Concrete Dams as a Structures - Coursework Example

Name Instructor Course Date Introduction Concrete dams are hydraulic structures constructed using masonry stones or concrete. These dams are designed to store water by using only its material weight to resist horizontal water pressure. These dams are designed to ensure that each component is not only stable but also independent of any other component. There are a several types of concrete dams, which are grouped according to how they transfer hydrostatic pressure. They include Buttress dams, Arch dams, and gravity dams. Gravity dams are structures made from concrete, and that use the strength and mass of the construction material and their geometric shape to resist design loads. They can be constructed in a curved, straight, or angled manner depending on the site condition and the design specifications. Earthquakes are natural or manmade disaster that produce energy within the earth’s crust produces seismic waves. For decades, earthquakes have had adverse effects on a number of large dams. They can cause catastrophic failures thus the design of gravity dams using finite element analysis and seismic analysis has become critical. Essentially when designing a concrete gravity dam one should meet the following general criteria. The design should ensure that the dam is safe from against the risk of overturning at any of its horizontal positions in contact with the foundation of the dam or located in the foundation itself. The dam should safe from any form of sliding that may occur within any part of its horizontal plane, at the point of contact with the foundation or within the foundations geological features. The components of the structures should be designed to be proportional enough that allowable stresses experienced by the foundation of the dam and the concrete used are not exceeded. Loading To ensure that the dam is safe has to be checked ageist all the classes of loadings. These types include primary, secondary, and exceptional loading. Primary loads are applicable in all types of structures and are considered the most important type of loading in the dam and example is dead loads. Secondary loads are categorized as discretional loads and are of lesser importance than the primary loads. Examples of secondary loads are thermal stresses. Exceptional are loads that are less likes to occur. They are designed in special cases or when the site conditions necessities. An example is an earthquake or seismic activity. When designing a concrete gravity dam, the designer should first evaluate the type of loads that can act on the structure. There are several types namely temperature, dead weight, sand and silt, ice, internal hydraulic pressure, earthquake, hydraulic pressure from the reservoir and the tail water. The Dead loads include the weight of the body of the gravity dam structure together with the bridge and pier gates. The pressure created by water that it at the upstream part of the dam is described as hydraulic pressure from the reservoir. The pressure created by water at the downstream part of the dam is described as hydraulic pressure from the tail of the water. The uplift pressure is the pressure that is derived from water in the body and foundation of the dam, and it pushes the dam upwards increasing the risk of sliding. It is described as the internal hydraulic pressure. Sand and silt are forces that act on the upstream part of the dam in the form of earth pressure. The loading exerted by the pressure of ice acts at the upstream of the dam; the pressure increases as the ice freezes, and if it becomes too big, it can become a problem. Earthquake or seismic activity is a particular type of loading different from the ones mentioned above. It is very difficult to predict. Earthquake loading prediction is crucial especially in earthquake prone regions. They are caused by ground motions and complicated acceleration that form oscillating patterns and that produce dynamic forces in the dam that are transient in nature. The retained water and the inertia of the structure cause these forces. The horizontal acceleration of the complex forces is much greater than the vertical acceleration. Gravity dams obtain their stability from the forces of gravity of the materials used. The forces that give the dam its stability include the thrust from the tail of the water. Forces that contribute to the destabilization of the dam include; the water pressure from the reservoir, temperature stresses, seismic forces ice pressure, silt pressure, uplift, forces from the waves in the reservoir, and wind pressures. These forces can be categorized into two groups namely forces directly derive from the dams materials unit weight and fluid pressure such as the dams weight and the pressure exerted by the water. The second group is forces obtained based on the varying degree of their reliability; such forces include earthquake loads, ice and silt pressure or the uplift. Load combination The designs of a concrete gravity dam must be based on the extreme load combinations given below. The combination below uses the IS: 6512-1984 safety factors. The choice of loading combination is based on the site conditions of the project and the type of material to based. Load Combination A Used when the dam structure is complete but does not into account the water in the tail and the reservoir. The loading combination only looks at the condition of the construction. Load Combination B Used when the dam structure is complete with a full reservoir elevation and the tail water at dry