二級(jí)公路設(shè)計(jì)專業(yè)外文翻譯-公路隧道-文庫吧
2025-04-17 17:20 本頁面
【正文】 uter edge and lowering the inner edge to produce the desired super elevation is attained some distance beyond the 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 unfortable. As the car approaches a curve, super elevation begins and the vehicle is tilted inward, but the passenger must remain vertical since there is on centrifugal force requiring pensation. 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 elevation takes effect, further 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. By careful application of super elevation along the spiral, a smooth and gradual application of centrifugal force can be had and the roughness avoided. 河南理工大學(xué)本科畢業(yè)設(shè)計(jì) 專業(yè)外文翻譯 3 Easement curves have been used by the railroads for many years, but their adoption by highway agencies has e only recently. This is understandable. Railroad trains must follow the precise alignment of the tracks, and the disfort described here can be avoided only by adopting easement curves. On the other hand, the motorvehicle operator is free to alter his lateral position on the road and can provide his own easement curves by steering into circular 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 circular 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 of 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 mon notation is, as stationing increases: TS (tangent to spiral), SC (spiral to circular curve), CS (circular curve to spiral), and ST (spiral go tangent). On twolane 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 because of the slanted position of the front of the vehicle to the roadway centerline. For 24ft roadways, the added width is so small that it can be neglected. Only for 30mph design speeds and curves sharper than 22176。 does the added width reach 2 ft. For narrower pavements, however, widening assumes importance even on fairly flat curves. Remended amounts of and procedures for curve widening are given in Geometric Design for Highways. 2. Grades The 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 of 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 th
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