224 



PRACTICAL STRUCTURAL DESIGN 



a truss, for it is merely necessary to classify the web framing and 

 follow the methods given for that special framing. 



The shed-roof truss (Fig. 147) is so called because it slopes one 

 way, like the roof of a lean-to shed. In this type it is necessary to 



set off the whole 

 load line, as though 

 the roof had unequal 

 reactions. Notice 

 the difference in 

 the stresses in the 

 member QR and 

 the member OP. 



The scissors truss 

 shown in Fig. 148 

 is a very common 

 Fig. 146. -Hog Back Truss. tmss> ft f avori te 



with many builders and draftsmen. Setting off the loads on 

 the load line it is very quickly discovered that to make the 

 force diagram close it is necessary to commence with the load 

 on joint 3, instead of either joint 2 or joint 5. Completing 

 the diagram, which has to be done by drawing the dotted lines 

 aj and fj, it is discovered that all the members are in compression. 

 The dotted lines aj and fj represent the thrust of the truss against 

 the walls or tops of the buttresses. The dotted line qj repre- 

 sents the tension required to resist the thrust, consequently the 



Fig. 147. Shed Roof Truss. 



pull in a rod which can be run from joint 1 to joint 6 and convert 

 the diagonal thrust into a vertical reaction. 



A vertical rod may be used to connect joints 3 and 4, and this 

 will render the rod from 1 to 6 unnecessary and will convert the 



