Section 8 Structural idealisation

8.1 General

8.1.1 General structural idealisation is covered in Pt 4, Ch 3, 3 Structural idealisation. Additional approaches relevant to Pt 10 Ship Units are given in this Section.

8.2 Mixed steel grades

8.2.1 When a stiffener is of a higher strength material than the attached plate, the yield stress used for the calculation of the section modulus requirements in Pt 10, Ch 3 Scantling Requirements is, in general, not to be greater than 1,35 times the minimum specified yield stress of the attached plate. If the yield stress of the stiffener exceeds this limitation, the following criterion is to be satisfied:

where

specified minimum yield stress of the material of the stiffener, in N/mm²

specified minimum yield stress of the material of the attached plate, in N/mm²

maximum hull girder stress of sagging and hogging, in N/mm², as defined in Pt 10, Ch 3, 2.4 Hull envelope framing 2.4.2 and Pt 10, Ch 3, 4.7 Tank bulkheads 4.7.1 in Pt 10, Ch 3 Scantling Requirements, for stiffeners in cargo tank region and machinery spaces respectively and not to be taken as less than

net section modulus, in way of face-plate/free edge of the stiffener, in cm³

net section modulus, in way of the attached plate of stiffener, in cm³

8.3 Effective bending span of local support members

8.3.1 The effective bending span, l _bdg, of a stiffener is defined for typical arrangements in Pt 10, Ch 1, 8.3 Effective bending span of local support members 8.3.3 to Pt 10, Ch 1, 8.3 Effective bending span of local support members 8.3.7. Where arrangements differ from those shown in Figure 1.8.1 Effective Bending Span of Stiffeners Supported by Web Stiffeners (Double Skin Construction) through Figure 1.8.6 Effective Shear Span for Local Support Members with Continuous Face Plate along Bracket Edge, span definition may be specially considered.

8.3.2 The effective bending span may be reduced due to the presence of brackets, provided the brackets are effectively supported by the adjacent structure, otherwise the effective bending span is to be taken as the full length of the stiffener between primary member supports.

8.3.3 If the web stiffener is sniped at the end or not attached to the stiffener under consideration, the effective bending span is to be taken as the full length between primary member supports unless a backing bracket is fitted, see Figure 1.8.2 Effective Bending Span of Stiffeners Supported by Web Stiffeners (Single Skin Construction).

8.3.4 The effective bending span may only be reduced where brackets are fitted to the flange or free edge of the stiffener. Brackets fitted to the attached plating on the side opposite to that of the stiffener are not to be considered as effective in reducing the effective bending span.

8.3.5 The effective bending span, l _bdg, for stiffeners forming part of a double skin arrangement is to be taken as shown in Figure 1.8.1 Effective Bending Span of Stiffeners Supported by Web Stiffeners (Double Skin Construction).

8.3.6 The effective bending span, l _bdg, for stiffeners forming part of a single skin arrangement is to be taken as shown in Figure 1.8.2 Effective Bending Span of Stiffeners Supported by Web Stiffeners (Single Skin Construction).

8.3.7 For stiffeners supported by a bracket on one side of primary support members, the effective bending span is to be taken as the full distance between primary support members as shown in Figure 1.8.2 Effective Bending Span of Stiffeners Supported by Web Stiffeners (Single Skin Construction) (a). If brackets are fitted on both sides of the primary support member, the effective bending span is to be taken as in Figure 1.8.2 Effective Bending Span of Stiffeners Supported by Web Stiffeners (Single Skin Construction) (b), (c) and (d).

8.3.8 Where the face plate of the stiffener is continuous along the edge of the bracket, the effective bending span is to be taken to the position where the depth of the bracket is equal to one quarter of the depth of the stiffener, see Figure 1.8.3 Effective Bending Span for Local Support Members with Continuous Face Plate along Bracket Edge.

8.3.9 For the calculation of the span point, the bracket length is not to be taken greater than 1,5 times the length of the arm on the bulkhead or base.

