公路线形设计外文翻译(共14页).doc
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1、精选优质文档-倾情为你奉上Geometric Design of HighwaysA Alignment Design of a road is shown on the plane view and is a series of straight lines called tangents connected by circular. In modern practice it is common to interpose transition or spiral curves between tangents and circular curves.Alignment must be co
2、nsistent. Sudden changes from flat to sharp curves and long tangents followed by sharp curves must be avoided; otherwise, accident hazards will be created. Likewise, placing circular curves of different radii end to end (compound curves) or having a short tangent between two curves is poor practice
3、unless suitable transitions between them are provided. Long, flat curves are preferable at all times, as they are pleasing in appearance and decrease possibility of future obsolescence. However, alignment without tangents is undesirable on two-lane roads because some drivers hesitate to pass on curv
4、es. Long, flat curves should be used for small changes in direction, as short curves appear as “kink”. Also horizontal and vertical alignment must be considered together, not separately. For example, a sharp horizontal curve beginning near a crest can create a serious accident hazard.A vehicle trave
5、ling in a curved path is subject to centrifugal force. This is balanced by an equal and opposite force developed through cannot exceed certain maximums, and these controls place limits on the sharpness of curves that can be used with a design speed.Usually the sharpness of a given circular curve is
6、indicated by its radius. However, for alignment design, sharpness is commonly expressed in terms of degree of curve, which is the central angle subtended by a 100-ft length of curve. Degree of curve is inversely proportional to the radius.Tangent sections of highways carry normal cross slope; curved
7、 sections are super Provision must be made for gradual change from one to the other. This usually involves maintaining the center line of each individual roadway at profile grade while raising the outer edge and lowering the inner edge to produce the desired super is attained some distance beyond th
8、e point of curve. If a vehicle travels at high speed on a carefully restricted path made up of tangents connected by sharp circular curve, riding is extremely uncomfortable. As the car approaches a curve, super begins and the vehicle is tilted inward, but the passenger must remain vertical since the
9、re is on centrifugal force requiring compensation. When the vehicle reaches the curve, full centrifugal force develops at once, and pulls the rider outward from his vertical position. To achieve a position of equilibrium he must force his body far inward. As the remaining super takes effect, further
10、 adjustment in position is required. This process is repeated in reverse order as the vehicle leaves the curve. When easement curves are introduced, the change in radius from infinity on the tangent to that of the circular curve is effected gradually so that centrifugal force also develops gradually
11、. By careful application of super along the spiral, a smooth and gradual application of centrifugal force can be had and the roughness avoided.Easement curves have been used by the railroads for many years, but their adoption by highway agencies has come only recently. This is understandable. Railro
12、ad trains must follow the precise alignment of the tracks, and the discomfort described here can be avoided only by adopting easement curves. On the other hand, the motor-vehicle operator is free to alter his lateral position on the road and can provide his own easement curves by steering into circu
13、lar curves gradually. However, this weaving within a traffic lane (but sometimes into other lanes) is dangerous. Properly designed easement curves make weaving unnecessary. It is largely for safety reasons, then, that easement curves have been widely adopted by highway agencies.For the same radius c
14、ircular curve, the addition of easement curves at the ends changes the location of the curve with relation to its tangents; hence the decision regarding their use should be made before the final location survey. They point of beginning of an ordinary circular curve is usually labeled the PC (point o
15、f curve) or BC (beginning of curve). Its end is marked the PT (point of tangent) or EC (end of curve). For curves that include easements, the common notation is, as stationing increases: TS (tangent to spiral), SC (spiral to circular curve), CS (circular curve to spiral), and ST (spiral go tangent).
16、On two-lane pavements provision of a wilder roadway is advisable on sharp curves. This will allow for such factors as (1) the tendency for drivers to shy away from the pavement edge, (2) increased effective transverse vehicle width because the front and rear wheels do not track, and (3) added width
17、because of the slanted position of the front of the vehicle to the roadway center. For 24-ft roadways, the added width is so small that it can be neglected. Only for 30mph design speeds and curves sharper than 22does the added width reach 2 ft. For narrower pavements, however, widening assumes impor
18、tance even on fairly flat curves. Recommended amounts of and procedures for curve widening are given in Geometric Design for Highways.B GradesThe vertical alignment of the roadway and its effect on the safe and economical operation of the motor vehicle constitute one of the most important features o
19、f road design. The vertical alignment, which consists of a series of straight lines connected by vertical parabolic or circular curves, is known as the “grade line.” When the grade line is increasing from the horizontal it is known as a “plus grade,” and when it is decreasing from the horizontal it
20、is known as a “minus grade.” In analyzing grade and grade controls, the designer usually studies the effect of change in grade on the center profile.In the establishment of a grade, an ideal situation is one in which the cut is balanced against the fill without a great deal of borrow or an excess of
21、 cut to be wasted. All hauls should be downhill if possible and not too long. The grade should follow the general terrain and rise and fall in the direction of the existing drainage. In mountainous country the grade may be set to balance excavation against embankment as a clue toward least overall c
22、ost. In flat or prairie country it will be approximately parallel to the ground surface but sufficiently above it to allow surface drainage and, where necessary, to permit the wind to clear drifting snow. Where the road approaches or follows along streams, the height of the grade line may be dictate
23、d by the expected level of flood water. Under all conditions, smooth, flowing grade lines are preferable to choppy ones of many short straight sections connected with short vertical curves.Changes of grade from plus to minus should be placed in cuts, and changes from a minus grade to a plus grade sh
24、ould be placed in fills. This will generally give a good design, and many times it will avoid the appearance of building hills and producing depressions contrary to the general existing contours of the land. Other considerations for determining the grade line may be of more importance than the balan
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