LLOYD’SREGISTER OFSHIPPING
4
Equivalent mild steel H.T. steel line Extent of H.T. steel A A F.P. Plate butt 0,2L 0,075L
A – A centre of aftermost mild steel plate
B
S S
B = length of aftermost mild steel plate
Fig. 3.2.2 l b 0,5 1,0 1,5 2,0 2,5 3,0 f 0,19 0,30 0,39 0,48 0,55 0,62 l b 3,5 4,0 4,5 5,0 5,5 6 and above f 0,69 0,76 0,82 0,88 0,94 1,00 NOTE
Intermediate values to be obtained by linear interpolation.
3.2.5 The section modulus of a double plate bulkhead over a spacing b may be calculated as:
Z = (6fbtp+ dwtw) cm3
where dw, b, tpand tware measured, in mm, and are as shown in Fig. 3.3.2.
3.2.6 The effective section modulus of a built section may be taken as:
Z = +
(
1 +)
cm3where
a = the area of the face plate of the member, in cm2 dw = the depth, in mm, of the web between the inside of
the face plate and the attached plating. Where the member is at right angles to a line of corrugations, the minimum depth is to be taken
tw = the thickness of the web of the section, in mm
A = the area, in cm2, of the attached plating, see 3.2.7.
If the calculated value of A is less than the face area
a, then A is to be taken as equal to a.
3.2.7 The geometric properties of primary support members (i.e. girders, transverses, webs, stringers, etc.) are to be calculated in association with an effective area of attached load bearing plating, A, determined as follows: (a) For a member attached to plane plating:
A = 10fbtp cm2 200 (A – a) 200A + twdw twdw2 6000 adw 10 dw 6000 tp tw tp b dw
Structural Design
(b) For a member attached to corrugated plating and parallel to the corrugations:
A = 10btp cm2 See Fig. 3.3.1
(c) For a member attached to corrugated plating and at right angles to the corrugations:
A is to be taken as equivalent to the area of the face
plate of the member.
3.3 Determination of span point
3.3.1 The effective length, le, of a stiffening member is generally less than the overall length, l, by an amount which
depends on the design of the end connections. The span points, between which the value of leis measured, are to be
determined as follows:
(a) For rolled or built secondary stiffening members: The span point is to be taken at the point where the depth of the end bracket, measured from the face of the secondary stiffening member is equal to the depth of the member. Where there is no end bracket, the span point is to be measured between primary member webs. For double skin construction the span may be reduced by the depth of primary member web stiffener, see Fig. 3.3.3.
(b) For primary support members:
The span point is to be taken at a point distant from the end of the member,
where be = bb
(
1 –)
See also Fig. 3.3.3.
3.3.2 Where the end connections of longitudinals are designed with brackets to achieve compliance with the ShipRight FDA Procedure, no reduction in span is permitted for such brackets unless the fatigue life is subsequently reassessed and shown to be adequate for the resulting reduced scantlings.
3.3.3 Where the stiffener member is inclined to a vertical or horizontal axis and the inclination exceeds 10°, the span is to be measured along the member.
3.3.4 It is assumed that the ends of stiffening members are substantially fixed against rotation and displacement. If the arrangement of supporting structure is such that this condition is not achieved, consideration will be given to the effective span to be used for the stiffener.
3.4 Calculation of hull section modulus
3.4.1 All continuous longitudinal structural material is to be included in the calculation of the inertia of the hull midship section, and the lever z is, except where otherwise specified for particular ship types, to be measured vertically from the neutral axis to the top of keel and to the moulded strength deck line at the side. The strength deck is to be taken as follows:
(a) Where there is a complete upper deck and no effective superstructure, the strength deck is the upper deck.
dw db
Part 3, Chapter 3
Section 3 b tp dw p tw 0,5b c θ Fig. 3.3.1 Fig. 3.3.2Structural Design
(b) Where the upper deck is stepped, as in the case of raised quarter deck ships, or there is an effective super- structure on the upper deck, the strength deck is stepped as shown in Fig. 3.3.4.
3.4.2 An effective superstructure is one which exceeds 0,15L in length and extends inside the midship 0,5L region. Superstructure decks less than 12 m in length are not to be considered as strength deck.
3.4.3 Deck openings having a length in the fore and aft directions exceeding 2,5 m or a breadth exceeding 1,2 m or 0,04B m, whichever is the lesser, are always to be deducted from the sectional areas used in the section modulus calculation.
3.4.4 Deck openings smaller than stated in 3.4.3 includ- ing manholes, need not be deducted provided they are isolated and the sum of their breadths or shadow area breadths (see 3.4.5), in one transverse section does not exceed 0,06 (B1– å b1)
where
B1 = breadth of ship at section considered åb1 = sum of breadths of deductible openings
Where a large number of deck openings are proposed in any transverse space, special consideration will be required. RULES ANDREGULATIONS FOR THECLASSIFICATION OFSHIPS, July 2000
Part 3, Chapter 3
Section 3LLOYD’SREGISTER OFSHIPPING
6 Span points ds ds Span point Span point ds ds Span point db be bb dw l Span point Span point db be bb dw l
Structural Design
3.4.5 Where calculating deduction-free openings, the openings are assumed to have longitudinal extensions as shown by the shaded areas in Fig. 3.3.5. The shadow area is obtained by drawing two tangent lines to an opening angle of 30°. The section to be considered should be perpendicular to the centreline of the ship and should result in the maximum deduction in each transverse space.
3.4.6 Isolated openings in longitudinals or longitudinal girders need not be deducted if their depth does not exceed 25 per cent of the web depth with a maximum depth for scallops of 75 mm.
3.4.7 Openings are considered isolated if they are spaced not less than 1 m apart.
3.4.8 For compensation that may be required for
openings, see individual ship Chapters.
3.4.9 Where trunk decks or continuous hatch coamings are effectively supported by longitudinal bulkheads or deep girders, they are to be included in the longitudinal sectional area when calculating the hull section modulus. The lever zt is to be taken as:
zt = zc
(
0,9 + 0,2)
m but not less than zy = horizontal distance from top of continuous strength member to the centreline of the ship, in metres
z = vertical distance from the neutral axis to the moulded deck line at side, in metres
zc = vertical distance from the neutral axis to the top of the continuous strength member, in metres
zcand y are to be measured to the point giving the largest value of zt.
3.4.10 Where continuous hatch coamings are effectively supported (except inboard coamings of multi-hatch arrange- ments, see 3.4.12), 100 per cent of their sectional area may be included in the calculation of the hull section modulus.
y B
3.4.11 Where a continuous longitudinal underdeck girder, or girders, are arranged to support the inboard hatch coamings, 50 per cent of their sectional area may be included. If the girder is fitted in conjunction with a longitudi- nal centreline bulkhead, 100 per cent of the sectional area may be included. In cases where the girders are enclosed box sections, or where the girders are effectively tied to the bottom structure, the area to be included will be specially considered.
3.4.12 The percentage of the sectional area to be included for inboard continuous hatch side coamings should be the same percentage as that of the longitudinal girder under.
Part 3, Chapter 3
Section 3 Fig. 3.3.4 b3 b2 b1 X X Transverse TransverseTotal equivalent breadth of small openings at xx = b1 + b2 + b3
30
Structural Design
3.4.13 Where continuous deck longitudinals or deck girders are arranged above the strength deck, the sectional area may be included in the calculation of the hull section modulus. The lever is to be taken to a position corresponding to the depth of the longitudinal member above the moulded deckline at side amidships.