Figure 1.8.1 Effective Bending Span of Stiffeners Supported by Web Stiffeners (Double Skin Construction)

Figure 1.8.2 Effective Bending Span of Stiffeners Supported by Web Stiffeners (Single Skin Construction)

Figure 1.8.3 Effective Bending Span for Local Support Members with Continuous Face Plate along Bracket Edge

8.4 Effective shear span of local support members

8.4.1 The effective shear span, l _shr, of a stiffener is defined for typical arrangements in Pt 10, Ch 1, 8.4 Effective shear span of local support members 8.4.5 to Pt 10, Ch 1, 8.4 Effective shear span of local support members 8.4.7. Effective shear span for other arrangements will be specially considered.

8.4.2 The effective shear span may be reduced due to the presence of brackets provided the brackets are effectively supported by the adjacent structure, otherwise the effective shear span is to be as the full length as given in Pt 10, Ch 1, 8.4 Effective shear span of local support members 8.4.4.

8.4.3 The effective shear span may be reduced for brackets fitted on either the flange or the free edge of the stiffener, or for brackets fitted to the attached plating on the side opposite to that of the stiffener. If brackets are fitted at both the flange or free edge of the stiffener, and to the attached plating on the side opposite to that of the stiffener the effective shear span may be calculated using the longer effective bracket arm.

8.4.4 The effective shear span may be reduced by a minimum of s/4000 m at each end of the member, regardless of support detail, hence the effective shear span, l _shr, is not to be taken greater than:

Where:

full length of the stiffener between primary support members, in m

stiffener spacing, in mm

8.4.5 The effective shear span, l _shr, for stiffeners forming part of a double skin arrangement is to be taken as shown in Pt 10, Ch 1, 8.4 Effective shear span of local support members 8.4.8.

8.4.6 The effective shear span, l _shr, for stiffeners forming part of a single skin arrangement is to be taken as shown in Figure 1.8.5 Effective Shear Span of Stiffeners Supported by Web Stiffeners (Single Skin Construction).

8.4.7 Where the face plate of the stiffener is continuous along the curved edge of the bracket, the effective shear span is to be taken as shown in Figure 1.8.6 Effective Shear Span for Local Support Members with Continuous Face Plate along Bracket Edge.

8.4.8 For curved and/or long brackets (length/height ratio) the effective bracket length is to be taken as the maximum inscribed 1:1.5 bracket as shown in Figure 1.8.4 Effective Shear Span of Stiffeners Supported by Web Stiffeners (Double Skin Construction) (c) and Figure 1.8.5 Effective Shear Span of Stiffeners Supported by Web Stiffeners (Single Skin Construction) (c).

Figure 1.8.4 Effective Shear Span of Stiffeners Supported by Web Stiffeners (Double Skin Construction)

Figure 1.8.5 Effective Shear Span of Stiffeners Supported by Web Stiffeners (Single Skin Construction)

Figure 1.8.6 Effective Shear Span for Local Support Members with Continuous Face Plate along Bracket Edge

8.5 Effective shear span

8.5.1 The effective shear span of a stiffener may be reduced due to the presence of brackets, provided the brackets are effectively supported by the adjacent structure, otherwise the effective shear span is to be as the full length, as given in Pt 10, Ch 1, 8.5 Effective shear span 8.5.3.

8.5.2 The effective shear span may be reduced for brackets fitted on either the flange or the free edge of the stiffener, or for brackets fitted to the attached plating on the side opposite to that of the stiffener. If brackets are fitted at both the flange or free edge of the stiffener, and to the attached plating on the side opposite to that of the stiffener, the effective shear span may be calculated using the longer effective bracket arm.

8.5.3 The effective shear span may be reduced by a minimum of s/4000 m at each end of the member, regardless of support detail, hence the effective shear span is not to be taken greater than:

where

full length of the stiffener between primary support members, in metres

stiffener spacing, in mm.

8.6 Effective elastic sectional properties of local support members

8.6.1 The net elastic shear area of local support members is to be taken as:

where

stiffener height, including face-plate, in mm

net thickness of attached plate, in mm

net web thickness, in mm

angle between the stiffener web and attached plating, in degrees.

is to be taken as 90° if the angle is greater than or equal to 75°.

