GRAPHIC STATICS 229 



of wind on a number of forms of roof trusses would make it appear 

 complicated. A simple truss is sufficient. The principles are 

 as easy to grasp as any of the work in graphic statics and the 

 earnest student can go through all the trusses illustrated in this 

 chapter and make diagrams for the effect of wind on each truss. 



First obtain the direction of the resultant of the wind and the 

 directions of the reactions due to wind. Draw a triangle repre- 

 sent ing this. From the intersection where the reactions meet, 

 draw a reaction line, representing the forces in amount and 

 direction on the lower chord. On the inclined wind resultant 

 set off the wind load on each joint and from this load line draw 

 lines parallel to the members of the truss and complete the force 

 diagram. The force diagram for wind differs from that in which 

 vertical loads are considered, merely by having the load line 

 inclined and not vertical. All the other lines are parallel to the 

 truss members. 



Tables of stresses must be made for roof trusses when all loads 

 are separately considered. Such a table will have a number of 

 columns ruled on lined paper. Each system of loading will have 

 two columns, one for tension and one for compression. The 

 columns are as follows, from left to right : 



1. Designation of members between joints. 



2. Dead load on top chord (+) and (-). 



3. Snow load (+) and (-). 



4. Wind load (+) and (-). 



5. Uniform load on lower chord (+) and (-). 



6. Trolley, or other, moving loads (+) and (-). 



7. Total loading (+) and (-). 



Each member shown in the last column to have both tensile 

 and compressive forces to resist is designed accordingly. 



In Fig. 151 at (a) is shown a roof truss and the graphical 

 method for finding the reactions and the wind stresses, with the 

 two ends of the truss secured to the supports. First, referring to 

 (a), the reactions are found by multiplying the length of the slope 

 on one side by the distance between trusses, to obtain the area 

 acted upon by the wind. This area is multiplied by the wind 

 pressure per square foot. It acts at the center of area, as shown 

 by the arrow. 



To obtain the reactions graphically, prolong the line of the 

 wind resultant through the truss, and the length, ab, represents 



