GRAPHIC STATICS 



235 



o4- The intersection of these lines fixes the location of the re- 

 sultant wind force, which acts normally to the lie ou. 



The roof is assumed to be fast at the supports, so the resultant 

 wind force multiplied by y and divided by x gives the amount 

 of # 2 - If one end of the roof is free the reactions are found as in 

 Fig. 153 and Fig. 154, after finding the amount and location 

 of the resultant as just shown. 



The remainder of the process for ascertaining the forces in 

 the members is exactly as shown in other cases, the only difference 

 being that the load line is 

 parallel to the resultant of 

 the wind pressure. The dia- 

 gram will appear to be com- 

 plicated but it only needs care 

 and patience to make it right. 

 Some of the lines on the truss 

 diagram are very short and it 



may be advisable to plot them A ' 

 with a protractor, or compute 

 their direction by using tables, 

 as it is difficult to transfer a 

 short line and draw a long 

 line parallel with it. 



Cantilever Trusses 

 In Fig 157 is shown a can- 

 tilever truss that appears to 

 be a favorite with examiners, 

 for it is found in many exami- 

 nation papers. In this par- 

 ticular example the right re- 

 action is zero. Sometimes this 



Fig. 157 Cantilever Truss 



roof is shown with the left support under joint 7. Sometimes 

 the loads are varied so there is a negative reaction on the 

 right, which means the end of the truss must be tied down. 

 Sometimes there is a small positive reaction at the right, which 

 is ignored if the forces are small in the members. This roof is 

 for a grand stand. In a cantilever truss the loads at the ends 

 are used, whereas they are ignored in ordinary trusses because 

 there they are carried directly by the walls. 



