PART II.] APPLICATION TO BRIDGES. 85 



f the working strain in tons per sq. foot of cross-section. 

 (See Stoney, Theory of Strains, vol. ii., p. 441.) 



"We have also the rule : " Multiply the distributed load in 

 tons by 4 ; the product is the weight of the main girders, end- 

 pillars and cross-bracing in pounds per running foot." Iron is 

 taken at 5 tons per sq. inch tension, and 4 tons per sq. inch 

 compression. 



2d. The moving or live load / which is determined by the 

 purpose of the bridge. This load can take various positions 

 upon the bridge, and may even be divided into several por- 

 tions. It is therefore an important problem to determine that 

 distribution which shall cause the maximum strains. 



The live load is, as the term implies, in motion, so that, in 

 combination with the deflection, there is a centrifugal force, 

 or increase of pressure. This is, however, in practice disre- 

 garded, while such a coefficient of safety is chosen in propor- 

 tioning the parts, that the increase of strain due to this cause is 

 fully covered. 



3d. Horizontal forces, caused by the wind and the passage 

 of loads. 



4/i. Pressures at the supports. The known forces cause re- 

 actions at the supports, which evidently must also be considered 

 as forces acting upon the bridge girder. For straight girders, 

 these reactions are vertical, while in suspension and arch sys- 

 tems they are inclined. 



67. Bridge Loading. The heaviest load to which a railway 

 bridge can be subjected is when it is covered from end to end 

 with locomotives. " The standard locomotive is assumed to be 

 24 feet long, and to have six wheels with a 12-foot base ; to 

 have half its weight resting on the middle wheels, and one- 

 fourth on the leading and trailing pairs respectively, which are 

 supposed to be at equal distances on either side of the middle 

 wheels." (See Stoney, vol. ii., p. 405.) The standard engine 

 is assu'med to weigh 24 tons. 30 tons and 32 tons, according to 

 the construction. This makes the standard load 1 ton, 1 \ ton, or 

 \\ ton per foot of single line. Short bridges of less than 40 

 feet span must be considered as subject to concentrated loads 

 from single engines. 



The maximum load for public bridges is recommended by 

 Stoney at 100 Ibs. per sq. ft. 



6. In the Straight Truss all the Outer Forces act In a 



