Pressure exerted by a liquid

To design any underwater structure or tank bulkheads one need to know the liquid pressure/load exerted on it. This presentation gives all the information one needs to know to enable calculate liquid pressure exerted on a tank structure.

1. Pressure exerted by a liquid

2. Liquid pressure is the load per unit area exerted by the liquid Liquid pressure is the increase in pressure at increasing depths in a liquid . This pressure increases because the liquid at lower depths has to support all of the water above it.

3. Why to know liquid pressure or load for a ship structure To design any underwater structure or tank bulkheads one need to know the liquid pressure/load exerted on it.

4. Total load exerted by a liquid on a horizontal plane = P= p (Pressure)x A(Area)

5. Load on an immersed plane

6. Load on an immersed Plane P= ρ x g x A x H A= Area of the immersed plane H= The distance of centroid of the plane from the liquid surface

7. Example A rectangular bulkhead is 10 m wide and 8 m deep. It is loaded on one side only with oil of relative density of 0.8. Calculate the load on the bulkhead if the oil is: Just at the top of bulkhead 3 m up the sounding pipe

8. Centre of Pressure The centre of pressure on an immersed plane is the point at which the whole liquid load may be regarded as acting. Centre of Pressure from Surface = Second moment of area about surface/ first moment of area about surface

9. Second moment of area Second moment of area about any surface can be calculated by using parallel axis theorem i.e. I oo = I na + AH 2 Where, I na = Second moment of area about neutral axis (axis passing through centroid). H = distance between the surface and neutral axis .

10. Centre of Pressure C.P. = second moment of area about a surface/ first moment of area about a surface = I oo /AH = I na +AH 2 = I na / AH + H

11. Second moment of area of common shapes

12. Centre of pressure From top edge: For rectangle : 2/3D For triangle: D/2 For circular shape: 5/8D

13. Numericals on C.P. A peak bulkhead is in the form of a triangle, apex down, 6 m wide and 9 m deep. The tank is filled with sea water. Calculate the load on the bulkhead and the position of the centre of pressure relative to the top of the bulkhead if the water is At the top of the bulkhead 4 m up the sounding pipe

14. A triangular bulkhead is 7 m wide on top and vertical depth of 8 m. Calculate load on the bulkhead and the position of the centre of pressure if the bulkhead is flooded with sea water on only one side: To the top edge With 4 m head to the top edge

15. Numericals A ballast tank is 15 m long, 12 m wide and 1.4 m deep and is filled with fresh water. Calculate the load on the top and short side, if: The tank is just completely full There is a head of 7 m of water above the tank top.

16. Shearing force on bulkhead stiffeners

17. Formula Load on bulkhead stiffener = P SF at Top = P/3 SF at Bottom= - 2P/3 Point of zero shear = l/√3 I= length of stiffener

18. Numerical on S.F. A bulkhead 9 m deep is supported by vertical stiffeners 750 m apart. The bulkhead is flooded to the top edge with sea water on one side only. Calculate: Shearing force at top Shearing force at bottom Position of zero shear

19. Thank You