FIGURE 19. Parabolic arc.
r = rate of change in grade = 0.02 percent per
foot; L = (Gb - G 0 )Ir = [1.8 - (-4.6)]/0.02 =
320 ft (97.5 m).
- Locate the PC and PT
The station of the PC = station of the PI -
'L/2 = (54 + 20) - (1 + 60) = 52 + 60; station
of PT = (54 + 20) + (1 + 60) = 55 + 80; ele-
uT^TTTn?FIGURE IA 20. T Tangent offset. «? vation of PC = elevation of PI - GJ./2 =
2% 3Q + Q 046(16Q) = 3Q3 66 ft (92 £ 6 m);
elevation of PT - 296.30 + 0.018(160) =
299.18 ft (91.19Om).
- Use the average-grade method to find the elevation of
each station
Calculate the grade at the given station; calculate the average grade between the PI and
that station, and multiply the average grade by the horizontal distance to find the ordinate.
Equations used in analyzing a parabolic arc are
y= ^- +Gs (41)
G = rx + G 0 (42)
y = (Ga + G)| (43)
DT=-^y (44a)
