Structural Design Principles
Section 6 – STRUCTURAL ARRANGEMENT PRINCIPLES
4. Ordinary stiffener 1 Profile of stiffeners
4.1.1 Stiffener profile with a bulb section
The properties of bulb profile sections are to be determined by exact calculations. If it is not possible, a bulb section may be taken as equivalent to a built-up section. The dimensions of the equivalent angle section are to be obtained, in mm, from the following formulae.
2 2
Figure 1: Dimensions of stiffeners RCN 1 to July 2010 version (effective from 1July 2012)
4.2 Span of ordinary stiffeners
4.2.1 Ordinary stiffener
The span l of ordinary stiffeners is to be measured as shown in Fig 2. For curved stiffeners, the span is measured along the chord.
Figure 2: Span of ordinary stiffeners
Figure 3: Span of ordinary stiffeners within a double hull
4.2.2 Ordinary stiffener within a double hull
The span l of ordinary stiffeners fitted inside a double hull, i.e. when the web of the primary supporting members is connected with the inner hull and the outer shell acting as its flanges, is to be measured as shown in Fig 3.
4.2.3 Ordinary stiffeners supported by struts
The arrangement of ordinary stiffeners supported by struts is not allowed for ships over 120 m in length.
The span l of ordinary stiffeners supported by one strut fitted at mid distance of the primary supporting members is to be taken as 0.7l2.
In case where two struts are fitted between primary supporting members, the span l of ordinary stiffeners is to be taken as the greater of 1.4l1 and 0.7l2.
l1 and l2 are the spans defined in Figs 4 and 5.
l2 l1
Figure 4: Span of ordinary stiffeners with one strut
l2 l1
Figure 5: Span of ordinary stiffeners with two struts
4.3 Attached plating
4.3.1 Effective breadth for yielding check
The effective width bp of the attached plating to be considered in the actual net section modulus for the yielding check of ordinary stiffeners is to be obtained, in m, from the following formulae:
• where the plating extends on both sides of the ordinary stiffener:
l 2 .
=0
bp , or s bp=
whichever is lesser.
• where the plating extends on one side of the ordinary stiffener (i.e. ordinary stiffeners bounding openings):
s bp =0.5
l 1 .
=0 bp
whichever is lesser.
4.3.2 Effective width for buckling check
The effective width of the attached plating of ordinary stiffeners for checking the buckling of ordinary stiffeners is defined in Ch 6, Sec 3, [5].
4.4 Geometric property of ordinary stiffeners
4.4.1 General
Geometric properties of stiffeners such as moment of inertia, section modulus, shear sectional area, slenderness ratio of web plating, etc., are to be calculated based on the net thickness as defined in Ch 3, Sec 2.
4.4.2 Stiffener not perpendicular to the attached plating
The actual stiffener’s net section modulus is to be calculated about an axis parallel to the attached plating.
Where the stiffener is not perpendicular to the attached plating, the actual net section modulus can be obtained, in cm3, from the following formula:
α
0sin w w= where:
w0 : Actual net section modulus, in cm3, of the stiffener assumed to be perpendicular to the attached plating
α : Angle, in degrees, between the stiffener web and the attached plating, as shown in Fig 6, but not to be taken less than 50.
The correction is to be applied when α is between 50 and 75 degrees.
Figure 6: Angle between stiffener web and attached plating
Where the angle between the web plate of stiffener and the attached plating is less than 50 degrees, tripping bracket is to be fitted at suitable spacing. If the angle between the web plate of an unsymmetrical stiffener and the attached plating is less than 50 degrees, the face plate of the stiffener is to be fitted on the side of open bevel, as shown in Fig 7.
Figure 7: Orientation of stiffener when the angle is less than 50 degrees
4.5 End connections of ordinary stiffeners
4.5.1 General
Where ordinary stiffeners are to be continuous through primary supporting members, they are to be properly connected to the web plating so as to ensure proper transmission of loads. Some sample connections are shown in Fig 8 to Fig 11.
(a) (b)
Figure 8: (a) Connection without collar plate and (b) Connection with stiffener at side of longitudinal
Figure 9: Connection with collar plate
Figure 10: Connection with one large collar plate
Figure 11: Connection with two large collar plates
4.5.2 Structural continuity of stiffeners
Where ordinary stiffeners are cut at primary supporting members, brackets are to be fitted to ensure structural continuity. In this case, the net section modulus and net sectional area of the brackets are to be not less than those of the ordinary stiffener.
The minimum net thickness of brackets is to be not less than that required for web plate of ordinary stiffeners.
The brackets are to be flanged or stiffened by a welded face plate where:
• the net thickness of the bracket, in mm, is less than 15lb, where lb is the length, in m, of the free edge of the end bracket or brackets; or
• the longer arm of the bracket is greater than 800 mm.
The net sectional area, in cm2, of the flanged edge or faceplate is to be at least equal to 10lb. 4.5.3 End connections
End connection of stiffeners is to be sufficiently supported by the primary supporting members. Generally, a stiffener or a bracket to support the ordinary stiffener is to be provided.
Where slots for penetration of stiffeners are reinforced with collars, they are to be of the same materials as the primary supporting members.
Brackets or stiffeners to support the ordinary stiffeners are to be of sufficient sectional area and moment of inertia with respect to structural continuity, and are to have appropriate shape with respect to fatigue strength. If brackets or stiffeners to support the ordinary stiffeners are not fitted, or special slot configurations considering the fatigue strength are provided, fatigue strength assessment for slots are required by the Society.