forces specified in 24 ; and a lateral force of 100 pounds 

 for each vertical lineal foot of the trestle bents ; or 



2d. With both tracks unloaded, a lateral force of 500 

 pounds for each longitudinal lineal foot of the structure, act- 

 ing at the centre line of the girders ; and a lateral force of 

 200 pounds for each vertical lineal foot of the trestle bents. 



26. For determining the requisite anchorage for a loaded 

 structure, the train shall be assumed to weigh only 1,100 

 pounds per lineal foot. 



27. The strains produced in the bracing of the trestle Longitudinal 

 towers, in any members of the trusses or in the attachments 



of the girders or trusses to their bearings, by the greatest 

 tractive force of the engines or by suddenly stopping the 

 maximum trains on any part of the work must be provided 

 for; the coefficient of friction of the wheels on the rails 

 being assumed as 0.20. 



28. Variation in temperature, to the extent of 1 50 degrees, Temperature. 

 shall be provided for. 



29. When the structures are on curves, the additional centrifugal 

 effects due to the centrifugal force of trains on each track 



shall be considered as a live load. It will be assumed to act 5 

 feet above base of rail, and will be computed for a speed of 

 60-30 miles per hour; D being the degree of curve ( 109). 



30. All parts shall be so designed that the strains coming 

 upon them can be accurately calculated. 



PROPORTION OF PARTS. 



31. All parts of the structures shall be proportioned in ivn*,ie strain. 

 >n by the following allowed unit strains, net sections: 



For Medium Steel. 



Floor beam hangers, when permitted ( 94) ..... 6,000 



Longitudinal, lateral and sway bracing, tor lateral 



*. 24,25^ ....... 18,000 



Longitudinal, lateral and sway brat, m^, |.,r live 



load I?, 29, 109).. >00 Medium SweU 



Solid rolled beams, used as cross floor beams and 



stringers (g 45, 49). . . 10,000 