8.6.2 effective shear depth of stiffeners is to be taken as:

where

are defined in Pt 10, Ch 1, 8.6 Effective elastic sectional properties of local support members 8.6.1.

8.6.3 The elastic net section modulus of local support members is to be taken as:

cm³

where

Z_{el–ϕ–net}

net section modulus of corresponding upright stiffener, i.e. when ϕ_w is equal to 90°, in cm³

ϕ_w is defined in Pt 10, Ch 1, 8.6 Effective elastic sectional properties of local support members 8.6.1.

8.7 Effective plastic section modulus and shear area of stiffeners

8.7.1 The net plastic shear area of local support members is to be taken as:

where

h_stf , t_p-net , ϕ_w are defined inPt 10, Ch 1, 8.4 Effective shear span of local support members 8.4.1

t_w-net

net web thickness, in mm.

8.7.2 The effective net plastic section modulus of local support members is to be taken as:

where

f_w

web shear stress factor

0,75 for flanged profile cross-sections with n = 1 or 2

1,0 for flanged profile cross-sections with n = 0 and for flat bar stiffeners

number of moment effective end supports of each member

0, 1 or 2

A moment effective end support may be considered where:

the stiffener is continuous at the support;
the stiffener passes through the support plate while it is connected at its termination point by a carling (or equivalent) to adjacent stiffeners;
the stiffener is attached to an abutting stiffener effective in bending (not a buckling stiffener) or bracket. The bracket is assumed to be bending effective when it is attached to another stiffener (not a buckling stiffener).

	=	depth of stiffener web, in mm:
	=	for T, L (rolled and built-up) and L2 profiles
	=	for flat bar and L3 profiles to be taken as given in Table 1.8.1 Characteristic flange data for HP bulb profiles and Table 1.8.2 Characteristic flange data for JIS bulb profiles for bulb profiles
	=	h_stf for flat bar and L3 profiles to be taken as given in Table 1.8.1 Characteristic flange data for HP bulb profiles and Table 1.8.2 Characteristic flange data for JIS bulb profiles for bulb profiles

0,25 (1 +

)

β	=	0,5 for all cases, except L profiles without a mid span tripping bracket
β	=

but not to be taken greater than 0,5 for L (rolled and built-up) profiles without a mid span tripping bracket

	=	net cross-sectional area of flange, in mm²
	=	in general
	=	0 for flat bar stiffeners

breadth of flange, in mm. For bulb profiles, see Table 1.8.1 Characteristic flange data for HP bulb profiles and Table 1.8.2 Characteristic flange data for JIS bulb profiles

	=	distance from mid thickness of stiffener web to the centre of the flange area:
	=	0,5 () for rolled angle profiles
	=	0 for T profiles

as given in Table 1.8.1 Characteristic flange data for HP bulb profiles and Table 1.8.2 Characteristic flange data for JIS bulb profiles for bulb profiles

	=	height of stiffener measured to the mid thickness of the flange:
	=	for profiles with flange of rectangular shape except for L3 profiles
	=	for L3 profiles as given in Table 1.8.1 Characteristic flange data for HP bulb profiles and Table 1.8.2 Characteristic flange data for JIS bulb profiles for bulb profiles

distance from upper edge of web to the top of the flange, in mm

	=	1,0 in general
	=	0,8 for continuous flanges with end bracket(s). A continuous flange is defined as a flange that is not sniped and continuous through the primary support member
	=	0,7 for non-continuous flanges with end bracket(s). A non-continuous flange is defined as a flange that is sniped at the primary support member or terminated at the support without aligned structure on the other side of the support

length of stiffener flange between supporting webs, in metres, but reduced by the arm length of end bracket(s) for stiffeners with end bracket(s) fitted

	=	net flange thickness, in mm
	=	0 for flat bar stiffeners as given in Table 1.8.1 Characteristic flange data for HP bulb profiles and Table 1.8.2 Characteristic flange data for JIS bulb profiles for bulb profiles

are defined in Pt 10, Ch 1, 8.4 Effective shear span of local support members 8.4.1.

