120 RAILK<UD LOCATION AND CONSTRUCTION — MACKENZIE. 



Where piles are driven under abutments on land, 12 inches of 

 concrete is placed around pile-heads, 12"xl2" caps are then put 

 on, and a course of timber laid close, to carry the masonry. 



In calculating the weight on the earth under the abutment, 

 draw lines from outer edges of bed-plates on an angle of 30 

 degrees with the vertical, down to the foundation, and assume 

 that the live and dead loads, and also the weight of the masonry 

 within those lines are all concentrated on the space between the 

 points where these lines meet the foundation. This length mul- 

 tiplied hy the thickness of the wall, or width of grillage, etc., gives 

 the number of square feet over which the whole weight is sup- 

 posed to be distributed. 



The weight of rolling-stock has so increased in the last twenty 

 years that iron bridges are now being removed and replaced by 

 heavier steel structures, all over the country. The designer has 

 now to ask himself : " Am I designing for five, ten, or for fifty 

 years ?" If a thoughtful man, he will not sail too close to the 

 wind in proportioning his bridges, but provide a margin to meet 

 future increased weight of rolling-stock. 



Types. — For spans up to 15 ft., use rolled beams. 



" from 15 to 80 ft., use plate-girders 8 ft. c. toe. 



" from 80 to 100 ft., riveted Warren girders. 



" from 100 to 150 ft., single intersection pin- 



connected Pratt trusses, with parallel or 

 arched top chord, 14 ft. to 16 ft. clear 

 width inside. 

 " from 150 to 550 ft., double intersection pin- 



connected Pratt trusses, with parallel or 

 arched top chord. 

 Up to 225 ft., 14 ft. clear width min. Max. 16 ft. 

 225 to 320 ft., 18 ft. center to center. 

 320 to 420 ft., 21 ft. 

 420 to 550 ft., 25 ft. 



