英文翻译英文+中文文档格式.docx

1、 A radius of about one-fourth 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

2、 are usually small in comparison 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 o

3、verflow 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 Where Q=discharge, or L=

4、coefficienth=head on the spillway （vertical distance from the crest of the spillway to the reservoir level）, mThe coefficient 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 ac

5、tual length L. Square-cornered piers disturb the flow considerably and reduce the effective length by the width of the piers plus about 0.2h for each pier.Streamlining the piers or flaring the spillway entrance minimizes the flow disturbance. If the cross-sectional area of the reservoir just upstrea

6、m from the spillway is less than five times the area of flow over the spillway, the approach velocity with increase the discharge a noticeable amount. The effect of approach velocity can be accounted for by the equationwhere is the approach velocity. PROPERTIES OF CONCRETE The characteristics of con

7、crete should be considered in relation to the quality for any given construction purpose. The closest practicable approach to perfection in every property of the concrete would result in poor economy under many conditions, and the most desirable structure is that in which the concrete has been desig

8、ned with the correct emphasis on each of the various properties of the concrete, and not solely with a view to obtaining, say, the maximum possible strength. Although the attainment of the maximum strength should not be the sole criterion in design, the measurement of the crushing strength of concre

9、te cubes or cylinders provides a means of maintaining a uniform standard of quality, and, in fact, is the usual way of doing so. Since the other properties of any particular mix of concrete are related to the crushing strength in some manner, it is possible that as a single control test it is still

10、the most convenient and informative.The testing of the hardened concrete in prefabricated units presents no difficulty, since complete units can be selected and broken if necessary in the process of testing. Samples can be taken from some parts of a finished structure by cutting cores, but at consid

11、er one cost and with a possible weakening of the structure. It is customs, therefore, to estimate the properties of the concrete in the structure on the oasis of the tests made on specimens mounded from the fresh concrete as it is placed. These specimens are compacted and cured in a standard manner

12、given in BS 1881 in 1970 as in these two respects it is impossible to simulate exactly the conditions in the structure. Since the crushing structure is also affected by the size and shape of a specimen or part of a structure, it follows that the crushing strength of a cube is not necessary the same

13、as that of the mass of exactly the same concrete.Crushing strengthConcrete can be made having a strength in compression of up to about 80N/,or even more depending mainly on the relative proportions of water and cement, that is, the water/cement ratio, and the degree of compaction. Crushing strengths

14、 of between 20 and 50 N/ at 28 days are normally obtained on the site with reasonably good supervision, for mixes roughly equivalent to 1:2:4 of cement: sand: coarse aggregate. In some types of precast concrete such as railway sleepers, strengths ranging from 40 to 65 N/ at 28 days are obtained with

15、 rich mixes having a low water/cement ratio.The crushing strength of concrete is influenced by a number of factors in addition to the water/cement ratio and the degree of compaction. The more important factors are Type of cement and its quality. Both the rate of strength gain and the ultimate streng

16、th may be affected.Type and surface texture of aggregate. There is considerable evidence to suggest that some aggregates produce concrete of greater compressive and tensile strengths than obtained with smooth river gravels.Efficiency of curing. A loss in strength of up to about 40 per cent may resul

17、t from premature drying out. Curing is therefore of considerable, importance both in the field and in the making of tests. The method of curing concrete test cubes given in BS 1881 should, for this reason, be strictly adhered to.Temperature In general, the rate of hardening of concrete is increased

18、by an increase temperature. At freezing temperatures the crushing strength may remain low for some time.Age Under normal conditions increase in strength with age, the rate of increase depending on the type of cement with age. For instance, high alumina cement produces concrete with a crushing streng

19、th at 21 hours equal to that of normal Portland cement concrete at 28 days. Hardening continues but at a much slower rate for a number of years. The above refers to the static ultimate load. When subjected to repeated loads concrete fails at a load smaller than the ultimate static load, a fatigue ef

20、fect. A number of investigators have established that after several million cycles of loading, the fatigue strength in compression is 50-60 per cent of the ultimate static strength.Tensile and flexural strength The tensile strength of concrete varies from one-eighth of the compressive strength at ea

21、rly ages to about one- twentieth later, and is not usually taken into account in the design of reinforced concrete structures. The tensile strength is, however, of considerable importance in resisting cracking due to changes in moisture content or temperature. Tensile strength tests are used for con

22、crete roads and airfields. The measurement of the strength of concrete in direct tension is difficult and is rarely attempted. Two more practical methods of assessing tensile strength are available. One gives a measure of the tensile strength in bending, usually termed the flexural strength. BS 1881

23、:1970 gives details concerning the making and curing of flexure test specimens, and of the method test. The standard size of specimen is 150 150750 long for aggregate of maximum size 40. If the largest nominal size of the aggregate is 20, specimens 100100 long may be used. A load is applied through

24、two rollers at the third points of the span until the specimen breaks. The extreme fiber stresses, that is, compressive at the top and tensile at the bottom, can then be computed by the usual beam formulae. The beam will obviously fail in tension since the tensile strength is much lower than the com

25、pressive strength. Formulae for the calculation of the modulus of rupture are given in BS 1881:1970. Test specimens is the form of beams are sometimes used to measure the modulus of rupture or flexural strength quickly on the site. The two halves of the specimen may then be crushed so that besides the flexural strength the compressive strength can be approximately determined on the same sample. The test is described in BS 1881: Values of the modulus of rupture are utilized in some methods of design of unreinforced concrete roads and runways, in which reliance is placed on the f