TYPES OF HORIZONTAL CURVESThere are four types of horizontal curves. They aredescribed as follows:1. SIMPLE. The simple curve is an arc of a circle(view A, fig. 11-2). The radius of the circle determinesthe sharpness or flatness of the curve.2. COMPOUND. Frequently, the terrain willrequire the use of the compound curve. This curvenormally consists of two simple curves joined togetherand curving in the same direction (view B, fig. 11-2).3. REVERSE. A reverse curve consists of twosimple curves joined together, but curving in oppositedirection. For safety reasons, the use of this curveshould be avoided when possible (view C, fig. 11-2).4. SPIRAL. The spiral is a curve that has a varyingradius. It is used on railroads and most modemhighways. Its purpose is to provide a transition from thetangent to a simple curve or between simple curves ina compound curve (view D, fig. 11-2).ELEMENTS OF A HORIZONTAL CURVEThe elements of a circular curve are shown infigure 11-3. Each element is designated and explainedas follows:PI POINT OF INTERSECTION. The point ofintersection is the point where the back and for-ward tangents intersect. Sometimes, the pointof intersection is designated as V (vertex).I INTERSECTING ANGLE. The intersectingangle is the deflection angle at the PI. Its valueis either computed from the preliminarytraverse angles or measured in the field.A CENTRAL ANGLE. The central angle is theangle formed by two radii drawn from theFigure 11-2.—Horizontal curves.11-2

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