FORCES ACTING ON GRAVITY DAM
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.
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.
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.
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.
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.
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.
According to IS : 1893 - 2002, the design value of horizontal seismic coefficient
(Ah) may be determined by one of the two methods
(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.
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 ex
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
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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
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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
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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
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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
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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
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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
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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.
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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.
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20. According to Von
Kannan, the amount
of this hydrodynamic
pressure (Pf) is given
by :
2/3/2014 20
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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
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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
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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
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