Merchant Shipping (Safety and Load Line Conventions) Act 1932

RULE XXXVII Strength.

The Assigning Authority is to be satisfied with the structural strength of ships to which freeboards are assigned.

Ships which comply with the highest standard of the rules of a Classification Society recognised by the Administration, shall be regarded as having sufficient strength for the minimum freeboards allowed under the Rules.

Ships which do not comply with the highest standard of the rules of a Classification Society recognised by the Administration, shall be assigned such increased freeboards as shall be determined by the Assigning Authority, and for guidance the following strength moduli are formulated :—

• Material.—The strength moduli are based on the assumption that the structure is built of mild steel, manufactured by the open hearth process (acid or basic), and having a tensile strength of 26 to 32 tons per square inch, and an elongation of at least 16 per cent. on a length of 8 inches.

• Strength Deck.—The strength deck is the uppermost deck which is incorporated into and forms an integral part of the longitudinal girder within the half-length amidships.

• Depth to Strength Deck (Ds).—The depth to strength deck is the vertical distance in feet amidships from the top of the keel to the top of the strength deck beam at side.

• Draught (d).—The draught is the vertical distance in feet amidships from the top of the keel to the centre of the disc.

• Longitudinal Modulus.—The longitudinal modulus is the moment of inertia I of the midship section about the neutral axis divided by the distance y measured from the neutral axis to the top of the strength deck beam at side, calculated in way of openings but without deductions for rivet holes. Areas are measured in square inches and distances in feet.

Below the strength deck, all continuous longitudinal members other than such parts of under deck girders as are required entirely for supporting purposes, are included. Above the strength deck, the gunwale angle bar and the extension of the sheerstrake are the only members included.

The required longitudinal modulus for effective material is expressed by f.d.B., where f is the factor obtained from the following table :—

L.	f.
100	1.80
120	2.00
140	2.35
160	2.70
180	3.15
200	3.60
220	4.20
240	4.80
260	5.45
280	6.20
300	6.95
320	7.70
340	8.55
360	9.40
380	10.30
400	11.20
420	12.15
440	13.10
460	14.15
480	15.15
500	16.25
520	17.35
540	18.45
560	19.60
580	20.80
600	22.00

For intermediate lengths, the value of f is determined by interpolation.

This formula applies where L does not exceed 600 feet; B is between and , both inclusive, and is between 10 and 13.5, both inclusive.

• Frame.—For the purpose of the frame modulus, the frame is regarded as composed of a frame angle and a reverse angle each of the same size and thickness.

• Frame Modulus.— The modulus of the midship frame below the lowest tier of beams is the moment of inertia I of the frame section about the neutral axis divided by the distance y measured from the neutral axis to the extremity of the frame section, calculated without deduction for rivet and bolt holes. The modulus is measured in inch units.

The required frame modulus is expressed by , where—

• s is the frame spacing in inches.

• t is the vertical distance in feet measured at amidships from the top of the keel to a point midway between the top of the inner bottom at side and the top of the heel bracket (see Figure 2); where there is no double bottom, t is measured to a point midway between the top of the floor at centre and the top of the floor at side.

• f₁ is a coefficient depending on H, which, in ships fitted with double bottoms, is the vertical distance in feet from the middle of the beam bracket of the lowest tier of beams at side to a point midway between the top of the inner bottom at side and the top of the heel bracket (see Figure 2). Where there is no double bottom, H is measured to a point midway between the top of the floor at centre and the top of the floor at side. Where the frame obtains additional strength from the form of the ship, due allowance is made in the value of f₁

• f₂ is a coefficient depending on K, which is the vertical distance in feet from the top of the lowest tier of beams at side to a point 7 feet 6 inches above the freeboard deck at side, or, if there is a superstructure, to a point 12 feet 6 inches above the freeboard deck at side (see Figure 2). The values of f₁ and f₂ are obtained from the following tables :—

  H in feet	0	7	9	11	13	15	17	19	21	23	25
  f1	9	11	12.5	15	19	24	29.5	36	43	51	59

  Intermediate values are obtained by interpolation.
  K in feet	0	5	10	15	20	25	30	35	40
  f2	0	0.5	10	2.0	30	4.5	6.5	90	12.0

This formula applies where D is between 15 feet and 60 feet, both inclusive, B is between and both inclusive, is between 10 and 13.5, both inclusive; and the horizontal distance from the outside of the frame to the centre of the first row of pillars does not exceed 20 feet.

In single deck ships of ordinary form, where H does not exceed 18 feet, the frame modulus determined by the preceding method is multiplied by the factor f₃where

f₃ = .50 + .05(H − 8)

.

Where the horizontal distance from the outside of the frame to the centre of the first row of pillars exceeds 20 feet, the Assigning Authority is to be satisfied that sufficient additional strength is provided.