28 



PRACTICAL STRUCTURAL DESIGN 



A number of curves are used by engineers and scientists, the 

 most useful of which, and the only one used by structural de- 

 signers, is the parabola. This curve is formed by cutting a section 



,u- a-r J 

 Fig. 21 Section of Cone Developed into Parabola 



through a cone parallel to the slope, as shown by the line EF 

 in Fig. 21. The major axis in all curves is designated by the letter 

 A and the minor axis, perpendicular to the major axis, by the small 

 letter a. The axes of a parabola are infinite, so for this curve A 

 designates height and a one-half the base. 



x 



L. 



Fig. 22 The Parabola and its Equations 



In Fig. 22 let X and x = the abscissas and Y and y the ordinates 

 of the parabola. The abscissas are parallel to and the ordinates 

 are perpendicular to the major axis. Assuming X and Y each 



