28.6
縦断曲線の紹介
縦断曲線は、高速道路や鉄道の異なる勾配を結ぶ放物線状の遷移で、後方接線と前方接線の間の滑らかな配置を保証します。後方接線は最初の勾配を表し、前方接線はその後の勾配を定義します。これらの曲線は、接線の長さが等しい対称曲線、または長さが異なる非対称曲線にすることができます。縦断曲線を定義する重要なポイン…
Vertical curves provide smooth transitions between varying grades on highways and railroads, improving safety and comfort.
These curves are parabolic and can be symmetrical or nonsymmetrical, depending on the lengths of the tangents.
They connect the back tangent, representing the initial grade, to the forward tangent, representing the following grade.
The transition begins at the Point of Vertical Curvature or P.V.C., where the back tangent meets the curve. The curve rises or falls gently, guided by its geometry, and eventually connects to the forward tangent at the Point of Vertical Tangency or P.V.T.
The point where the extensions of the back and forward tangents intersect is called the Point of Vertical Intersection or P.V.I.
Tangent offsets, the vertical distances from tangents to the curve, help calculate elevations and define the curve's profile.
At the curve’s midpoint, the elevation is halfway between the P.V.I. and the midpoint of a straight line joining the P.V.C. and P.V.T.
Vertical curves are carefully designed to ensure smooth alignment, proper sight distance, and balance between cutting and filling, enhancing the functionality of roadways.
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Q1: What is the purpose of vertical curves on highways and railroads?
Vertical curves provide smooth transitions between varying grades on highways and railroads, improving safety and comfort for travelers. These parabolic curves connect the back tangent, representing the initial grade, to the forward tangent, representing the following grade, ensuring proper alignment and sight distance.
Q2: What are the key points that define a vertical curve?
A vertical curve is defined by three critical points: the Point of Vertical Curvature (P.V.C.) where the back tangent meets the curve, the Point of Vertical Tangency (P.V.T.) where the curve connects to the forward tangent, and the Point of Vertical Intersection (P.V.I.) where the extensions of both tangents intersect.
Q3: How do tangent offsets help in vertical curve design?
Tangent offsets are vertical distances measured from the tangents to the curve, essential for calculating elevations and defining the curve's profile. These offsets vary with the square of the distance from tangency points, allowing engineers to determine elevation of intermediate points on vertical curves and verify calculations using parabolic properties.
Q4: What is the difference between symmetrical and nonsymmetrical vertical curves?
Symmetrical vertical curves have equal tangent lengths on both sides, while nonsymmetrical curves have varying tangent lengths. The choice between them depends on the specific grade conditions and design requirements, allowing engineers to optimize alignment and earthwork balance for different roadway situations.
Q5: How is the elevation calculated at the midpoint of a vertical curve?
At the curve's midpoint, the elevation is determined by averaging the elevation at the Point of Vertical Intersection (P.V.I.) with the midpoint of a straight line connecting the P.V.C. and P.V.T. This property simplifies geometric layout calculations and ensures consistent parabolic curve geometry throughout the design.
Q6: What factors must be balanced when designing vertical curves?
Vertical curve design balances sight distance requirements, optimal earthwork considerations, and suitable grade selections. Engineers adjust curve length and gradients to ensure alignment efficiency, safety, and economical roadway construction while meeting modern transportation infrastructure demands and field procedure for staking out curves.
Q7: Why are parabolic properties important for vertical curve verification?
Parabolic properties are essential because tangent offsets vary predictably with the square of distance from tangency points, and second differences remain equal at evenly spaced stations. These mathematical relationships allow engineers to verify curve calculations and ensure accurate elevation profiles throughout the vertical alignment.