Fig. 1.—Grand Trunk Railway bridge across the Trent Valley Canal at Nassau, Ont. Central Bridge & Engineering Co., Peterborough, Ont., builders.
| June 30, 1898 | Engineering News. |
The Trent Valley Canal, now in process of construction by the Dominion of Canada, is projected to extend from Georgian Bay through the province of Ontario to Lake Ontario, and is expected to be of great public value as a waterway. Its construction has naturally demanded numerous high-level and swing bridges. Several of these were erected during the past year by the Central Bridge & Engineering Co., of Peterborough, Ont., and the purpose of the present paper is to describe certain peculiarities in the construction of the two more important of these, in the design and detail of which the present writer was directly interested.
Fig. 1.—Grand Trunk Railway bridge across the Trent Valley Canal at Nassau, Ont. Central Bridge & Engineering Co., Peterborough, Ont., builders.
These were the swing bridges to carry the Grand Trunk Ry. over the canal at Nassau, Ont., and the Canadian Pacific R. R., over the canal at Ashburnham, Ont. The former has a clear span of 217½ ft. c. to c. of end lifts, and the latter a span of 187 ft. c. to c. of end lifts. Both were of the same general design, riveted lattice trusses with minor differences due to the different lengths of span, and the idiosyncrasies of the men who framed the new Canadian tariff. When the material for the long span was ordered angle were most economical; when we came to detail the short span the tariff had ade it preferable to use channels. Both were figured for the loadings given under class II of the 1896 specifications of the Department of Railways and Canals, viz.: the dead load of the spans themselves, cross ties, rails, etc., at 00 lbs. per lin. ft. of span, and a rolling load of two 112-ton locomotives with a uniform train load of 3,000 lbs. per lin. ft. For the longer span this gives a loading on the turntable, when the bridge is swinging, of about 800,000 lbs. The general style of construction is shown in the diagram, Fig. 1, and need not detain use except to say that all connections were riveted with the exception of the top laterals and the pin connections for the eye-bars and sway-rods connecting the trusses to the central tower. The peculiarities of the construction were three: The turntable center, the central tower, and the end lifts. The design of the latter is the especial property of Mr. W. H. Law, at that time the engineer and manager of the company. The device is based on the use of the toggle-joint, is very simple to construct, and most effective in operation.
Central tower.—In most swing bridges of ordinary types, whether rim or center bearing, we have to do in the ultimate analysis with beams of complete or partial continuity, and have to take care of shearing stresses transmitted across pivot or drum, and provide special devices to prevent hammering of the truss ends. In the bridge under consideration the rolling load can produce stresses only in the span on which it may be; and the trusses when closed many be figured as simple spans resting on their own supports and completely discontinuous. The turntable is surmounted by a braced tower, Fig. 2, on which rests forged steel links turning on 4 15-16-in. pins, and themselves carrying similar pins to receive the ends of the eye-bars. When the bridge is closed these eye-bars can receive no stress; when the bridge swings, the trusses are simply hung by them to the central tower—a form of construction most simple, effective and economical, easily computed and most practicable in the shop.
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Fig. 2.—Center tower construction for Grand Trunk Ry. bridge over the Trent Valley Canal.
It is quite possible, of course, that by some accident or other—a knock from a boat, say—the links at the top of the tower might be drawn over so far to one side as to fail to return to their normal position when the bridge is swung back to its position when closed. To obviate any mishap of this kind, diagonal sway rods 1 in. square are introduced extending from the pins at the hip to the central tower. Here they connect to 2-in. pins which travel in slotted holes 4 ins. in length, giving each pin a movement of 1 in. each way from the center. These rods only come into play in the case of accident to the links, and are emergency safeguards and wind braces.
The central portal is double, as shown; one set of bracing acting with the links, the other set giving rigidity to the tower; the whole forming very efficient protection against accidents common to canals as well as against high winds.
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Fig. 3.—Turntable and center pivot construction for Grand Trunk Ry. bridge over Trent Valley Canal.
Turntable center.—This was designed for the express purpose of reducing shop cost by keeping the radial girders of full depth throughout their length. The load from the bridge is delivered to the drum by 16 radial girders which receive it from 8 bearing beams—that is, from 8 points of support. The turntable is combined rim and center bearing—and 250,000 lbs. reach the center, while 550,000 lbs. go to the 36 rollers. The center, Fig. 3, of cast-iron or steel, terminates in its own pin, and the form of construction reduces somewhat the amount of power required to turn the bridge; with this additional feature that the necessity of using bolts is entirely done away. The steel center plate was riveted o the cast-iron center in the shop and the field riveting was then easily done without any special danger to the center.
The whole structure as thus designed merits attention from the manufacturer's standpoint, and its description may be of use on the further perfecting of shop detail, most centers being an outgrowth from the design of locomotive turntables, while this is an original creation out of hand.
*Draftsman, Carnegie Steel Co., Pittsburg, Pa.