Concrete dam lecture 4
- 1. FORCES ACTING ON GRAVITY DAM 2/3/2014 1 PREPARED BY V.H.KHOKHANI,ASSISTANT PROFESSOR, DIET,
- 2. 5. Wave Pressure : Wind blowing over the water surface in the reservoir exerts a drag on the surface due to which ripples and waves are formed. The impact of these waves Produces a pressure on the upper portion of the dam. The magnitude of the wave pressure mainly depends on the dimensions of the waves which in turn depend on the extent of water surface and the wind velocity. 2/3/2014 2 PREPARED BY V.H.KHOKHANI,ASSISTANT PROFESSOR, DIET,
- 5. 2/3/2014 5/3 Hw 1/8 Hw Pw = 2.4 YwHw Still water surface 5 PREPARED BY V.H.KHOKHANI,ASSISTANT PROFESSOR, DIET,
- 8. Silt pressure The weight and the pressure of the submerged silt are to be considered in addition to weight and pressure of water. The weight of the silt acts vertically on the slope and pressure horizontally, in a similar fashion to the corresponding forces due to water. It is recommended that the submerged density of silt for calculating horizontal pressure may be taken as 1360 kg/m³. Equivalently, for calculating vertical force, the same may be taken as 1925 kg/m³. 2/3/2014 8 PREPARED BY V.H.KHOKHANI,ASSISTANT PROFESSOR, DIET,
- 10. Wind Pressure : Wind pressure is required to be consider only on that portion of the dam structure which is exposed to the action of wind. Normally wind pressure is taken as 1 to 1.5 kN/m2 for the area exposed to the wind pressure. Wind pressure is not a significant force and therefore, sometimes, not considered in design of a dam. 2/3/2014 10 PREPARED BY V.H.KHOKHANI,ASSISTANT PROFESSOR, DIET,
- 11. Earthquake Forces (Seismic Forces) : Earthquake or seismic activity is associated with complex oscillating patterns of acceleration and ground motions, which generate transient dynamic loads due to inertia of the dam and the retained body of water. Horizontal and vertical accelerations are not equal, the former being of greater intensity. 2/3/2014 11 PREPARED BY V.H.KHOKHANI,ASSISTANT PROFESSOR, DIET,
- 12. The earthquake acceleration is usually designated as a fraction of the acceleration due to gravity and is expressed as α⋅g, where α is the Seismic Coefficient. The seismic coefficient depends on various factors, like the intensity of the earthquake, the part or zone of the country in which the structure is located, the elasticity of the material of the dam and its foundation, etc. 2/3/2014 12 PREPARED BY V.H.KHOKHANI,ASSISTANT PROFESSOR, DIET,
- 13. For the purpose of determining the value of the seismic coefficient which has to be adopted in the design of a dam, India has been divided into five seismic zones, depending upon the severity of the earthquakes which may occur in different places. A map showing these zones is given in the Bureau of Indian Standards code IS: 1893-2002 (Part-1) “Criteria for earthquake resistant design of Structures (fourth revision)”, and has been reproduced in Figure 28. 2/3/2014 13 PREPARED BY V.H.KHOKHANI,ASSISTANT PROFESSOR, DIET,
- 14. According to IS : 1893 - 2002, the design value of horizontal seismic coefficient (Ah) may be determined by one of the two methods 2/3/2014 14 PREPARED BY V.H.KHOKHANI,ASSISTANT PROFESSOR, DIET,
- 15. (a) Seismic coefficient method (b) Response spectrum method 2/3/2014 15 PREPARED BY V.H.KHOKHANI,ASSISTANT PROFESSOR, DIET,
- 16. (a) Seismic coefficient method The total design lateral force or design seismic base shear (VB) along any principal direction shall be determined by the following expression. 2/3/2014 16 PREPARED BY V.H.KHOKHANI,ASSISTANT PROFESSOR, DIET,
- 18. Hydrodynamic Pressure : Horizontal acceleration acting towards the reservoir causes a momentary increase ln the water pressure as the foundation and dam accelerate towards the reservoir and the water resists movement owing to its inertia. The extra pressure exerted by this process is known as hydrodynamic pressure. 2/3/2014 18 PREPARED BY V.H.KHOKHANI,ASSISTANT PROFESSOR, DIET,
- 20. According to Von Kannan, the amount of this hydrodynamic pressure (Pf) is given by : 2/3/2014 20 PREPARED BY V.H.KHOKHANI,ASSISTANT PROFESSOR, DIET,
- 25. Weight of dam 2/3/2014 25 PREPARED BY V.H.KHOKHANI,ASSISTANT PROFESSOR, DIET,
- 31. 2/3/2014 W1 W2 56 TOE 31 PREPARED BY V.H.KHOKHANI,ASSISTANT PROFESSOR, DIET, 211 205 6m ½ * (4) *(6) 2 3 2/3 (6)
- 32. Name of force Designati on Megnitud e of force Lever arm (distance of C.G. from toe) Vertical forces Weight of dam W 1 6*84*23.5* 1 (6/2) + 50 W2 0.5*50*75* 23.5 (2/3) 50 Weight of water Supported on d/s (1/2)*4*6* 10 (1/3) 4 2/3/2014 32 PREPARED BY V.H.KHOKHANI,ASSISTANT PROFESSOR, DIET,
- 33. 2/3/2014 8 u3 u1 u2 u4 48 (10)(80) 10(6) 56 48 80 6 307 3 680 610 3 hdhu hd 8 33 PREPARED BY V.H.KHOKHANI,ASSISTANT PROFESSOR, DIET,
- 35. Name of force Designation Megnitude of force Lever arm (distance of C.G. from toe) Vertical forces +Uplift forces u1 300.8*8*1 (48)+4 u2 (1/2)*8*484*1 48 + (2/3) 8 u3 48*59*1 48/2 u4 (1/2) *48*(300.8 - 59) (2/3)48 HORIZONTAL FORCE P (1/2)*784.8*80 *1 (1/3)80 P’ (1/2)*58.9*6 (1/3)6 2/3/2014 35 PREPARED BY V.H.KHOKHANI,ASSISTANT PROFESSOR, DIET,
- 37. Name of force Designation Megnitude of force Lever arm (distance of C.G. from toe)Vertical forces Weight of dam W 1 6*84*23.5*1 (6/2) + 50 W2 0.5*50*75*23.5 (2/3) 50 Weight of water Supported on d/s (1/2)*4*6*10 (1/3) 4 +Uplift forces u1 300.8*8*1 (48)+4 u2 (1/2)*8*484*1 48 + (2/3) 8 u3 48*59*1 48/2 u4 (1/2) *48*(300.8 - 59) (2/3)48 HORIZONTAL FORCE P (1/2)*784.8*80*1 (1/3)80 P’ (1/2)*58.9*6 (1/3)6 2/3/2014 37 PREPARED BY V.H.KHOKHANI,ASSISTANT PROFESSOR, DIET,