224 



MODERN BRIDGE TRUSSES. 



Art. 123. 



The web bracing may be of either the Warren or the Pratt 



type. 



Fig. 162. 



As the span increases, economic construction requires the 

 depth of the truss to increase. The panel length must also in- 

 crease if an economical inclination of the diagonal members is to 

 be had; but there is a limit also to the panel length. This diffi- 

 culty was avoided in the Whipple truss by running the diagonals 

 over two panels. The same object is accomplished by putting a 

 sub-truss in each panel of a Pratt truss to carry every other panel 

 load to the main truss. 



This forms the Baltimore truss which was introduced in 1871. 

 (Fig. 163.) It is a sub-trussed Pratt truss in which the stresses 

 are staticallv determinate. 



BALTIMORE TRUSS 



Fig. 163. 



As in the case of the Pratt truss, it is economical to have a 

 curved upper chord for very long spans as shown in Fig. 164, and 

 then it is called the Pettit truss. 



PETTIT TRUSS 



Fig. 164. 



