM web stiffener in way of the connection including backing bracket where fitted, as shown in Figure 9. If the PSM web stiffener incorporates a soft heel ending or soft heel and soft toe ending, A_w is to be measured at the throat of the connection, as shown in Figure 9.

5.2.3 The load, W₂, in kN, transmitted through the PSM web stiffener is to be taken as:

• If the web stiffener is connected to the intersecting stiffener:

  

• If the web stiffener is not connected to the intersecting stiffener:

  W₂ = 0

The values of A_w, A_wc and A₁ are to be such that the calculated stresses satisfy the following criteria:

• For the connection to the PSM web stiffener not in way of the weld: σ_w ≤ σ_perm
• For the connection to the PSM web stiffener in way of the weld: σ_wc ≤ σ_perm
• For the shear connection to the PSM web: τ_w ≤ τ_perm

where:

W : Load, in kN, as defined in [5.2.2].

f_c : Collar load factor as defined in [5.2.2].

α_a : Panel aspect ratio, as defined in [5.2.2].

A₁ : Effective net shear area, in cm², as defined in [5.2.2].

A_w : Effective net cross sectional area, in cm², as defined in [5.2.2].

A_wc : Effective net area, in cm², of the PSM web stiffener in way of the weld as shown in Figure 9.

σ_perm : Permissible direct stress given in Table 1 for AC-S and AC-SD, in N/mm².

τ_perm : Permissible shear stress given in Table 1 for AC-S and AC-SD, in N/mm².

5.2.4 Bottom slamming and bow impact loads

For bottom slamming or bow impact loads, the load W, in kN, transmitted through the PSM web stiffener is to comply with the following criteria instead of those defined in [5.2.2] and [5.2.3]:

where:

W : Load, in kN, as defined in [5.2.2].

A₁ : Effective net shear area, in cm², as defined in [5.2.2].

A_w : Effective net cross sectional area, in cm², as defined in [5.2.2].

σ_perm : Permissible direct stress given in Table 1 for AC-I, in N/mm².

τ_perm : Permissible shear stress given in Table 1 for AC-I, in N/mm².

5.2.5 Where a backing bracket is fitted in addition to the PSM web stiffener, it is to be aligned with the web stiffener. The arm length of the backing bracket is not to be less than the depth of the web stiffener. The net cross sectional area through the throat of the bracket is to be included in the calculation of A_w as shown in Figure 9.

5.2.6 Lapped connections of PSM web stiffeners or tripping brackets to stiffeners are not permitted in the cargo hold region.

5.2.7 Where built-up stiffeners have their face plate welded to the side of the web, a symmetrical arrangement of connection to the PSM is to be fitted. This may be achieved by fitting backing brackets on the opposite side of the PSM or bulkhead. In way of the cargo hold region, the PSM web stiffener and backing brackets are to be butt welded to the intersecting stiffener web.

5.2.8 Where the web stiffener of the PSM is parallel to the web of the intersecting stiffener, but not connected to it, the offset PSM web stiffener is to be located in close proximity to the slot edge as shown in Figure 10. The ends of the offset web stiffeners are to be suitably tapered and softened.

Locations where the web stiffener of the PSM are not connected to the intersecting stiffeners as well as the detail arrangements are to be specially considered on the basis of their ability to transmit load with equivalent effectiveness to that of [5.2.2] through [5.2.7]. Details of calculations made and/or testing procedures and results are to be submitted.

5.2.9 The size of the fillet welds is to be calculated according to Ch 12, Sec 3, [2.5] based on the weld factors given in Table 2. For the welding in way of the shear connection the size is not to be less than that required for the PSM web plate for the location under consideration.

Figure 10 : Offset PSM web stiffeners