1 General

1.1 Limit states

1.1.1 Definition

A limit state is defined as a state beyond which the structure no longer satisfies the requirements. The following categories of limit states are relevant for structures:

Serviceability limit state (SLS), which corresponds to conditions beyond which specified requirements are no longer met.
Ultimate limit state (ULS), which corresponds to the maximum load carrying-capacity or, in some cases, to the maximum applicable strain or deformation, under intact (undamaged) conditions.
Fatigue limit state (FLS), which corresponds to degradation due to effect of time varying (cyclic) loading.
Accidental limit state (ALS), which concerns the ability of the structure to resist accident situations.

1.1.2 Serviceability limit state

Serviceability limit state, which concerns the normal use, includes:

Local damage which may reduce the working life of the structure or affect the efficiency or appearance of structural members or non-structural elements.
Unacceptable deformations which affect the efficient use and appearance of structural or nonstructural elements or the functioning of safety equipment.

In the context of serviceability limit state, the term ‘appearance’ is concerned with such criteria as high deflection and extensive cracking, rather than aesthetics.

1.1.3 Ultimate limit state

Ultimate limit state, which corresponds to the maximum load-carrying capacity, or in some cases, the maximum applicable strain or deformation, includes:

Attainment of the maximum resistance capacity of sections, members or connections by rupture or excessive deformations or instability (buckling).
Excessive yielding, transforming the structure or part of it into a plastic mechanism.

1.1.4 Fatigue limit state

Fatigue limit states assess that the fatigue capacity of structural members due to cyclic loads is greater than the design fatigue life.

1.1.5 Accidental limit state

Accidental limit states are concerned with the ability of the structure to resist accident situations or abnormal events. Flooded conditions of any compartment without progression of the flooding to another compartment are considered. The limit states are concerned with the following in intact (undamaged) conditions with accidental or abnormal loads, or in damaged conditions with environmental loads the ship meets during a limited time frame:

The safety of life.
Environment.
Property (ship and cargo).

Accidental limit state includes:

Loss of structural strength without loss of containment.
Loss of structural strength and loss of containment.

1.2 Failure modes

1.2.1 A number of possible failure modes may be relevant for the various parts of the ship structure. For each failure mode, one or more limit states may be relevant. The failure modes to be considered for the assessment of ship structural safety with relation to the limit states are shown in Table 1.

Table 1 : Failure modes in relation to the limit states to be considered

Possible failure modes to be considered	Limit states ⁽¹⁾
Possible failure modes to be considered	SLS	ULS	FLS	ALS
Yielding	Y	Y	-	Y
Plastic collapse	-	Y	-	Y
Buckling	Y	Y	-	Y
Rupture	-	Y	-	Y
Fatigue cracking	-	-	Y	-
Brittle fracture ⁽²⁾	-	-	-	-
⁽¹⁾ “Y” indicates that the structural assessment is to be carried out. ⁽²⁾ Controlled by the material rule requirement of steel grade.

1.2.2 Yielding

The yielding failure mode is the mode in which plastic strain locally occurs in the structural members to be considered under combined in-plane and normal stresses. Local plastic strain is controlled in SLS, ULS and ALS by checking that the stresses caused in the structural members remains below a permissible value.

1.2.3 Plastic collapse

The plastic collapse failure mode usually appears in the local structural members under large lateral impact pressure. In this failure mode, permanent lateral deflection in the local structural members occurs, but does not influence the global strength. This mode is controlled in ULS and ALS by using conventional plastic design method.

1.2.4 Buckling

The buckling failure mode is the instability phenomena of structural members under compressive loads. When the stress in structural members just attains the elastic buckling stress, elastic (reversible) buckling occurs during the compressive load. This buckling failure mode is controlled in SLS. By further increasing the compressive load, stress redistribution occurs due to buckling of the weakest structural member and the stress in some structural members reaches the yield stress. This buckling failure mode with large elastic deflection is controlled in ULS or ALS. When compression is unloaded, no consequence of failure due to buckling is seen.

On the other hand, plastic (irreversible) buckling occurs when the stress in structural members exceeds the yield stress. As a result, the substantial permanent deflections due to plastic buckling appear. This irreversible buckling failure mode is controlled only in ULS or ALS for global hull girder strength.

1.2.5 Rupture

The rupture failure mode is the mode in which breaking occurs in the structural members to be considered under large tensile stress beyond the yield stress of the material. This failure mode is controlled in ULS or ALS, but the assessment of this failure mode is covered by controlling the yielding failure.

1.2.6 Brittle fracture

Brittle fracture is dependent upon the material, temperature and thickness. Therefore, this mode is controlled by the material rule requirement of steel grade.

1.2.7 Fatigue cracking

This failure mode is different from the failure modes mentioned above and is controlled in FLS.