Each span consists of two reinforced-concrete slabs or girders, each slab forming half the width of the floor and having a curb wall to retain the ballast. Drain-holes are cored in the slabs, and steel stirrups are embedded in them, with the loops projecting so as to give a direct attachment for chains in handling.
The concrete used for the trestles is of different com position for different parts of the work. For the piles is used a 1:3 mixture of cement and fine screened gravel, or a 1 :2:3 mixture using sand and fine stone screenings. The concrete for the Chenoweth piles is made compara tively dry, while that for the cast piles is made very wet. For the caps and girders, the mixture is as follows: with gravel (pit or screened gravel, according to service) ; 1:2:4 with sand and stone. A 2-in. size is the maximum for both stone and gravel. The concrete is well spaded at the sides of the forms, in order to give a smooth sur face. The quantities and weights are about as follows: The piles are allowed to season for about 30 days, the girders for 60 days, before handling. The caps are cast in place, as already described, and are allowed to season for 30 days before the girders are placed upon them.
For structures of over five or six spans in length, longitudinal rigidity is obtained by the use of double bents at suitable intervals. The double-bent consists of two rows of piles carrying a single cap of twice the usual width. In the Salt River trestle (see Plate 8), a solid pier was used in place of the piles and cap at every sixth bent, but the double-bent construction is considered pref erable. Where longer spans are used (about 25 ft.), and where the trestles cross streams in which floating ice has to be resisted, thin piers are used in preference to the pile bents. These are carried down to footings on a solid •Each slab forms half the width of one span.
foundation, or may be supported by wooden or concrete foundation piles.
For the construction of the first of these structures, falsework was used. Wooden piles were driven at each side, in line with the old bents, and the 16-ft. stringers were swung round to form 16-ft. caps for a temporary floor or deck. This enabled the old structure to be re moved entirely, and the concrete trestle built, without interference with traffic. In later work, the floor of the
timber trestle is partly dismantled, and the concrete piles are driven to form bents intermediate with the old timber bents. The forms for the caps are then erected, and the concrete caps are cast in place, without interfering with traffic. This concrete is allowed to set for about 30 days. A part of the timber structure is then torn out by a wrecking crane or derrick car, and the same machine sets the concrete girder slabs in place upon the concrete caps. The track ties are supported temporarily on 6-in. blocking on the floor, ready for ballasting.
The longest structure is 435 ft. long; the maximum height, 18 ft. from ground to rail level.
Viaducts. Viaducts are long bridges, of many spans, and may be divided into two classes: (1) high structures, usually carrying railway traffic; (2) viaducts carrying a roadway or street, often with street-car tracks and sidewalks, over rivers or railroad yards. There are notable ex amples of both of these kinds of viaducts built entirely of reinforced concrete.
From the point of view of the concrete con structor, these structures may be distinguished according to the type of span employed— namely: girder viaducts and arch viaducts. The discussions on the design and construction of gir der and arch bridges already given will apply to these two kinds of viaducts, but descriptions of one or two examples will prove interesting be cause of the magnitude of the structures.
The Richmond and Chesapeake Bay Railway Viaduct, illustrated in Plates 9 and 10 (A) is an example of the girder type, and its design and erection form one of the most wonderful accom plishments in reinforced concrete construction in existence to-day. This viaduct is 2,800 feet long, and varies in height from 18 ft. at either end to 70 feet at the highest point.
The viaduct was designed to carry a train of cars, each 54 ft. long over all and weighing 150,000 lbs., on four-wheeled trucks placed 33 ft. c. to c. The wheels on each truck were 7 ft. on centers, and thus the nearest wheels of the adjoining cars were 14 ft. c. to c.