RAILROAD CURVES 179 



TABLES FOR TRANSITION SPIRALS 



The following tables contain the data required for the 

 laying out of eleven different spirals. The unit degree of spiral 

 is marked at the top of each table. The column headed / 

 contains the length, in feet, between the P. Si and the points 

 on the spiral, and the one headed d gives the degrees of curve 

 of spiral at these points. The third column gives the corre- 

 sponding deviation angles; the fourth the deflection angle; and 

 the remaining columns give the values of the spiral offset F, 

 the coordinate y, and the corrections x and t, all in feet. As 

 an illustration of the use of these tables, let the preceding 

 example be solved by means of them. Since a = f , reference 

 is made to the table for a = 30', where it is found that for 

 / = 400 ft., the corresponding value of ^ = 2.33, and that of 

 I cor. = .03. Then, as before, F = 2.33 ft. and CV= 199.97 ft. 



LAYING'OUT A SPIRAL IN THE FIELD 



Let RT and R'T, in the accompanying illustration, be the 

 two tangents that are to be connected with the circular curve 



AB by the two spirals CA and C'B. It will be assumed that 

 the two spirals are of equal length. 



Compute the unit degree of curve of spiral, the spiral offset 

 VE= V'E', and the distance CV = CV f , or obtain these quan- 

 tities with the help of the tables and compute the distance 



