30 



PRACTICAL STRUCTURAL DESIGN 



Example. Compute the moments at the points shown in 

 Fig. 23. 



1000 x 6 x 4 

 5x5 



1000 x 8 x 2 

 5x5 



= 960 



640 



1000 x 7 x 3 

 5x5 



1000 x 9 x 1 

 5x5 



= 840 



360 



The parabola in Fig. 23 was constructed by using perpendiculars 

 computed as shown and the curve drawn with a French curve. 



The middle perpendi- 

 cular line was divided 

 into 10 equal parts 

 and the ordinates to 

 the major axis mea- 

 sured off on the hori- 

 zontal lines to check 

 the accuracy of the 

 curve. This is recom- 

 mended as an exercise 

 for the student. 



A graphical method 

 for constructing a pa- 

 rabola is shown in Fig. 

 24. The perpendicular 

 representing the bend- 

 ing moment at the 



Fig. 23 



Ordinate Method for Constructing 

 Parabola 



middle of the beam is 

 set up and a rectangle 

 drawn, with a height equal to the bending moment and a width 

 equal to the span. The horizontal lines of the rectangle are 

 divided into any number of equal parts and the vertical end lines 

 into half this number. In the example the horizontal lines are 

 divided into 8 parts and the vertical lines into 4 parts. From the 

 apex radiating lines are drawn to the end divisions and vertical lines 

 are drawn through the horizontal divisions. A curve is drawn 

 through the intersections of the radiating and vertical lines. 



All the common methods in use for drawing parabolas have 

 been given in order that the students may have a choice of methods 

 as well as to show that in even the most ordinary matters there 

 are several ways of accomplishing a result. The man who knows 



