2 Hull Structural Steel

2.1 General

2.1.1 Young’s modulus and Poisson’s ratio

The Young’s modulus for Carbon steel materials is equal to 206,000 N/mm² and the Poisson’s ratio equal to 0.3.

2.1.2 Steel material grades and mechanical properties

Steel having a specified minimum yield stress of 235 N/mm² is regarded as normal strength hull structural steel and is denoted by ‘MS’ for mild steel. Steel having a higher specified minimum yield stress is regarded as higher strength hull structural steel and is denoted ‘HT’ for high tensile steel.

Material grades of hull structural steels are referred to as follows:

a) A, B, D and E denote normal strength steel grades.
b) AH, DH and EH denote higher strength steel grades.

Table 1 gives the mechanical characteristics of steels generally used in the construction of ships.

Table 1 : Mechanical properties of hull steels

*Steel grades for plates with t_{as_built}* ≤ 100 mm**	R_eH, specified minimum yield stress, in N/mm²	R_m, specified tensile strength, in N/mm²
A-B-D-E	235	400 – 520
AH32-DH32-EH32-FH32	315	440 – 570
AH36-DH36-EH36-FH36	355	490 – 630
AH40-DH40-EH40-FH40	390	510 – 660

2.1.3 Higher strength steels other than those indicated in Table 1 are considered by the Society on a case-by-case basis.

2.1.4 High tensile steel

When steels with a specified minimum yield stress R_eH other than 235 N/mm² are used, hull girder strength and hull scantlings are to be determined by taking into account the material factor, k defined in [2.2].

2.1.5 Onboard documents

It is required to keep onboard a plan indicating the steel types and grades adopted for the hull structures. Where steels other than those indicated in Table 1 are used, their mechanical and chemical properties, as well as any workmanship requirements or recommendations, are to be available onboard together with the above plan.

2.2 Material factor, k

2.2.1 Unless otherwise specified, the material factor, k of normal and higher strength steel for hull girder strength and scantling purposes is to be taken as defined in Table 2, as a function of the specified minimum yield stress R_eH.

For intermediate values of R_eH, k is obtained by linear interpolation.

Steels with a specified minimum yield stress R_eH, greater than 390 N/mm² are considered by the Society on a case-by-case basis.

Table 2 : Material factor, k

R_eH, specified minimum yield stress, in N/mm²	k
235	1.00
315	0.78
355	0.72
390	0.68

2.3 Steel grades

2.3.1 Materials in the various strength members are not to be of lower grade than those corresponding to the material classes and grades specified in Table 3 to Table 8. General requirements are given in Table 3, while additional minimum requirements for ships with length exceeding 150 m and 250 m, single side bulk carriers with length exceeding 150 m, are given in Table 4 to Table 6. The material grade requirements for hull members of each class depending on the thickness are defined in Table 8.

Table 3 : Material classes and grades

Structural member category		Material class/grade
Secondary	A1. Longitudinal bulkhead strakes, other than those belonging to the Primary category A2. Deck plating exposed to weather, other than that belonging to the Primary or Special category A3. Side plating	- Class I within 0.4 L amidships - Grade A/AH outside 0.4 L amidships
Primary	B1. Bottom plating, including keel plate B2. Strength deck plating, excluding that belonging to the Special category B3. Continuous longitudinal plating of strength members above strength deck, excluding hatch coamings B4. Uppermost strake in longitudinal bulkhead B5. Vertical strake (hatch side girder) and uppermost sloped strake in topside tank	- Class II within 0.4 L amidships - Grade A/AH outside 0.4 L amidships
Special	C1. Sheer strake at strength deck ⁽¹⁾ C2. Stringer plate in strength deck ⁽¹⁾ C3. Deck strake at longitudinal bulkhead, excluding deck plating in way of inner-skin bulkhead of double-hull ships ⁽¹⁾	- Class III within 0.4 L amidships - Class II outside 0.4 L amidships - Class I outside 0.6 L amidships
	C4. Strength deck plating at outboard corners of cargo hatch openings for ships with hatch opening configurations similar to those of container carriers	- Class III within 0.4 L amidships - Class II outside 0.4 L amidships - Class I outside 0.6 L amidships - Min. Class III within cargo hold region
	C5. Strength deck plating at corners of cargo hatch openings	- Class III within 0.6 L amidships - Class II within rest of cargo hold region
	C6. Bilge strake of ships with double bottom over the full breadth and with length less than 150 m	- Class II within 0.6 L amidships - Class I outside 0.6 L amidships
	C7. Bilge strake in other ships ⁽¹⁾	- Class III within 0.4 L amidships - Class II outside 0.4 L amidships - Class I outside 0.6 L amidships
	C8. Longitudinal hatch coamings of length greater than 0.15 L including coaming top plate and flange C9. End brackets and deckhouse transition of longitudinal cargo hatch coamings	- Class III within 0.4 L amidships - Class II outside 0.4 L amidships - Class I outside 0.6 L amidships - Not to be less than Grade D/DH
⁽¹⁾ Single strakes required to be of class III within 0.4L amidships are to have breadths not less than 800+5L, in mm, need not be greater than 1800 mm, unless limited by the geometry of the ship’s design.

