CHAPTER XII. 



STRESSES IN SIMPLE BRIDGE TRUSSES FROM UNIFORM 



LOADS. 



125. Dead Loads for Bridges. The dead load con- 

 sists of the weight of the structure itself. It is estimated by 

 comparison with similar structures which have been built or 

 from empirical formulas. The dead load is usually assumed to 

 be uniformly distributed over the length of the bridge. In a 

 simple bridge truss (one supported at its ends), the chord mem- 

 bers increase in weight toward the middle, while the web mem- 

 bers increase toward the ends. The weight of the floor and 

 floor system is uniform longitudinally except the weight of the 

 floor beams, which is concentrated at the panel points. The 

 wind bracing increases in weight toward the ends. Thus it is 

 seen that the assumption of uniform load for the dead load is 

 not far wrong. The construction reduces the entire dead load 

 to concentrated loads at the panel points of the trusses, and 

 the assumption of its uniform distribution makes these panel 

 loads equal when the panels are equal. 



The dead load is estimated in two parts, the weight of the 

 floor and the weight of the steel work. The floor as here con- 

 sidered does not include any of the supporting steel work. For 

 ordinary railroad bridges it includes the rails, ties, guard-rails 

 and their fastenings, and the steel work consists of the weight of 

 the trusses, the floor system and the bracing between the trusses. 

 The floor system consists of the stringers supporting the ties, 

 and the floor beams supporting the stringers. The floor beams 

 are usually riveted to the posts. For unusual- constructions and 

 for highway bridges the weight of the floor may be easily cal- 

 culated. 



The weight of all the steel work must be estimated from 

 a similar bridge whose weight is known. For ordinary bridges 

 of a given type and loading, an empirical formula of the form 

 w=aL+b is often used for estimating the weight of the steel 

 work per foot of bridge. L is the span length and a and 6 are 



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