118 



PRACTICAL STRUCTURAL DESIGN 



reinforced concrete. The moment diagram for obtaining the 

 lengths of cover plates may be used without change for determin- 

 ing rivet spacing by adopting a scale for the vertical lines pro- 

 portioned to the ratio the total stress bears to the moment. By 

 total stress is meant the product of the area of the flange angles 



plus the area of the 



generally made with a " belly," for the material can be distributed 

 at will since the beam is cast in a mold. This effects some saving 

 in the cost of patterns and castings. Plate girders are sometimes 

 made in this form, a familiar example being the main girders 

 along the under side of railway cars. The stress equals the mo- 

 ment at any point divided by the depth of the beam at that 

 point. 



Trussed Beams 



The trussed beam shown in Fig. 76 is the most simple form of 

 truss. One-half of a uniformly distributed load is assumed to be 

 concentrated over the strut in the middle, so, letting 



W 



P= 

 2 



P L PL 



