GRAPHIC STATICS 



231 



WIHD 



In Fig. 153 the roof is assumed to rest at one end on rollers, 

 in order to take care of temperature changes, which, in trusses 

 secured at both ends, often cause tremendous changes in the 

 stresses. The re- 

 action at the free 

 end is vertical and 

 the wind is from the 

 fast end. At the 



t 



free end drop a ver- 

 tical line. Through 

 the center of area 

 on the windward 

 side of the roof 

 draw a line, normal 

 to the slope, down- 

 ward to an inter- 

 section with the 

 vertical reaction 

 line. The point of 

 intersection is then 

 connected to the 

 fast end by a line, 

 which gives the di- 

 rection of result- 



Fig. 



153. Wind Pressure on Roof -wind on Fast 

 Side. 



ant Hi. 



At (6) is the force diagram. First draw a load line parallel 

 to the wind resultant and lay off the amount of wind at each 

 end joint and at each joint on the truss. From the ends draw 

 lines parallel to the two reaction lines. This forms a triangle 

 alf, the side fl being equal to #3, and the side al being equal to 

 Ri. The remainder of the diagram is readily drawn, all the lines 

 on the windward side being parallel to the members of the truss, 

 with the load line (the wind) inclined. 



In Fig. 154 is shown the method to use when the wind is blow- 

 ing from the free end towards the fast end. No explanation is 

 required for this figure if the explanations given for Fig. 153 are 

 understood. 



The free end of a truss may rest on steel plates instead of 

 rollers. The only difference between this method and that when 

 the free end rests on rollers is that the reaction under the free 



