GIRDERS AND TRUSSES 133 



sion. Sometimes no intermediate rafters are used, the roofing 

 being carried by purlins resting on the joints. 



To obtain proper results the sloping rafter of the truss is divided 

 into equal spaces and verticals are dropped to the bottom chord 

 (or tie). Braces extend from the foot of one vertical to the top of 

 another. If, through any error or because it is considered best, 

 intermediate purlins rest on the truss rafters, or the roof is carried 

 directly on these rafters, it will be necessary to design them to 

 carry the bending stress in addition to the direct compression. 



In the King truss, Fig. SOA (a), the load, P, when applied at 

 the upper vertex causes no stress in the rod BB. When applied at 

 the lower end the stress is tension and equal to the load. The stress 

 AB = hah* the load, so the coefficient = \. The stress in the 



, , , . j , ,- , , , j half the span , , , . 

 horizontal tie rod = half the load x - > that is 



height 



P L__PL 



" 2 X 2d 4d 



In the Queen truss the action resembles an arch in that the com- 

 pressive and tensile stresses increase towards the supports in the 

 rafters and tie, and the stresses in the verticals and diagonals 

 decrease toward the supports, for the inclined rafters carry part of 

 the shear. Half of the load on each end panel is carried by the 

 abutments and creates no stress in the truss. 



In Fig. 85A (6) a load is assumed to be applied at the upper end 

 of BC. If the load is at the lower end the rod BC carries this 

 load to the rafter at the vertex of the triangle. If the load is 

 applied directly to the rafter at the vertex there is no stress in BC. 

 This will not again be referred to, as it applies to the rod in the 

 end triangle in all trusses. Half the load is carried on.AB and half 

 goes down CD to the tie. The load on the top of the truss is in- 

 creased by the load coming to the tie by the braces CD on both 

 sides of the center; therefore it is 2P, if the load on a joint is called 

 P. The vertical rod, DD, however, carries only the load at the 

 lower end. This load 2P from the top of the truss is carried half- 

 way down the truss to the joint and there it has added to it the 



P * 



load -~ of the brace AB, this brace therefore carrying a load 



m 

 op 



- The tie rod is in tension by an amount 



3P half the span T 3P jk SPL 

 2 : height ' 2 X 2d " 4d ' 



