210 



PRACTICAL STRUCTURAL DESIGN 



wind load will also act at the joints, but not vertically, this, how- 

 ever making no difference in the method of treatment, for the 

 wind loads on the joints will be parallel, although not vertical. 

 In an earlier chapter the wind effect on roofs was assumed to be a 

 vertical load, but when finding the stresses in a roof truss by 

 graphic statics one diagram is drawn for the vertical loads and 

 a separate diagram for the effect of wind. Stresses are tabulated 

 for each system of loading and added. There are several "com- 

 bined " methods for making a single diagram serve for the vertical 

 loads and wind loads, but they are not easy to remember and the 

 separate diagrams cannot be forgotten once they are mastered. 



Stress 



Diagram 



Fig. 126. Forces in King Truss by Graphic Statics. 



In Fig. 126 is shown a king truss with the reciprocal diagram. 

 The loads are marked at each joint. The two end loads are carried 

 directly by the walls, so do not affect the stresses in the members 

 of the truss. The spaces between members are lettered. A verti- 

 cal line is drawn on one side of the truss. This is usually on the 

 right side, but it may be on the left if most convenient. Begin- 

 ning at the top, all the loads are set off as shown. The loads are 

 between the lettered portions of the members, so the spaces on 

 the vertical load line drawn to scale represent the loads between 

 the letters. After setting off all the loads the lines of the truss 

 members are transferred by means of triangles, or parallel rule, 

 to connect with the load line. This gives the sloping lines meet- 

 ing on the line CF. 



The construction here described is for a truss uniformly loaded 

 with equal reactions. The amount of the reaction at each end 



