212 



PRACTICAL STRUCTURAL DESIGN 



From E' to E the arrow point is away from the joint, so the stress 

 is tensile. From E to D the arrow point is towards the joint, so 

 the stress is compressive. 



The truss is symmetrical with symmetrical loads, so having 

 obtained the stresses and their character in the members on one 

 side of the center, as well as in the center vertical member, all 

 the required information has been found. 



In Fig. 127 is shown a Queen truss with its reciprocal diagram. 

 The truss is symmetrical with symmetrical loading, so the end 

 reactions are equal. It is only necessary to draw one half of the 

 stress diagram, for there is no center vertical member. ABC I A 



k- -9000 - 



Fig. 127. Forces in Queen Truss by Graphic Statics. 



is the reciprocal for the joint between A and B. BCEDB is the 

 reciprocal for the joint between B and D. ICE HI is the recip- 

 rocal for the joint CEHI. DFGHED is the reciprocal for the 

 joint EGH. FF'GF is the reciprocal of the joint between F and 

 F'. The point on the load line is below the point I, a distance 

 equal to FI. 



In the examples shown no account has been taken of wind. 

 The reactions are equal and when wind is considered as acting 

 normal to the surface of the roof the reactions are unequal. 

 Therefore until the graphical method for obtaining reactions 

 with unequal loading is shown, we will assume the wind to be 

 reduced to a load acting vertically. For slopes not exceeding 

 30 degrees from the horizontal this will not make much difference 

 and is fairly common practice. To avoid confusing the student 

 vertical loading will be considered first and after illustrating the 

 work on the forms of trusses in common use the general method 

 for obtaining stresses caused by wind will be taken up. 



The word " stress " has been used because it is so commonly 

 used in connection with the members of a truss. It is not strictly 

 correct. " Force " is the proper word and forces act on the mem- 



