CHAPTER XI. 

 BRIDGE TRUSSES. 



120. Construction. Bridge trusses differ from roof trusses 

 in form and in the character of the loads which they have to 

 carry. In the past, in America, the principal consideration in 

 selecting types of bridge trusses has been economy in first cost. 

 Little attention was given to the aesthetic side of the question. 

 But recently more thought has been given to this subject, espe- 

 cially for bridges in the more thickly populated districts of our 

 country. Frequently, a structure of pleasing outline and propor- 

 tion can be constructed which will cost no more than some un- 

 gainly structure which will forever be an eyesore. 



The primary loads on bridge trusses are moving loads, 

 and these cause the stresses in certain members to reverse and 

 render certain forms of truss desirable for the usual materials 

 of construction. 



The loads on bridge trusses are carried to the panel points 

 by the floor joists or stringers, so that in all except a few old 

 types now out of date, no loads are appiled to the trusses be- 

 tween the panel points. In the truss of Fig. 148, unequal loads 

 at D and F are produced in connection with the dead load 

 when the live load covers a part of the span. As shown, a 

 diagonal member DE would be in tension, but if the load 

 P 2 were greater than P 1; a diagonal member CF would be 

 in tension. 



Members made of eyebars or rods have practically no stiff- 

 ness and can not, therefore, carry any compressive stress. If 

 such members are used for the above diagonals, there will be two 

 in the panel ; one acts when P t is the greater load, and the other 

 when P 2 is the greater. Both diagonals can not be in action at 

 the same time and, therefore, the truss is always an assemblage 

 of triangles. If both diagonals were made stiff members so that 



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