Section 2 Calculation of Strength

201. Structural analysis

The unit is to be analysed by the method deemed appropriate by the Society for a sufficient num- ber of conditions including all conditions specified in Ch 1, 107.

Ch 4 Design Condition Ch 4

202. Analysis of units resting on the sea bed

Units designed to rest on the sea bed are to be analysed assuming the overturning moment due to the combined environmental forces from any direction and the sufficient downward gravity loadings on the support footings or mat to withstand the moment.

203. Plastic analysis

Scantlings of structural members designed on the basis of plastic analysis are to be at the dis- cretion of the Society.

204. Buckling strength

Structural members subject to in-plane loads are to have the sufficient strength against buckling in consideration of their shapes, scantlings, boundary conditions, etc.

205. Fatigue strength

1. The possibility of fatigue damage due to cyclic loading should be considered in the design of self-elevating and column-stabilized units.

2. The area anticipated stress concentration is to be considered to fatigue strength, the fatigue analysis is to be based on the intended mode and area of operations to be considered in the unit's design.

3. The fatigue life is to be based on a period of time equal to the specified design life of the unit.

The period is normally not to be taken as less than 20 years.

206. Stress concentration

1. The effect of local stress concentrations is to be considered for notches in members or dis- continuous parts of structure.

2. Where the tensile stresses acting on the thickness direction of plating, plate material with suitable

through-thickness properties is required in accordance with Pt 2, Classification of Steel Ships.

Ch 1 of Rules for the

207. Bending stress

1. When calculating bending stresses of structural members, the effective width of the plate is to be

determined in accordance with the requirements in Pt 3, Ch 1, 602. of Rules Classification of Steel Ships.

for the

2. Where subjected to eccentric loadings, an increase of bending stress due to the deflections of the structural members is to be taken into account.

208. Shearing stress

When calculating shearing stresses in bulkheads, plate girder webs, hull side plating, etc., only the effective shear area of web is to be considered as being effective. In this regard, the total depth of the girder may be considered as the web depth.

209. Combination of stresses

1. In obtaining respective local stresses of the structural members, all the stress components concerned are to be summed up. In this case, for tubular members, the effect of circumferential stress due to external pressure is to be considered.

2. The scantlings are to be determined on the basis of criteria which combine, in a rational manner deemed appropriate by the Society, the individual stress components acting on the respective struc- tural members.(See 210.)

Ch 4 Design Condition Ch 4

210. Equivalent stress

1. For plate structures, members may be designed according to the equivalent stress criterion, where the equivalent stress is obtained from the following formula.

← ← ← ← ←←

Ρ̓ÜſſĪ Ñ ſſĪ Ň ſſ ſſ ÑĮſſiĪ

ſſ← , ſſ← : Stress in the ← - and ← -directions at the centre of thickness of the plate, respectively (NÕmm Ī).

ſſi←← : Shearing stress in the ← Ň ← plane(NÕmm Ī).

2. The equivalent stress specified in Par 1 is not to exceed 0.7 and 0.9 times the yield strength of the material, for the static loading and combined loading condition specified in 301., respectively.

211. Corrosion allowance

1. In case where the unit is fitted with a corrosion protection system deemed appropriate by the Society, with regard to the corrosion allowance specified in Par 2, reduction may be made as deemed adequate by the Society.

2. Where the unit is not fitted with a corrosion protection system deemed appropriate by the Society, the scantlings determined by the analysing method specified in Sec 2 in conjunction with the al- lowable stresses specified in Sec 3 are to be added by a proper corrosion allowance. In this case, the corrosion allowance is, as a rule, not to be less than 2.5 mm and is to be determined consider- ing the environmental condition, the means and degree of corrosion protection specified in Ch 3, Sec 5 and the process of its maintenance. And further, where the requirements in Rules for the Classification of Steel Ships or Rules for the Classification of Steel Barges are applied, the scantlings are not to be less than those specified in the relevant requirements.