【正文】 ods of demand). Such reservoirs are, therefore, helpful in permitting the pumps or water treatment plants to work at a uniform rate, and they store water during the hours of no demand or less demand and supply water from their ‘storage’ during the critical periods of maximum demand. (4) Multipurpose Reservoirs A reservoir planned and constructed to serve not only one purpose but various purposes together is called a multipurpose reservoir. Reservoir, designed for one purpose, incidentally serving other purpose, shall not be called a multipurpose reservoir, but will be called so, only if designed to serve those purposes also in addition to its main purpose. Hence, a reservoir designed to protect the downstream areas from floods and also to conserve water for water supply, irrigation, industrial needs, hydroelectric purposes, etc. shall be called a multipurpose reservoir. THE ELECTRIC POWER SYSTEM A great amount of effort is necessary to maintain an electric power supply within the requirement of the various types of customers served. Large investments are necessary, and continuing advancements in methods must be made as loads steadily increase from year to year. Some of the requirements for electric power supply are recognized by most consumers, such as proper voltage, availability of power on demand, reliability, and reasonable cost. Other characteristics, such as frequency, wave shape, and phase balance, are seldom recognized by the customer but are given constant attention by the utility power engineers. The voltage of the power supply at the customer’s service entrance must be held substantially constant. Variations in supply voltage are, from the customer’s view, detrimental in various respects. For example, belownormal voltage substantially reduces the light output from incandescent lamps. Abovenormal voltage increase the light output but substantially reduces the life of the lamp. Motor operate at belownormal voltage draw abnormally high current and may overheat, even when carrying no more than the rated horsepower load. Over voltage 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。 Details of the method are given in BS 1881:1970. Values of the modulus of rupture are utilized in some methods of design of unreinforced concrete roads and runways, in which reliance is place on the flexural strength of the concrete to distribute concentrated loads over a wide area. More recently introduced is a test made by splitting cylinders by pression across the diameter, to give what is termed the splitting tensile strength。 provision of additional reserve facilities。 A radius of about onefourth of the spillway height has proved satisfactory. Structural design of an ogee spillway is essentially the same as the design of a concrete gravity section. The pressure exerted on the crest of the spillway by the flowing water and the drag forces caused by fluid friction are usually small in parison with the other forces acting on the section. The change in momentum of the flow in the vicinity of the reverse curve may, however, create a force which must be considered. The requirements of the ogee shape usually necessitate a thicker section than the adjacent no overflow sections. A saving of concrete can be effected by providing a projecting corbel on the upstream face to control the flow in outlet conduits through the dam, a corbel will interfere with gate operation. The discharge of an overflow spillway is given by the weir equation 23CQ Lh?? Where Q=discharge, or sec/3m tcoefficienC ?? L=coefficient h=head on the spillway (vertical distance from the crest of the spillway to the reservoir level), 2 m The coefficient ?C varies with the design and head. Experimental models are often used to determine spillway coefficient. End contractions on a spillway reduce the effective length below the actual length L. Squarecornered piers d