DISCUSSION. 



147 



stresses due to dead load, even if arrived at correctly, by the pro- 

 portion between the two classes of loading is certainly incorrect 

 as regards the bracing and verticals, though correct as regards 

 the booms. I cannot, without further data, give a decided opinion 

 as to the accuracy of the maxima stresses due to rolling load, such 

 as the traction engine mentioned, but if my surmise is correct, 

 that they have been calculated on the supposition that its weight 

 can be concentrated at each apex separately, they are inaccurate. 

 An item in the design, which I do not consider good practice, is 

 the fact that even supposing the stresses in the verticals to be 

 correct as given in the paper, viz : varying from 25 to 80 tons, 

 these members are made identical in section throughout the truss, 

 being four 2 J inch diameter rods. I am an advocate of uniformity 

 in section of groups of truss members within certain limits, though 

 the stresses in those members vary in amount, still I consider the 

 limits exceeded in this instance. The four rods, under a working 

 stress of 5 tons, will support 80 tons with a factor of safety of 4 ; 

 this factor would be unduly increased to more than 12 in the case 

 of the verticals, with 25 ton stress, which is not economy in 

 design. Appended will be found a check calculation by the method 

 of sections, of maxima stresses in the booms. 



Maximum Stresses in Booms by Method of Sections. 



X x (101) = 88-375 x 20 .-. X x = 167-95 = X a 



X z (24) = 88-375 x 60-251(40 + 20) .-. X 3 = 158-23 = X 4 

 From symmetry X 5 = X 6 = 158-23 

 X 7 = X 8 = 167-95 



20 lx oo.**, on r , Z 



Z x (101 



(20 2 + 10-5 



J} 1) = 88-375 x 20 



Z, x 18-5 = 88-375 x 40-251 x 20 



Z„ = 



190-04 = Z 8 

 163-78 = Z 7 



20-7 

 26-5 



= 88-375 



= 88 



•375 



40 - 251 x 20 



80 



251 



Z 3 = 146-37 = Z 6 

 Z± = 152-45 = Z, 



4 (60+40 + 20) 

 Comparative table of Maxima Stresses in semi-truss : 



Member. Stress as given. 



Z x ... 190-94 



Z^ ... 163-35 



Z* ... 131-80 



137-30 

 168-35 

 168-35 

 172-58 

 172-53 

 25-25 

 71-59 

 60-28 

 79-79 

 27-22 



^4 



X, 



x, 



x, 



v, 



v 2 

 v, 



Stress as proposed. 



190-0 



164-4 



147-0 



153-0 



168-0 



168-0 



159-0 



159-0 

 25-25 

 55-0 

 25-25 

 45-0 

 25-25 



