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24. Tripping of framing

24.1 Framing members consisting of tee sections must satisfy one of the following criteria;

or

24.2 Framing members consisting of angle sections must satisfy the criterion;

24.3 Framing members consisting of flat bar sections must satisfy one of the following criteria;

or

24.4 Framing members consisting of offset bulb bars must comply with the following flange slenderness ratio :-

24.5 where:

"ZpAF" is the as fitted modulus calculated in accordance with section 22;

"B" is the width of the bulb outstand in centimetres ( see figure);

"Zp" is the required frame section modulus in accordance with sections 18 or 19 and 20;

"fy" is the nominal yield stress;

"Hw" is the height of the web in centimetres;

"LU" is the unbraced span in centimetres between tripping brackets or runners, or the span (LB) where no tripping brackets or runners are fitted;

"N" equals one when is equal to or greater than 85 degrees and (1-cos ) when is less than 85 degrees; where

"" is the acute angle at the mid point of the span between:

• a line joining the mid point of the root of the web and the centroid of the section, and
• the tangent plane at the root of the web.

"V" equals

"WF" is the width of the flange in centimetres;

"Tw" is the thickness of the web in centimetres;

24.6 The tangent plane is the plane at the mid point of the moulded span at the shell that is parallel to the plane defined by the chord of the span and the chord of the two frame spaces in way.

24.7 The thickness of the web of any section must not exceed the thickness of the contiguous shell plating, and not be less than

where
"t" is the thickness of the shell plate; and

"fy" is the nominal yield stress of the contiguous shell plating.

25. Hull girder strength ^

25.1 The design bending moment Bmice for the hull girder of a ship is obtained from the formula:-

where
"Bmram" is the ice ramming bending moment obtained from section 25.2; and

"Bmasw" is the maximum Arctic operating still water bending moment in meganewton metres for the sagging condition;

"CF" is the factor for the Category from Table 5.

25.2 The ice ramming bending moment (BMram) is obtained from the formula :

meganewton metres

where
"L" is the distance, in metres, on the summer load water-line from the forward side of the stem to the after side of the rudder post, or to the centre of the rudder stock if there is no rudder post. L is to be not less than 96 per cent, and need not be greater than 97 per cent, of the extreme length on the summer load water-line. In ships with unusual stem of stern arrangements the length L will be specially considered.

"" is the displacement in thousands of tonnes, and

"P" is the shaft power in megawatts.

25.3 The section modulus of the hull girder of a CAC1 ship must not be less than that obtained

• for the deck, from the formula:-

• for the bottom, from the formula:-

where
"fy" is the nominal yield strength in megapascals of the steel which is distributed in the upper part of the hull girder in accordance with Recognized Standards; and

"f" is the material factor, where the material factor is obtained from the relationship:

25.4 The section modulus obtained according to paragraph 25.3 must extend from at least 0.1´L aft of amidships to at least 0.3´L forward of amidships.

25.5 The structure above the neutral axis must be designed against buckling according to Recognized Standards.

26. Ice-skegs ^

26.1 Ice-skegs must be fitted on

• all CAC1 ships,
• CAC2 ships of 50,000 tonnes displacement or less,
• CAC3 ships of 20,000 tonnes displacement or less, and
• CAC4 ships of 2,000 tonnes displacement or less.

26.2 The horizontal load to be used in the delineation of an iceskeg is obtained from the formula:-

where
"Fmax" is the maximum force developed when ramming, and is obtained from the following relationship:-

"HL" is the horizontal load in megapascals;

"" is the displacement in thousands of tonnes;

"P" is the shaft power in megawatts; and

"CF" is the Factor from Table 5.

26.3 The stopping force (SF) at any section of an iceskeg is obtained from the formula

meganewtons

where
"AF" is the area factor from Table 2;

"PAV" is derived from Table 5 using the maximum width of the iceskeg at the section in question;

"AS" is the sectional area of the iceskeg at the section in question but the depth down from the top need not exceed 2 x VP; and

"VP" is determined according to section 14.2.

