An investigation was made in 1901 by a com mittee of the Railway Engineering Association in regard to the present practice respecting the degree of completeness of the plans and specifi cations furnished by the railroads. It was found that of the 72 railroads replying defi nitely to the inquiry, 33 per cent prepare 'plans of more or less detail, but sufficiently full and precise to allow the bidder to figure the weight correctly and if awarded the contract to at once list the mill orders for material"; 18 per cent prepare "general outline drawings showing the composition of members, hut no details of joints and connections"; while 49 per cent pre pare 'full specifications with survey plan only, leaving the bidder to submit a design with his If, however, the comparison be made on the basis of mileage represented by these 72 railroads, the corresponding percentages are 48, 24 and 28 respectively. The total mileage rep resented was 117,245 miles. A large majority of the engineers and bridge companies that responded were in favor of making detail plans.
The shop drawings, which show the form of the bridge, the character and relations of all its parts, give the section and length, of every member and the size and position of every de tail, 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 department of the shops, make 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 equipment of the bridge shops and the increasing experi ence of the manufacturers who devote their entire time and attention to the study of better methods of transforming plates, bars, shapes, rivets and pins into bridges, constitute import ant factors in the development of bridge con struction.
In 1911-15 a new tendency in bridge con struction was shown in the introduction of various alloy steels of considerably higher strength than the grade of structural steel which has been the recognized standard during the previous decade. Nickel steel was used for some members of the Municipal Bridge at Saint Louis and the new Quebec cantilever bridge, Mayan steel in the second Memphis cantilever bridge, high carbon steel in the Hell Gate Arch and the continuous truss bridge at Sciotoville, and silicon steel in the Metropolis Bridge.
As the length of span for the different classes of bridges gives a general indication of the progress in the science and art of bridge building, the following references are made to the longest existing span for each class.
In plate girder bridges the girders, as their name implies, have solid webs composed of steel plates. The longest plate-girder span is 130 feet 6% inches between centres of bearings, was erected in 1916 at 79th street, Chicago, where the Nickel Plate Railroad crosses the tracks of the Illinois Central Railroad. The bridge is a part of the scheme to eliminate grade crossings at Grand Crossing. Previous to that the longest span was 128 feet 4 inches, located on the Mahoning division of the Erie Railroad and erected in 1902. The longest ones in a highway bridge are those of the via duct on the Riverside Drive in New York, erected in 1900, the span being 126 feet. The heaviest plate girder is in the skew span of the Boston & Albany Railroad where it crosses the New York, New Haven & Hartford Railroad at Mass. A single girder weighs 170 tons, is feet long and 10% feet deep. In 1901 the heaviest girder weighed 103 tons.
The large amount of new construction and the corresponding increase in the weight of rolling stock have combined to secure a more extensive adoption of plate girders and the de signs of many new details for them. These affect chiefly the composition of the flanges, the web splices, the expansion bearings and the solid floor system. Solid bridge floors, in which the ordinary ballasted track is carried by trans verse metal troughs, were introduced in 1887, while those in which reinforced concrete was first used were built in 1903. Recent develop ment in solid floors relates mainly to those types in which steel beams supply the necessary strength and concrete protects the beams and supports the ballast. Solid floors may not only be made much shallower than the ordinary open type, thereby reducing the total cost of the track elevation, but they also permit the ordi nary 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 extraordinary care.
The necessity for bridges of greater stiffness under the increased live loads has also led to the use of riveted bridges for considerably longer spans than were in use formerly. The use of pin-connected trusses for spans less than about 150 feet is undesirable for railroad bridges, on account of the excessive vibration due to the large ratio of the moving load to the dead load or weight of the bridge itself.