Table 1.8.1 Characteristic flange data for HP bulb profiles

(mm)	(mm)	(mm)	(mm)	(mm)	(mm)
200	171	40	14,4	10,9	188
220	188	44	16,2	12,1	206
240	205	49	17,7	13,3	225
260	221	53	19,5	14,5	244
280	238	57	21,3	15,8	263
300	255	62	22,8	16,9	281
320	271	65	25,0	18,1	300
340	288	70	26,4	19,3	318
370	313	77	28,8	21,1	346
400	338	83	31,5	22,0	374
430	363	90	33,9	24,7	402
NOTE
Characteristic flange data converted to net scantlings are given as:



see Fig. 1.8.1

Table 1.8.2 Characteristic flange data for JIS bulb profiles

(mm)	(mm)	(mm)	(mm)	(mm)	(mm)
180	156	34	11,9	9,0	170
200	172	39	13,7	10,4	188
230	198	45	15,2	11,7	217
250	215	49	17,1	12,9	235
NOTE
Characteristic flange data converted to net scantlings are as given in Table 1.8.1 Characteristic flange data for HP bulb profiles
see Fig. 1.8.1

Figure 1.8.7 Characteristic data for bulb profiles

8.8 Load calculation point for the determination of scantlings of plate panels for scantling requirements

8.8.1 Scantlings of plate strakes are to be derived based on the idealisation of the as-built structure as a series of elementary plate panels (EPPs).

8.8.2 An EPP is the unstiffened part of the plating between stiffeners. The plate panel length, l_epp, and breadth, s_epp, of the EPP are defined in relation to the longest and shortest plate edges respectively, as shown in Figure 1.8.8 Elementary plate panel definition

Figure 1.8.8 Elementary plate panel definition

8.8.3 The required scantling of each EPP is to be calculated based on a load calculation point (LCP) defined as:

for longitudinal framing, at the mid-length of the EPP measured along the global x-axis at its lower edge. For horizontal plating the LCP is to be taken at the outboard y-value of the EPP. See Figure 1.8.9 Example of load calculation points for typical structural configurations – longitudinal framing;
for transverse framing, at the mid-length of the EPP measured along the global x-axis at the lower edge of strake. For horizontal plating the LCP is to be taken at the outboard y-value of the EPP. See Figure 1.8.10 Example of load calculation points for typical structural configurations – transverse framing;
for horizontal framing on vertical transverse structure, at the lower edge of the EPP at the point of outboard y-value of the EPP. See Figure 1.8.11 Example of load calculation points for typical structural configurations – horizontal framing on transverse structure;
for vertical framing on vertical transverse structure, at the greatest y-value of the lower edge of the EPP or at the lower edge of strake. See Figure 1.8.12 Example of load calculation points for typical structural configurations – vertical framing on transverse structure.

8.8.4 Both the local pressure and hull girder stress used for the calculation of the local scantling requirements are to be taken at the LCP.

Figure 1.8.9 Example of load calculation points for typical structural configurations – longitudinal framing

Figure 1.8.10 Example of load calculation points for typical structural configurations – transverse framing

Figure 1.8.11 Example of load calculation points for typical structural configurations – horizontal framing on transverse structure

Figure 1.8.12 Example of load calculation points for typical structural configurations – vertical framing on transverse structure

8.9 Load calculation point for the determination of scantlings of plate panels for hull girder strength

8.9.1 The required scantlings of the EPPs are to satisfy the hull girder bending and hull girder shear requirements of Pt 10, Ch 3 Scantling Requirements.

8.9.2 The required thickness of each EPP, with respect to buckling, is to be calculated based on stresses taken at the mid-length of the EPP measured along the global x-axis.

8.9.3 The buckling evaluation is to be calculated using the stress distribution across the width of the panel defined with a reference stress taken at the edge with maximum stress and reduced stress at the other edge given as a fraction, ψ, defined in Table 1.17.1 Buckling factor and reduction factor for plane plate panels, of the reference stress.