16 



SCIENCE. 



[N. S. Vol. XVI. No. 392. 



age represented was 117,245 miles. A large 

 majority of the engineers and bridge com- 

 panies that responded were in favor of 

 making detail plans. 



The shop drawings, which show the form 

 of the bridge, the character and relations 

 ( f all its parts, give the section and length 

 ( f every member, and the size and position 

 cf every detail whether it be a reinforcing 

 plate, a pin, a bolt, a rivet or a lacing bar. 

 All dimensions on the drawings are checked 

 independently so as to avoid any chance 

 for errors. The systematic manner in 

 which the drawings are made and checked, 

 and the thorough organization of every de- 

 partment of the shops, makes it possible 

 to manufacture the largest bridge, to ship 

 the pieces to a distant site and find on 

 erecting the structure in place that all 

 the parts fit together, although they had 

 not been assembled at the works. 



The constant improvement in the equip- 

 ment of the bridge shops, and the increas- 

 ing experience of the manufacturers who 

 devote their entire time and attention to 

 the study of better methods for transform- 

 ing plates, bars, shapes, rivets and pins 

 into bridges, constitute important factors 

 in the development of bridge construction. 



As the length of span for the different 

 classes of bridges gives a general indica- 

 tion of the progress in the science and art 

 of bridge building, the following refer- 

 ences are made to the longest existing span 

 for each class, together with the increase 

 in span which has been effected approx- 

 imately during the past decade. 



In plate girder bridges the girders, as 

 their name implies, have solid webs com- 

 posed of steel plates. A dozen years ago 

 but few plate girders were built whose 

 span exceeded 100 feet, the maximiun 

 span being but a few feet longer than this. 

 To-day such large girders are very fre- 

 quently constructed, and the maximum 

 span has been increased to 126 feet be- 



tween centers of bearings. This is the 

 span of the large plate girders of the 

 viaduct on the Riverside drive in New 

 York City, erected in 1900. The longest 

 railroad plate girder was erected about 

 the same time on the Bradford Division 

 of the Erie Railroad, its span being 125 

 feet 24 inches. The heaviest plate girder 

 is the middle one of a four-track bridge 

 on the New York Central Railroad erected 

 last year near Lyons, N. Y. Its weight is 

 103 tons, its span 107 feet 8 inches, and its 

 depth out to out 12 feet 2 inches. 



The large amount of new construction 

 and the corresponding increase in the 

 weight of the rolling stock have combined 

 to secure a more extensive adoption of plate 

 girders and the designs of many new de- 

 tails for them. These affect chiefly the 

 composition of the flanges, the web splices, 

 the expansion bearings and the solid floor 

 system. Although solid metal floors built 

 up of special shapes were first introduced 

 into this country fifteen years ago, their 

 general adoption has taken place largely 

 within the past decade on account of their 

 special adaptation to the requirements of 

 the elevation of tracks in cities. Solid 

 floors may not only be made much shal- 

 lower than the ordinary open type, thereby 

 reducing the total cost of the track eleva- 

 tion, but they also permit the ordinary 

 track construction with cross-ties in ballast 

 to be extended across the bridge, thus 

 avoiding the jar which otherwise results 

 as the train enters and leaves the bridge, 

 unless the track is maintained with ex- 

 traordinary care. 



The necessity for bridges of greater stitf- 

 ness under the increased live loads has also 

 led to the use of riveted bridges for con- 

 siderably longer spans than were in use six 

 or seven years ago. The use of pin-con- 

 nected trusses for spans less than about 150 

 feet is undesirable for railroad bridges, on 

 account of the excessive vibration due to 



