GIRDERS AND TRUSSES 123 



goes to each support. If the load is uniformly distributed one- 

 half a panel load will be concentrated at /, but this is carried 

 directly on the supports and has no effect on the stresses in the 

 framework. 



In Fig. 78 (c) two more panels have been added. If the load is 

 on the top chord the coefficients are as follows: 



If the load is on the bottom chord : 



W = o = i /0=f 



ad=\ ef = \ gh = i 



Merely for illustration the end panels have been completed by 

 dotted lines. The coefficient for hi = f (that is, it carries the reac- 

 tion). The difference, f - | = \ at h, acts vertically and creates 

 no stress in the truss. The member gi carries no load when the 

 weights all act vertically, but in case of wind or rolling loads caus- 

 ing horizontal or diagonal action on the frame there will be com- 

 pression on the member gi at the end where the load is applied 

 and tension in the same member at the opposite end. When the 

 vertical post hi is omitted, the end h rests on the abutment and 

 the truss is said to be suspended. 



Fig. 79 shows the development of the Howe truss, which is merely 

 the Pratt truss inverted. The verticals are in tension and the 

 diagonals are in compression. The Pratt truss is usually the more 

 economical and may be built of metal, or of metal and wood. The 

 Howe truss is usually a combination of metal for tension members 

 and wood for compression members. For maximum economy in 

 metal trusses the compression members should be as short as 

 possible, so the Howe truss is not well adapted for all metal 

 construction. 



Coefficients for the Howe truss are written as explained for 

 the Pratt truss, with the stresses reversed in kind. The middle 

 vertical, however, is opposite in character as affected by the load. 

 That is, when the load is on the lower chord the coefficient - 1, 

 but when it is on the upper chord the coefficient - 0. Practically, 

 however, in the latter case the vertical does carry a portion of the 

 weight of the lower chord in the middle panel. This is very small 

 and the smallest sized rod used will more than take care of it. 



A coefficient represents the proportion of panel load carried 

 by the member on which it is written. Coefficients are used only 



