BUTTRESS DAM

• By

• ADITYA SINGH

• T.E (CIVIL)

BUTTRESS DAMS

                                                  

   Buttress dams were first regions where materials were scarce or expensive but labour was cheap. Dams were used for irrigation and mining purposes. As designs have become more sophisticated, the virtues and weaknesses of the buttress type dams have become apparent. •The pressure of water on the inclined upstream at face adds to the stability of the dam, both by its magnitude and direction. •With free drainage of the foundations between the buttresses, uplift on their bases is considerably reduced. •The general flexibility of the dam can accommodate differential movement of the foundations. •Unless the foundation material was erodible minor leakage should not endanger the dam. •developed to conserve water in

A minimum of materials is required but its accurate placement involves skilled tradesmen and higher unit costs. Whilst construction is at low levels, the work can be overtopped by floods without serious damage - with considerable saving in river diversion works. The buttress type of dam finds particular application in wide valleys where sound rock would be the exception rather than the rule. Thorough investigations are therefore essential particularly if the dam is to be rigid.

Lateral stability of buttresses is not now considered to be serious except for high dams, but it should be checked, especially in areas of known seismicity. For large dams the stress distribution in the buttresses [from water load, own weight, thermal effects and foundation movements] is complex and does not conform to linear distribution on horizontal planes. Models show tensile stresses near the foundation of buttress heads in the case of good foundations - though such stresses are not evident from conventional analytical analysis. Preliminary designs should therefore be supplemented by detailed studies using finite elements or photoelastic methods.

• If a buttress dam is of slender dimensions, especially a multiple arch, and flood waters are to pass over it, a very careful examination is necessary of possible modes of vibration. What may not be serious for a gravity dam could be disastrous for a buttress dam.

• For large dams the stress distribution in the buttresses [from water load, own weight, thermal effects and foundation movements] is complex and does not conform to linear distribution on horizontal planes. Models show tensile stresses near the foundation of buttress heads in the case of good foundations - though such stresses are not evident from conventional analytical analysis. Preliminary designs should therefore be supplemented by detailed studies using finite

elements or photoelastic methods.

• There appears to be a case for studying large span multiple arches in wide valleys, i.e. the arches would be thick, unreinforced, and constructed by mass concrete methods.

• There is considerable scope for the application for prestressing to modify stresses within buttress dams as well as to improve their stability.

• A buttress dam is also commonly known as a hollow dam because the buttresses do not join together to form an actual solid wall across the river valley. The shape of the actual buttress or support is mostly flat or curved with most buttress dams today being constructed out of concrete and reinforced steel. Another common characteristic of the buttress dam is that the upstream face of the dam is inclined at

about 45 degrees.

DAMSBUTTRESS DAMS

Buttress dams are dams in which the face is held up by a series of supports.

Buttress dams can take many forms -- the face may be flat or curved.

Usually, buttress dams are made of concrete and may be reinforced with steel bars.

DAMSBUTTRESS DAMS

sloping membrane that transmits the water load to a series of buttresses @ right angles to axis of dam

-Increased formwork & reinforced steel compared w/gravity dam

-Less massive than gravity dam (requires 1/3 to 1/2 as much concrete)

-Use on weaker foundation

-Same forces as gravity and arch dams, however, ice pressures not as prevalent; gaps b/t buttresses relieve majority of uplift forces

Types Water Supporting Membrane

1. Flat-slab flat, concrete-reinforced slabs

2. Multiple-arch series of arches

DAMSBUTTRESS DAMS

Daniel-Johnson Dam, Quebec

DAMSBUTTRESS DAMS

Bartlett Dam, Colorado

Buttress Dams• Buttress Dams use multiple reinforced columns to support a dam that has a relatively thin

structure. Because of this, these dams often use half as much concrete as gravity dams

• How it Works • buttress dam are exactly the same as those that act on a gravity dam except the vertical load

The forces or laws of physics working against a presented by the water on a buttress dam is greater. The advantage of a buttress dam is that it typically requires less concrete to construct than a gravity dam. The formwork and reinforced steel used in the building of buttresses is expensive, however, and will ultimately offset any costs saved.

• In a buttress dam, stability is secured by making it of such a size and shape that it will resist overturning, sliding and crushing at the toe. The dam will not overturn provided that the moment around the turning point, caused by the water pressure, is smaller than the moment caused by the weight of the dam. This is the case if the resultant force of water pressure and weight falls within the base of the dam. However, in order to prevent tensile stress at the upstream face and excessive compressive stress at the downstream face, the dam cross section is usually designed so that the resultant falls within the middle at all elevations of the cross section (the core). For this type of

dam, impervious foundations with high bearing strength are essential.

What are the forcesacting on a buttress dam?

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• These are the main forces on a dam. • forces of the reservoir water• uplift force • weight of concrete•

• There are many other forces that may act on a buttress dam:

• internal hydrostatic pressure: in pores, cracks, joints, and seams

• temperature variations

• chemical reactions

• silt pressure; silt will build up over time on the upstream side; silt provides about 1.5 times the horizontal pressure of water and twice the vertical pressure of water

• wave load on the upstream side

• earthquake loads

• settlement of the foundation or abutments

• other structures on top of the dam -- gates, a bridge, cars

• creep of concrete: deformation of the concrete when under a constant load for a long period of time.

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• Basic forces on a concrete dam• forces of the reservoir water, Ww and Hw.

– Ww: vertical component of force of water -- weight of water

Ww = (unit weight of water) * (volume of water)the unit weight of water = 9810 N/m3

The weight of water should be disregarded for simulations.

– Hw: the horizontal component of the force of the waterWater applies pressure (*) in the shape of a triangle. The deeper the water, the more horizontal pressure it exerts on the dam. So at the surface of the reservoir, the water is exerting no pressure and at the bottom of the reservoir, the water is exerting maximum pressure.

• The base of the triangle is equal to the unit weight of water. The total force exerted by the triangle equals the area of the triangle:

• Hw = 0.5*(the unit weight of water)*height2height = the height of the water

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The force acts at the center of gravity of the triangle -- one-third of the way up from the bottom. The maroon arrow shows where the total force of the water acts.

• uplift force, U: if there is no drainage, the uplift force is the result of the water pressure under the dam pushing up on the dam

• weight of concrete, WcWc = (the specific weight of concrete)*(volume of concrete)the specific weight of concrete = 23.6 N/m3

Buttress Dam: ForcesWater pushes against the buttress dam, but the buttresses push back and prevent the dam from toppling

over. The weight of the buttress dam also pushes down into the ground.

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• What are the partsof a buttress dam?

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The buttress head (*) may be flat, as shown above, or rounded.• Usually, buttress dams are made of concrete and may be reinforced with steel bars.• The buttress (*) may be hollow or solid. The buttress of Bartlett Dam (pictured above) are hollow.

Bartlett is the highest concrete buttress dam in the U.S., at 287 feet. •

cross section of a buttress dam    

reservoir    Basin formed by the construction of a dam; it holds back a very large volume of water so that the flow rate can be controlled.

foundation   Concrete structure that supports the weight of the dam and transmits it to the ground to provide stability to the dam.

foundation blockage    Block of concrete that anchors the foundation in the ground to prevent movement.

buttress    Block of concrete reinforcing a wall that has to stand up to the thrust of the water; it provides stability to the dam.