WIND LOAD STRESSES 51 



The stress diagram is constructed in the same manner as that for dead 

 loads. Heavy lines in truss and stress diagram indicate compression, 

 and light lines indicate tension. 



The ambiguity at joint P z is removed by means of the dotted mem- 

 ber as in the case of the dead load stress diagram. It will be seen that 

 there are no stresses in the dotted web members in the right segment 

 of the truss. It is necessary to carry the solution entirely through the 

 truss, beginning at the left reaction and checking up at the right re- 

 action. It will be seen that the load P has no effect on the stresses in the 

 truss in this case. 



Wind Load Stresses: Rollers. Trusses longer than 70 feet 

 are usually fixed at one end, and are supported on rollers at the other 

 end. The reaction at the roller end is then vertical the horizontal com- 

 ponent of the external wind force being all taken by the fixed end. The 

 wind may come on either side of the truss giving rise to two conditions ; 

 (i) rollers leeward and (2) rollers windward, each requiring a separate 

 solution. 



Rollers Leeward 'The wind load stresses in a triangular Pratt 

 truss with rollers 'under the leeward side are calculated by graphic 

 resolution in Fig. 33. 



The reactions in Fig. 33 were first determined by means of force 

 and equilibrium polygons, on the assumption that they were parallel to 

 each other and to the resultant of the external loads. Then since the 

 reaction at the roller end is vertical and the horizontal component at the 

 fixed end is equal to the horizontal component of the external wind 

 forces, the true reactions were obtained by closing the force polygon. 



In order that the truss be in equilibrium under the action of the 

 three external forces R lf R 2 and the resultant of the wind loads, 

 the three external forces must meet in a point if produced. This fur- 

 nishes a method for determining the reactions, where the direction and 

 line of action of one and a point in the line of action of the other are 

 known, providing the point of intersection of the three forces comes 

 within the limits of the drawing board. 