weather level. The scenario assumes normal weather conditions ice, slit, and uplift effect are also normal. The loading combination looks at the normal operating conditions. Load Combination C Used when the dam structure is complete with a reservoir elevation that is at the maximum flood pool. The scenario assumes adverse weather conditions but with normal ice, slit and uplift effect conditions. The loading combination looks at the flood discharge conditions. Load Combination D It is the loading combination of A while taking into account earthquake effects. The loading combination looks at the earthquake conditions. Load combination E It is the loading combination B while taking into account earthquake effects but without the effects of ice. The loading combination looks at the earthquake conditions but without the effects of ice Load combination F It is the loading combination B while taking into account extreme cases uplifts and with the assumption that the drainage holes are not working. Finite Element Method The finite element method allows its users to model a structure's geometry correctly and examine its interaction with the foundation of the structure. It also has the capability of modeling the foundation rock that is found below the dam’s surface. The method uses STAAD.PROV8i to simulate how structures respond when subjected to blast or impact loading. It divides a structure that is under investigation into small elements that are then separately calculated by the use of a computer. Concrete gravity dam modeling through FEM When creating Finite element models several considerations should be made namely compressive stress, tension stress displacement, and sliding stability Sliding stability To ensure that the calculation of critical areas such as the area near or within the dam’s foundation element standards has to be taken into consideration. It will ensure that the dam is stable enough to all the loads without excessive deformation, and it will reduce the risk of sliding. Tension Stresses Tension stresses are very common in areas around the dam’s heel, and thus the designer must take extra care while coming up with the elements of these areas. To improve accuracy, the elements of these regions must be much smaller than usual it will also allow the elements to be much closer to the sensitive regions. To accurately analyze these elements a simplified finite element modeling method is used which states that amount of elements in the dams bottom layer with tension should not me more than seven percent of all the elements in that layer. These are because all the elements in the foundation are almost the same size thus the percentage of tension elements is taken to be the same the percentage. Compression stresses To determine compression stresses, the designer must first examine the whole model and then identify the area with the highest risk of the compressive area. The characteristics of the materials must be considered and compared to the values of the compressive stress calculated. Displacement control The control of the dam’s displacement should be based on the entire model. It is because displacement experienced at the bottom is dependent on the displacement experienced at the top. Modeling the elements The shape and size of the elements are of great importance. There are several standards to ensure that designers create elements that function properly. Tri and Quad are the most commonly used two-dimensional elements. Quad consists of four integration points and four nodes while Tri have one integration point with three nodes. It makes quad elements more accurate and the best two-dimensional element to use. While using the quad element the height to width ratio should not exceed there to one. In addition, the inner angle should not be more than forty-five degrees. Strain and stress in finite element analysis The elements used in processes of finite element analysis can be either plan strain or plain stress. In both cases, a designer must take into account three crucial considerations in the XY-plane. The first component is shear stress, and the last two are both normal stresses one vertical while the other is horizontal. Never the less, the stresses and strains have differences between them. All components in the analysis of plane stress are considered to be zero apart from the three mentioned above. It means that the elements will not be added any additional internal work. Finite element analysis of a concrete gravity dam In order to understand the finite element method will consider a concrete gravity dam with a height of 50m and a base of 31m. The two-dimensional plane stress element will be used to model the sections of the dam. The frequency calculations were done using the subspace Eigen solver. The dam’s base was assumed to be fixed. The impounded water at the upstream of the dam is taken as a vibrating lamp mass. In the calculations combined fluid-interface equation of motion was used and is given below as Conclusion The finite elect analysis was calculated alongside Saini-Viswanath method and IS: 1893-1984 methods. From the results, we can see that the finite element analysis produced values that were fifteen percent less than the finite element analysis. References Krishnamoorthy, C S. Finite Element Analysis: Theory and Programming. New Delhi [u.a.: Tata McGraw-Hill, 1995. Print. Ramamurty, G. Applied Finite Element Analysis. S.l.: I K International Pub. House Pvt, 2010. Print. Szabo, B A, and Ivo Babuška. Finite Element Analysis. New York: Wiley, 1991. Print. Read More