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Bending and shear stresses 369
The beam of the ship is 10 m and the depth is 6 m. All plating is 1.5 cm thick. Find the maximum tensile and compressive stresses when the ship is subjected to a sagging moment of 6000 tonnes metres.
Assume initially that Neutral Axis is at 12 depth, i.e. 3 m above base.
Item
Upper Deck 2nd Deck TankTop BottomShell Sideshell
P&S
Double Bottom
Girders
Item
Upper Deck
2nd Deck
Tank Top
Bottom Shell
Sideshell P&S
Double Bottom Girders
Area (sq m)
10 0:015 0:15
10 0:015 0:15 10 0:015 0:15 2 10 0:015 0:15 3
Lever
Moment Lever
0.45 3
0.15 1 0.30 2 0.45 3
00 0.075 2.5
I
1.35 0.15 0.60 1.35 0
0.1875 3.6375
2 6 0:015 0:18 2 1 0:015 0:03
0 2.5
3 1
0.81
0.225
Own inertia 1 Ah2
1 0:15 0:0152 12
1 0:15 0:0152 12
1 0:15 0:0152 12
1 0:15 0:0152 12
1 0:18 36 12
1 0:03 1 12
12
2:8 10 6
2:8 10 6 g 2:8 10 6 * 2:8 10 6
0:54
2500 10 6 0.5425
* It can be seen from this table that the second moment of area of horizontal members in the structure such as decks, tank tops and outer bottom, about their own neutral axes are small and in practice these values are ignored in the calculation.
Depth of Neutral Axis below half depth 0:225 0:81
0:28 m
Total second moment about half depth 3:6375 0:5425
4:18m4
Total second moment about neutral axis 4:18 0:81 0:282
INA 4:18 0:06 4:12m4