Concrete dam lecture 2
- 1. FORCES ACTING ON GRAVITY DAM 30/1/2014 1 PREPARED BY V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET.
- 2. 30/1/2014 2 PREPARED BY V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET.
- 3. 30/1/2014 3 PREPARED BY V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET.
- 4. Forces Acting on a Gravity Dam : (May 2012, October 2012 forces acting on a gravity dam are as follows : Water Pressure Weight of the Dam Uplift Pressure Ice Pressure Wave Pressure Silt Pressure Wind Pressure Earth quake pressure 30/1/2014 4 PREPARED BY V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET.
- 5. The forces to the dam include AS per IS 6512: 30/1/2014 5 PREPARED BY V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET. Stabilize forces 1. Weight of the dam 2. Thrust of the tail water Destabilizing force 1. water pressure 2. Uplift 3. Forces due to waves 4. Ice pressure 5. Temperature stresses 6. Silt pressure 7. Seismic forces 8. Wind pressure
- 6. 30/1/2014 PREPARED BY V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET. 6
- 7. The Bureau of Indian Standards code IS 6512-1984 “Criteria for design of solid gravity dams” recommends that a gravity dam should be designed for the most adverse load condition of the seven given type using the safety factors prescribed. 30/1/2014 PREPARED BY V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET. 7
- 8. 1. Load combination A (construction condition): Dam completed but no water in reservoir or tail water 2. Load combination B (normal operating conditions): Full reservoir elevation, normal dry weather tail water, normal uplift, ice and silt (if applicable) 3. Load combination C: (Flood discharge condition) - Reservoir at maximum flood pool elevation ,all gates open, tail water at flood elevation, normal uplift, and silt (if applicable)30/1/2014 PREPARED BY V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET. 8
- 9. 4. Load combination D: Combination of A and earthquake 5. Load combination E: Combination B, with earthquake but no ice 6. Load combination F: Combination C, but with extreme uplift, assuming the drainage holes to be Inoperative 7. Load combination G: Combination E but with extreme uplift (drains inoperative)30/1/2014 PREPARED BY V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET. 9
- 10. Water Pressure : Water Pressure (P) is the major external force exerted by the water stored in the Reservoir on the upstream face of the dam. It can be calculated by the law of hydrostatic pressure distribution; which is triangular in shape as shown in Fig. 3.3. (a) When u/s face is vertical : When the upstream face is vertical, the intensity of pressure is zero at the water surface and equal to γw • H at the base. 30/1/2014 PREPARED BY V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET. 10
- 11. 30/1/2014 PREPARED BY V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET. 11 γw • H2 /2 H H/3 γw • H
- 12. When the u/s face is partly vertical and partly inclined : Forces can be resolved into two components. 30/1/2014 PREPARED BY V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET. 12 (i) Horizontal Component(P H) = γw • H2 /2 (ii) Vertical Component(PV) = Weight of water in ABCD portion
- 13. 30/1/2014 PREPARED BY V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET. 13 A B C D
- 14. Tail Water Pressure : If there is tail water of height H' it will have 30/1/2014 PREPARED BY V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET. 14 (i) Horizontal Component(P H) = γw • H’2 /2 (ii) Vertical Component(PV) = Weight of water in EFG portion
- 15. 30/1/2014 PREPARED BY V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET. 15 PV PH H’ E F G
- 16. 2. Weight of the Dam : The weight of the dam per unit length is given by the product of the area of crosssection of the dam and the specific weight of the Construction material, i.e. concrete, and masonary it acts vertically downwards at the centre of gravity of the section. 30/1/2014 PREPARED BY V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET. 16
- 17. dam may be divided into smaller sections of simple geometrical shapes such as triangles,rectangles, etc. weight of each of these acting at its centre of gravity may be considered. Weight of any part of dam = cross- sectional area of that part x specific weight of material 30/1/2014 PREPARED BY V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET. 17
- 18. 30/1/2014 PREPARED BY V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET. 18 W2 W3 W1
- 19. 3. Uplift Pressure : Uplift pressure is defined as the force exerted by water penetrating through the pores, cracks, fissures within the body of the dam, at the contact between the dam and its foundation, and within the foundation. 30/1/2014 PREPARED BY V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET. 19
- 20. acts vertically upwards it causes a reduction in the effective weight 30/1/2014 PREPARED BY V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET. 20
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- 23. 30/1/2014 PREPARED BY V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET. 23
- 24. Ice Pressure : Ice pressure is exerted on a dam by a sheet of ice formed on the entire water surface of the reservoir, when it is subjected to expansion and contraction with changes in temperature. The coefficient of thermal expansion of ice being five times more than that of concrete, the dam face has to resist the force due to expansion of ice. This force acts linearly along the length of the dam, at the reservoir level. As per IS : 6512 - 1984, ice pressure may be taken equal to 250 kN/m2 applied to the face of the dam over the anticipated area of contact of ice with the face of the dam. 30/1/2014 PREPARED BY V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET. 24
- 25. 30/1/2014 PREPARED BY V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET. 25