【正文】 ltage on a motor may cause excessive heat loss in the iron of the motor, wasting energy and perhaps damaging the machine. Service voltages are usually specified by a nominal value and the voltage than 7 maintained close to this value, deviating perhaps less than 5 percent above or below the nominal value. For example, in a 120volt residential supply circuit, the voltage might normally vary between the limits of 115 and 125 volts as customer load and system conditions change throughout the day. Power must be available to the consumer in any amount that be may require from minute to minute. For example, motors may be turned on or off, without advance warning to the electric power pany. As electrical energy cannot be stored (except to a limited extent in storage batteries), the changing loads impose severe demands on the control equipment of any electrical power system. The operating staff must continually study load patterns to predict in advance those major load changes that follow known schedules, such as the starting and shutting down of factories at prescribed hours each day. The demands for reliability of service increase daily as our industrial and social environment bees more plex. Modern industry is almost locally dependent on electric power for its operation. Homes and office buildings are lighted, heated, and ventilated by electric power. In some instances loss of electric power may even pose a threat a life itself. Electric power, like everything else that is manmade can never be absolutely reliable. Occasional interruptions to service in limited areas will continue. Interruptions to large areas remain a possibility, although such occurrences may be very infrequent. Further interconnection of electric supply systems over wide areas, continuing development of reliable automated control systems and apparatus。 1 OVERFLOW SPILLWAY An overflow spillway is a section of dam designed to permit water to pass over its crest. Overflow spillways are widely used on gravity, arch, and buttress dams. Some earth dams have a concrete gravity section designed to serve as a spillway. The design of the spillway for tow dams is not usually critical, and a variety of simple crest patterns are used. In the case of large dams it is important that the overflowing water be guided smoothly over the crest with a minimum of turbulence. If the overflowing water breaks contact with the spillway surface, a vacuum will form at the point of separation and cavitations may occur. Cavitations plus the vibration from the alternates making and breaking of contact between the water and the face of the dam may result in serious structural damage. Cavities filled with vapor, air, and other gases will form in a liquid whenever the absolute pressure of the liquid is close to the vapor pressure. This phenomenon, cavitations, is likely to occur where high velocities cause reduced pressure. Such conditions may arise if the walls of a passage are so sharply curved as to cause separation of flow from the boundary. The cavity, on moving downstream, may enter a region where the absolute is much higher. This causes the vapor in the cavity to condense and return to liquid with a resulting implosion, or collapse, extremely high pressure result. Some of the implosive activity will occur at the surfaces of the passage and in the crevices and pores of the boundary material. Under a continual bombardment of these implosions, the surface undergoes fatigue failure and small particles are broken away, giving the surface a spongy appearance. This damaging action of cavitations is called pitting. The ideal spillway would take the form of the underside of the napped of a sharpcrested weir when the flow rate corresponds to the maximum design capacity of the spillway. More exact profiles may be found in more extensive treatments of the subject. The reverse curve on the downstream face of the spillway should be smooth and gradual。 Details of the method are given in BS 1881:1970. the testing machine is fitted with an extra bearing bar to distribute the load along the full length of the cylinder Plywood strips, 12mm wide and 3mm thick are inserted between the cylinder and the testing machine bearing surfaces top and bottom. From the maximum applied load at failure the tensile splitting strength is calculated as follows: ldp2ft ?? Where ?tf splitting tensile strength, N/ 2mm P=maximum applied load in N l=length of cylinder in mm d=diameter in mm As in the case of the pressive strength, repeated loading reduces the ultimate strength so that the fatigue strength in flexure is 5060 per cent of the static strength. Shear strength In practice, shearing of concrete is always acpany pression and tension caused by bending, and even in testing is impossible to staminate an element of bending. RESERVOIRS When a barrier is constructed across some river in the form of a dam, water gets stored up on the upstream side of the barrier, forming a pool of water, generally called a reservoir. 5 Broadly speaking, any water collected in a pool or a lake may be termed as a reservoir. The water stored in reservoir may be used for various purposes. Depending upon the purposes served, the reservoirs may be classified as follows: Storage or Conservation Reservoirs. Flood Control Reservoirs. Distribution Reservoirs. Multipurpose reservoirs. (1) Storage or Conservation Reservoirs. A city water supply, irrigation water supply or a hydroelectric project drawing water directly from a river or a stream may fail to satisfy the consumers’ demands during extremely low flows, while during high flow