234 



DEAD LOADS FOR BRIDGES. 



Art. 125. 



constants which vary with the style of bridge, the loading, the 

 specifications, etc. 



In highway bridges, the dead load consists of the floor and 

 its support, the joists, the car tracks (if any), handrailing, and 

 the remainder of the steel work. Ordinary country highway 

 bridges have plank floors, the planks being laid transversely on 

 longitudinal beams called joists, placed two to three feet apart. 

 The joists rest on transverse beams called floor beams, which 

 are suspended from the trusses at the panel points or riveted 

 to the posts. City bridges frequently have paved floors, the 

 paving being supported by the joists through the means of 

 buckle plates, reinforced concrete slabs, brick or concrete arches, 

 etc. The floor being designed first, its weight may be calcu- 

 lated. The weight of the floor beams, trusses and bracing is 

 estimated from a similar bridge or by means of an empirical 

 formula. 



The panel load for dead load is equal to the uniform dead 

 load per foot per truss multiplied by the panel length. In pony 

 trusses (121) this may be considered as all applied at the lower 

 or loaded chord panel points, but when horizontal bracing is 

 used in the plane of both chords the dead load should be divided 

 between them. It is sometimes assumed that two-thirds of the 

 dead load acts at the " loaded chord " and one-third at the 

 " unloaded chord." 



After all the stresses are calculated and the design is fin- 

 ished, a careful estimate of the weight should be made from the 

 design. If the total suspended weight differs materially from 

 the dead load assumed in the original calculations, the stresses 

 and the design should be revised. 



126. Live Loads. For railway bridges the live load is 

 usually specified as a series of wheel loads representing two 

 locomotives, followed by a uniform train load (see Chapter XIII), 

 but an " equivalent uniform live load " is sometimes used (142). 

 For highway bridges both concentrated and uniform moving loads 

 are usually specified. 1 The loads for which to design highway 

 bridges are not nearly so definite as those for railway bridges. 

 The floor system is usually designed for a road roller or traction 

 engine and the trusses for a uniform moving load per foot. The 



Cooper's General Specifications for Steel Highway Bridges and 



Viaducts. 



