RAILROAD BRIDGES 35 



Members shall be packed on pins, using filling rings where 

 necessary, in a manner that will prevent any lateral move- 

 ment. 



Pins shall be long enough to insure a full bearing of all the 

 parts connected upon the turned body of the pin. They shall 

 be secured by chamfered nuts or be provided with washers if 

 solid nuts are used. The screw ends shall be long enough to- 

 admit of burring the threads. 



147. Pin Holes. Pin holes shall be reinforced by plates 

 where necessary, and at least one plate shall be as wide as the 

 flanges will allow and be on the same side as the angles. They 

 shall contain sufficient rivets to distribute their portion of the 

 pin pressure to the full cross-section of the main member. 



148. Bolts. Where members are connected by bolts the 

 turned body of these shall be long enough to extend through 

 the metal. A washer at least J in. thick shall be used under the 

 nut. Bolts shall not be used in place of rivets except by special 

 permission. Heads and nuts shall be hexagonal. 



For anchor bolts see Par. 46. 



149. Column Bases in Trestles. (See Par. 101.) 



150. Camber. Ordinary truss spans shall be given a camber 

 by making the panel length of the top chords, or their horizontal 

 projections, longer than the corresponding panels of the bottom 

 chord in the proportion of J in. in 10 ft. For truss spans of 

 unusual length or loading, draw spans, cantilevers, etc., the 

 camber shall be obtained from the calculated distortion of the 

 various members under their assumed stress. 



151. Bearings. Bearing plates may be of cast steel or built 

 up. All bearing plates, built-up pedestals and built-up bolsters 

 or shoes shall be so designed and set upon the masonry that the 

 load will be distributed over the entire bearing area without 

 causing any of the parts to be overstressed. 



Movable bearings shall be designed to permit motion in one 

 direction only; fixed bearings shall be firmly anchored to the 

 masonry. 



Bridges on inclined grade without pin shoes shall have the 

 sole plates beveled so that the masonry and expansion surfaces 

 may be level. 



152. Temperature. Provision shall be made for a free expan- 

 sion and contraction of all bridge structures to the extent 



