GRAPHIC STATICS 



213 



bers. These forces set up strains in the members. A strain is the 

 amount of deformation caused by the action of an external 

 force and all internal stresses are caused by strains. From this 

 point the word " force " will be used instead of the word " stress." 



The loads on the panel joints are found by assuming an area 

 equal to the width of a panel and a length equal to the spacing 

 between trusses. This is based on each truss carrying the load 

 on a distance measured halfway between adjacent trusses and 

 each panel in a truss carrying a load on a width measured half 

 way between adjacent verticals. Half the load on the end panels 

 of a truss is carried by the wall. 



Fig. 128 shows a truss with a middle vertical and two side 

 verticals, together with one-half the force diagram. The con- 



Fig. 128. Forces by Graphic Statics in Queen Truss with Center Rod. 



struction is not different from the trusses already illustrated, but 

 attention is called to the fact that the side verticals, in the recipro- 

 cal diagram, end at the tie rod. The middle vertical, on the con- 

 trary, ties the truss together speaking in a general way so 

 it is measured from G to G'. The student should be able without 

 trouble to trace the reciprocals on Fig. 128. 



Fig. 129 illustrates a truss with a cambered tie rod. Notice 

 that the loads are plotted in order on the reciprocal diagram and 

 the sloping lines transferred as before. From the middle point 

 there are two reaction lines, representing the camber in the tie 

 rod, instead of one horizontal line. The side verticals end on the 

 lines representing the tie rod, but the middle vertical goes across 

 the open space to. tie the two main rafters together in the top 

 chord and carry the stress to the tie rod. If this vertical line 