26.4 The design parameter (DPH) for determining the pressure from Table 8 for the stopping force (SF) at any section of an iceskeg is obtained from the formula:-

where
"WS" is the width at the top of the iceskeg at the section in question; and

"LDL" is the applicable horizontal length of the design ice load from section 14.1.

26.5 Ice-skegs must be designed so that the maximum stopping force (SF) is not less than however it need not be greater than HL.

26.6 The critical section (CS) of an iceskeg is the section of maximum stopping force.

26.7 The critical distance (CL) is the fore and aft distance from the skeg forward point SP to the critical section CS.

26.8 The fore end structure must be designed to transmit the iceskeg loads into the main structure of the ship.

27. Design loads for rudders ^

27.1 The design pressure PAV for a rudder is to be determined from Table 6 [Page 31] using the value of parameter (DPT) obtained from the formula :-

where
"C" is the length of the chord of the rudder at the level in question, and

"LDL" is the horizontal length of the design load determined according to section 14.

27.2 The design pressure PAV for the lower two-thirds of a rudder may be reduced linearly from 1 ´ PAV at the upper one-third point to 0.3 ´ PAV at the bottom.

27.3 The design load (RDL) for a rudder is determined from the formula:

meganewtons

where:
"CF" is a factor for the Arctic Class from Table 5,

"AF" is a factor equal to 0.5 ,

"VP" is the vertical height of the design ice load from section 14.2,

"C" is the chord of the rudder at the level in question, and

"PAV" is the design pressure obtained according to section 10.

28. Design loads for propeller nozzles ^

A propeller nozzle is a device shrouding a propeller designed to improve propulsion efficiency and may also protect the propeller from damage by large ice pieces.

Figure 8 ^
Nozzle definitions

The requirements for nozzle strength are developed in the same manner as other structure. The design loads are obtained from the following, a direct lateral or transverse load, a symmetrical longitudinal load on the lower part of the nozzle, and an asymmetric longitudinal load at the shaft height. These loads are applied independently to determine the worst case to obtain the shell and framing scantlings in a similar manner to the main hull.

28.1 The lateral design load (NTDL) for a propeller nozzle is determined from the formula:

meganewtons

where:
"CF" is a factor for the Arctic Class from Table 5 [Page31],

"AF" is a factor equal to 0.5 ,

"VP" is the vertical height of the design ice load from section 14.2,

"NL" is the length of the nozzle at the axis of the propeller shaft, and

"PAV" is the design pressure obtained from Table 6 [Page 31 ], with the value of the design parameter (DPT) for determining the design pressure obtained from the formula:-

where
"NL" is as defined above, and

"LDL" is the horizontal length of the design load determined according to section 14.1.

28.2 The longitudinal asymmetric design load (NLASDL) for a nozzle at the level of the propeller shaft is determined from the formula:

meganewtons

where:
"CF" is a factor for the class from Table 5 [Page29],

"AF" is a factor equal to 1.0 ,

"VP" is the vertical height of the design ice load from section 14.2,

"NT" is the thickness of the nozzle, and

"PAV" is the design pressure obtained from Table 6, with the design parameter (DPH) obtained from the formula:-

where
"NT" is as defined above, and

"LDL" is the horizontal length of the design load determined according to section 14.1.

28.3 The longitudinal symmetric design loads (NLSDL) for a height VP above the bottom of a nozzle is determined from the formula:

meganewtons

where:
"CF" is a factor for the Arctic Class from Table 5,

"AF" is a factor equal to 0.5 ,

"NPA" is the longitudinal projected area of the nozzle for a height VP up from the bottom of the nozzle, and

"PAV" is the design pressure obtained from Table 6, with the value of the design parameter DPH for on the bottom of nozzle obtained from the formula:-

where
"NPA" is as defined above,

"LDL" is the horizontal length of the design load determined according to section 14.1, and

"VP" is the vertical height of the design ice load as defined in section 14.2,

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