1、中文 3018 字 Comparison of Design and Analysis of Concrete Gravity Dam ABSTRACT Gravity dams are solid concrete structures that maintain their stability against design loads from the geometric shape, mass and strength of the concrete.The purposes of dam construction may include navigation, flood damage
2、 reduction,hydroelectric power generation, fish and wildlife enhancement,water quality,water supply,and recreation.The design and evaluation of concrete gravity dam for earthquake loading must be based on appropriate criteria that reflect both the desired level of safety and the choice of the design
3、 and evaluation procedures.In Bangladesh, the entire country is divided into 3 seismic zones, depending upon the severity of the earthquake intensity. Thus, the main aim of this study is to design high concrete gravity dams based on the U.S.B.R. recommendations in seismic zone II of Bangladesh, for
4、varying horizontal earthquake intensities from 0.10 g - 0.30 g with 0.05 g increment to take into account the uncertainty and severity of earthquake intensities and constant other design loads, and to analyze its stability and stress conditions using analytical 2D gravity method and finite element m
5、ethod. The results of the horizontal earthquake intensity perturbation suggest that the stabilizing moments are found to decrease significantly with the increment of horizontal earthquake intensity while dealing with the U.S.B.R. Recommended initial dam section, indicating endanger to the dam stabil
6、ity, thus larger dam section is provided to increase the stabilizing moments and to make it safe against failure. The vertical, principal and shear stresses obtained using ANSYS 5.4 analyses are compared with those obtained using 2D gravity method and found less compares to 2D gravity method, except
7、 the principal stresses at the toe of the gravity dam for 0.10 g - 0.15 g. Although, it seems apparently that smaller dam section may be sufficient for stress analyses using ANSYS 5.4, it would not be possible to achieve the required factors of safety with smaller dam section.It is observed during s
8、tability analyses that the factor of safety against sliding is satisfied at last than other factors of safety, resulting huge dam section to make it safe against sliding. Thus, it can be concluded that it would not be feasible to construct a concrete gravity dam for horizontal earthquake intensity g
9、reater than 0.30 g without changing other loads and or dimension of the dam and keeping provision for drainage gallery to reduce the uplift pressure significantly. Keywords: Comparison Concrete Gravity Dam Dam Failure Design Earthquake Intensity Perturbation Stability and Stress 1.Introduction Basic
10、ally, a gravity concrete dam is defined as a structure,which is designed in such a way that its own weight resists the external forces. It is primarily the weight of a gravity dam which prevents it from being overturned when subjected to the thrust of impounded water 1. This type of structure is dur
11、able, and requires very little maintenance. Gravity dams typically consist of a non overflow section(s) and an overflow section or spillway. The two general concrete construction methods for concrete gravity dams are conventional placed mass concrete and RCC. Gravity dams, constructed in stone mason
12、ry, were built even in ancient times, most often in Egypt, Greece, and the Roman Empire 2,3. However, concrete gravity dams are preferred these days and mostly constructed. They can be constructed with ease on any dam site, where there exists a natural foundation strong enough to bear the enormous w
13、eight of the dam. Such a dam is generally straight in plan, although sometimes, it may be slightly curve. The line of the upstream face of the dam or the line of the crown of the dam if the upstream face in sloping, is taken as the reference line for layout purposes, etc. and is known as the “Base l
14、ine of the Dam” or the “Axis of the Dam”. When suitable conditions are available, such dams can be constructed up to great heights. The ratio of base width to height of high gravity dams is generally less than 1:1. A typical cross-section of a high concrete gravity dam is shown in Figure . The upstr
15、eam face may be kept throughout vertical or partly slanting for some of its length. A drainage gallery is generally provided in order to relieve the uplift pressure exerted by the seeping water.Purposes applicable to dam construction may include navigation, flood damage reduction, hydroelectric powe
16、r generation, fish and wildlife enhancement, water quality, water supply, and recreation. Many concrete gravity dams have been in service for over 50 years, and over this period important advances in the methodologies for evaluation of natural phenomena hazards have caused the design-basis events fo
17、r these dams to be revised upwards. Older existing dams may fail to meet revised safety criteria and structural rehabilitation to meet such criteria may be costly and difficult. The identified causes of failure, based on a study of over 1600 dams 4 are: Foundation problems (40%), Inadequate spillway
18、 (23%), Poor construction (12%), Uneven settlement (10%), High poor pressure (5%), Acts of war (3%), Embankment slips (2%), Defective ma terials(2%), Incorrect operation (2%), and Earthquakes (1%).Other surveys of dam failure have been cited by 5, who estimated failure rates from 210-4to7 10-4per da
19、myear based on these surveys. 2.Loads In the design of gravity concrete, it is essential to determine the loads required in the stability and stress analyses. The forces which may affect the design are: 1) Dead load or stabilizing force； 2) Headwater and tailwater pressures； 3) Uplift； 4) Temperatur
20、e； 5) Earth and silt pressures； 6) Ice pressure； 7) Earth quake forces； 8) Wind pressure； 9) Subatmospheric pressure； 10) Wave pressure, and 11) Reaction of foundation. The seismic safety of such dams has been a serious concern since damage to the Koyna Dam in India in 1967 which has been regarded a
21、s a watershed event in the development of seismic analysis and design of concrete gravity dams all over the world. It is essential that those responsible must implement policies and proce dures to ensure seismic safety of dams through sound professional practices and state-of-the-art in related tech
22、nical areas. Seismic safety of dams concerns public safety and therefore demands a higher degree of public confidence. The Estimations and descriptions of various forces are provided briefly in the following sections. 2.1. Water Pressure Water pressure (P) is the most major external force acting on
23、gravity dams. The horizontal water pressure exerted by the weight of water stored on the upstream and downstream sides of the dam can be estimated from the rule of hydrostatic pr essure distribution and can be expressed by where, H is the depth of water and w is the unit weight of water. 2.2. Uplift
24、 Pressure Water seepage through the pores, cracks and fissures of the foundation materials, and water seepage through dam body and then to the bottom through the joints between the body of the dam and its foundation at the base exert an uplift pressure on the base of the dam. According to the 6, the uplift pressure intensities at the heel and toe of the dam should be taken equal to their respective hydrostatic pressures and joined the intensity ordinates by a straight line. When 221 Hp w