Table 4 : Minimum material grades for ships with length exceeding 150 m

Structural member category	Material grade
Longitudinal plating of strength deck where contributing to the longitudinal strength	Grade B/AH within 0.4 L amidships
Continuous longitudinal plating of strength members above strength deck	Grade B/AH within 0.4 L amidships
Single side strakes for ships without inner continuous longitudinal bulkhead(s) between bottom and the strength deck	Grade B/AH within cargo hold region

Table 5 : Minimum material grades for ships with length exceeding 250 m

Structural member category ⁽¹⁾	Material grade
Shear strake at strength deck	Grade E/EH within 0.4 L amidships
Stringer plate in strength deck	Grade E/EH within 0.4 L amidships
Bilge strake	Grade D/DH within 0.4 L amidships
⁽¹⁾ Single strakes required to be of Grade E/EH and within 0.4 L amidships are to have breadths not less than 800+5 L (mm), need not be greater than 1800 (mm), unless limited by the geometry of the ship’s design.

Table 6 : Minimum material grades for single side skin bulk carriers with length exceeding 150 m

Structural member category	Material grade
Lower bracket of ordinary side frame ^{(1), (2)}	Grade D/DH
Side shell strakes included totally or partially between the two points located to 0.125 above and below the intersection of side shell and bilge hopper sloping plate or inner bottom plate ⁽²⁾	Grade D/DH
⁽¹⁾ The term ‘lower bracket’ means webs of lower brackets and webs of the lower part of side frames up to the point of 0.125 above the intersection of side shell and bilge hopper sloping plate or inner bottom plate. ⁽²⁾ The span of the side frame, , is defined as the distance between the supporting structures (see Pt 2, Ch 1, Sec 2, Figure 2).

Table 7 : DELETED

DELETED

Table 8 : Material grade requirements for classes I, II and III

Class	I		II		III
As-built thickness, in mm	MS	HT	MS	HT	MS	HT
t ≤ 15	A	AH	A	AH	A	AH
15 < t ≤ 20	A	AH	A	AH	B	AH
20 < t ≤ 25	A	AH	B	AH	D	DH
25 < t ≤ 30	A	AH	D	DH	D	DH
30 < t ≤ 35	B	AH	D	DH	E	EH
35 < t ≤ 40	B	AH	D	DH	E	EH
40 < t ≤ 50	D	DH	E	EH	E	EH

2.3.2 For strength members not mentioned in Table 3 to Table 7, Grade A/AH may be used upon agreement of the Society.

2.3.3 Plating materials for stern frames and shaft brackets are in general not to be of lower grades than corresponding to Class II.

2.4 Structures exposed to low air temperature

2.4.1 For ships intended to operate in areas with low air temperatures refer to Ch 1, Sec 2, [3.4.4].

2.5 Through thickness property

2.5.1 Where tee or cruciform connections employ partial or full penetration welds, and the plate material is subject to significant tensile strain in a direction perpendicular to the rolled surfaces, consideration is to be given to the use of special material with specified through thickness properties, in accordance with the Rules for Materials of the Society. These steels are to be designated on the approved plan by the required steel strength grade followed by the letter Z (e.g. EH36Z).

2.6 Stainless steel

2.6.1 The reduction of strength of stainless steel with increasing temperature is to be taken into account in the calculation of the material factor, k and in the material Young’s modulus, E.

Stainless steels are considered by the Society on a case-by-case basis.