238 



PRACTICAL STRUCTURAL DESIGN 



that the supports are at the same level and remain there. If 

 one support settles the stresses are increased enormously. It is 

 customary to use the "Three-moment Theorem" in dealing 

 with continuous beams, but it is rather involved and a great many 

 men do not take the trouble to investigate ,-eiiref ully the mo- 

 ments on continuous beams. The following graphical method 

 was proposed many years ago by T. Claxton Fidler in "A Prac- 

 tical Treatise on Bridge Construction." Another graphical 

 method is demonstrated in DuBois "Graphical Statics" and 

 Church's "Mechanics of Engineering." 



The Fidler method is known as the " Method of Characteristic 

 Points." Refer to Fig. 44 on page 49, where the effect of restrain- 



Fig. 160 A Graphical Treatment of Continuous Beams. Fidler method 



ing the ends of a uniformly loaded beam is discussed. To extend 

 this to a continuous girder uniformly loaded compute the bend- 

 ing moment on each span separately as though it were simply 

 supported at the ends. The bending /noment = WL -*- 8 and a 

 parabola must be drawn on each span with the middle ordinate 

 equal to the moment, as in Fig. 160. 



Divide each span into three equal parts and at the third points 

 erect vertical lines. Make each vertical line equal to two-thirds 

 the height of the parabola within which it is situated. Draw a 

 circle around the end of the line, the characteristic point be- 

 ing in the center of the circle. These characteristic points in 

 Fig. 160 are numbered from 1 to 8 inclusive. 



The end spans, of the series here shown, rest freely on the outer 

 end supports, hence characteristic points 1 and 8 are disregarded. 



