Section 4 Structural design factors

4.1.1 This Section gives the allowable design criteria to be used to assess the structure for maximum and minimum stresses and other values derived in accordance with the TLA method.

4.1.2 The allowable design criteria are also to be used when direct calculation or similar methods are used as an alternative to the TLA stress analysis model in Vol 1, Pt 7, Ch 3, 3 Stress analysis model.

4.1.3 The allowable design criteria are applicable for all ships including NS1 ships.

4.2.1 The stresses in the plating and stiffeners that have been derived using the TLA method are to be less than the allowable stresses and other design criteria given in this Section. For all areas of the structure, the modes of failures specified in Table 3.4.1 Design criteria for plating to Table 3.4.4 Design criteria for primary members are to be satisfied. See also Vol 1, Pt 7, Ch 3, 4.3 Presentation of results for presentation of results.

4.2.2 In order to satisfy the requirements, the ratio of the actual stress to the allowable stress or load utilisation factor is to be less than 1,0, similarly for the deflection and buckling utilisation factors. Hence

where

4.2.3 The allowable stresses are to be derived using the following formulae:

• Direct or bending allowable stress
  σ_a = f ₁ f _hts σ_o N/mm²
• Shear allowable stress
  τ_a = f ₁ f _hts τ_o N/mm²

where

f ₁ is taken from Table 5.3.2 Allowable stress factors f 1 in Pt 6, Ch 5, as specified below:

• Table 3.4.1 Design criteria for plating for plating.
• Table 3.4.2 Design criteria for stiffeners for stiffeners.
• Table 3.4.3 Design criteria for stiffened panels for plate panels.
• Table 3.4.4 Design criteria for primary members for primary members.

4.2.4 The buckling design safety factors are given in Table 5.3.2 Allowable stress factors f 1 in Pt 6, Ch 5.3 and are to be used in conjunction with the buckling requirements specified in Vol 1, Pt 6, Ch 2, 3 Buckling. The required values of safety factors λ _σ and λ _τ are as specified by f ₁ in Vol 1, Pt 7, Ch 3, 4.2 Design criteria for the Total Load Assessment (TLA) approach 4.2.3.

1. λ_a = f ₁

where

4.2.5 The limiting deflection requirements for stiffening members specified in Table 3.4.1 Design criteria for plating to Table 3.4.4 Design criteria for primary members are to be satisfied. The allowable deflection criteria are expressed as a deflection/span ratio and the actual deflection is to be less than the allowable deflection, δ_a, where δ_a is as follows:

where

f ₁ is taken from Table 5.3.2 Allowable stress factors f 1 in Pt 6, Ch 5 as specified in Vol 1, Pt 7, Ch 3, 4.2 Design criteria for the Total Load Assessment (TLA) approach 4.2.3

4.2.6 The assessment of scantling requirements to satisfy the impact or slamming pressure loads for plating and stiffening is given in Vol 1, Pt 6, Ch 3, 14 Strengthening for bottom slamming and Vol 1, Pt 6, Ch 3, 15 Strengthening for wave impact loads above waterline.

4.3.1 It is recommended that the capability of the structure is represented by one of the following methods:

1. Load utilisation factor, LUF, derived as follows:

   

   e.g. the stress LUF =

2. Adequacy parameter, AP, defined as follows:

   

   where

   LUF is given above.

4.3.2  Table 3.4.5 Adequacy parameter (AP) shows a comparison of the adequacy parameter with the load utilisation factor.