LONGITUDINAL STRENGTH/STRESS By Capt. J.Isbester, Bulk Carrier Practice, Ch.10 Loading Calculations. Nautical Institute, 1993 Longitudinal Shear Force tends to break or shear the ship across.

LONGITUDINAL STRENGTH/STRESS

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LONGITUDINAL STRENGTH/STRESS

LONGITUDINAL STRENGTH/STRESS

By Capt. J.Isbester, Bulk Carrier Practice, Ch.10 Loading Calculations.

Nautical Institute, 1993

Longitudinal Shear Force tends to break or shear the ship across.

The vessel is exposed to forces due to:

  • the weight of the structure,
  • the weight of the cargo,
  • the forces of buoyancy,
  • direct hydrostatic pressure.

Then these forces are not in balance at every point along the length of the vessel a Shear Force will exist.

Longitudinal Bending Moment tends to bend ship along her length, causing her to hog or sag.

The Bending Moment at any point along the length of the vessel is equal to the algebraic sum of the moments of all loads acting between that point and one end of the vessel (normally this is forward end).

Both are consequence of the irregular way that the hull weight, cargo and buoyancy are distributed along the length of the ship.

The Shear Force acting at a bulkhead (or other station) is the algebraic sum of loads acting on either side of the bulkhead, whilst the Bending Moment acting at a bulkhead is the algebraic sum of the moments acting on either side of the bulkhead.

Then the ship is in equilibrium in still water, values of Shear Force (SF) and Bending Moment (BM) at a specified bulkhead can be expressed as follows:

SF = W1 + W2 – W3

BM = M1 + M2 – M3

Where

W1 = the constant lightship weight abaft (i.e., aft of) the specified bulkhead, in tones

W2 = the deadweight carried abaft the specified bulkhead, in tones

W3 = the buoyancy acting abaft the specified bulkhead, in tones

M1 = the moment of W1 about the specified bulkhead, in tones.metres

M2 = the moment of W2 about the specified bulkhead, in tones.metres

M3 = the moment of W3 about the specified bulkhead, in tones.metres

W1 and M1 are constant values reflecting the ship’s lightweight and are contained in tables provided in the Loading Manual.

W3 and M3 the values of buoyancy, depend upon draft and trim and must be extracted from tables provided in the Loading Manual.

W2 and M2 depend upon the weights loaded and their positions. These values must be compiled and entered by the person making the calculation.

All the weights abaft each bulkhead must be listed and totaled.

The moments of each of these weight must also be listed and totaled and for this the distance of the center of gravity (CG) of each weight from the relevant bulkhead is required. This distance is obtained by comparing the distance of the CG and of the bulkhead from midships.

Bulkhead correction.

When the values of the shear forces have been obtained from the above calculations they can be reduced by a correction known as the bulkhead correction. The correction allows for the fact that some of the loaded caused by cargo in adjacent holds is transferred to the transverse bulkhead through the double bottom structure instead of through the side shell plating. Because this is so stress on the ship is reduced: the bulkhead correction is recognition of this fact.

The correction is only significant when the ship is loaded in alternate holds. When she is loaded in all holds the correction will be small and will not be needed.

For example: all the holds and almost all the tanks are abaft the collision (forepeak) bulkhead;

All loaded compartments except those forward of the collision bulkhead are listed in the column for the bulkhead. Each of the weights is then also entered in each of the remaining columns if it is abaft the bulkhead under consideration.

Frame 175 Distance Moment Frame 219 Distance Moment

Weight, W LCG – P W * O Weight, W LCG – P W * O

Cargo Hold No.1 8450.0 17.17 145086.5

No.3 9400.0 41.09 386246.0 9400.0 72.02 679988.0

No.5 127.95

_____________________________________________________________________________

38 Deadweight Weight XXXX XXXX

39 Ship Weight XXXX

40 Buoyancy Even Keel XXXX

41 Trim Factor XXXX

42 Trim Correction 4 * 41 XXXX

43 Buoyancy 40+42 XXXX

44 Shear Force 38 + 39 – 43

45 Difference SF – SF

46 Bulkhead Factor

47 Correction 45 + 46

48 Lesser Correction

49 Corrected Shear Force 44 +/- 48

50 Maximum permitted Shear Force

51 Shear Force as Percentage of Maximum Permitted 49/50*100

_____________________________________________________________________________

53 Deadweight Moment

54 Ship Moment

55 Buoyancy Moment

56 Trim Factor

57 Trim Correction 4 * 56

58 Buoyancy Moment 55 + 57

59 Bending Moment 53 + 54 – 58

60 Maximum Permitted Bending Moment

61 Bending Moment as Percentage of Maximum Permitted 59/60*100

LOADING GUIDANCE AND STABILITY INFORMATION BOOKLET

FINISHED PLAN

NKK CORPORATION TSU WORKS

m/v CHANNEL POTERNE , SNO.180, Feb.-7. 1997

CORRECTED SHEARING FORCE BY THE TRANSVERSE BULKHEAD

The actual Shearing Force obtained from the Longitudinal strength calculations may be corrected for the Effect of local force at the transverse bulkhead.

If the Searing Force at a bulkhead, calculated by the “Longitudinal Strength Calculation” exceeds the maximum permissible value shown in Page 1-13 then the Shearing Force at the bulkhead correction can be reduced by the following formula :

FS : actual Shearing Force calculated by the Longitudinal strength calculation at fore and aft bulkhead respectively.

C : Bulkhead factor

No.1 Cargo Hold ………………….0.404

No.2 Cargo Hold ………………….0.546

No.3 Cargo Hold ………………….0.557

No.4 Cargo Hold ………………….0.557

No.5 Cargo Hold ………………….0.557

No.6 Cargo Hold ………………….0.610

No.7 Cargo Hold ………………….0.610

No.8 Cargo Hold ………………….0.541

No.9 Cargo Hold ………………….0.346

dF : Shearing Force correction in mt

dF = ( C * ( FS(fore) – FS(aft) ) ) / 2

FC : corrected Shearing Force at fore and aft bulkhead in mt

FC(fore) = FC(fore) - dF

FC(aft) = FC(aft) - dF


Bulkhead correction.

When the values of the shear forces have been obtained from the above calculations they can be reduced by a correction known as the bulkhead correction.

The correction allows for the fact that some of the loaded caused by cargo in adjacent holds is transferred to the transverse bulkhead through the double bottom structure instead of through the side shell plating. Because this is so stress on the ship is reduced: the bulkhead correction is recognition of this fact.

The correction is only significant when the ship is loaded in alternate holds. When she is loaded in all holds the correction will be small and will not be needed.

(By Capt. J.Isbester, Bulk Carrier Practice, Ch.10 Loading Calculations.

Nautical Institute, 1993)

Shear Force

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