Tub′u-lar bridge.
A bridge formed by a great tube or hollow beam, through the center of which a roadway or railway passes.
A similar principle — that of the truss-beam — had been applied, varying, of course, in details of construction, to timber bridges, both in
Switzerland and
America; but the idea of the wrought-iron tubular bridge, and the carrying out of the details of construction which made it a success, are due to
Fairbairn, to whom is due the credit of devising the form and proportions of the tubular bridges of
Conway and Menai.
The rectangular cellular beam laid on its edge and constructed of wrought and cast iron in their respective places was
Fairbairn's, as was also the idea of making it self-supporting.
Stephenson very reluctantly gave up the idea of supporting it by chains.
The most remarkable one ever constructed is that across the
Menai Straits, on the
Chester and
Holyhead line of railway, and which unites the island of Anglesea with the mainland of
Wales.
Plate LXXII.
is a general view of the
Britannia Bridge from the Caernarvon side, showing also the Menai suspension-bridge.
A mile distant from it is the famous
Telford suspension-bridge, built in 1829 by that prince of road-makers.
Telford died in 1834.
Several requirements were made in regard to the tubular bridge to be erected over the strait.
Trains of all kinds were to cross it at full speed.
The Lords of the
Admiralty stipulated that it should not be less than 105 feet above high-water level.
And that neither scaffolding nor centering should be used, as they would temporarily impede navigation at all points.
Stephenson, at first, designed a cast-iron bridge of two arches, in which the necessity for centering was obviated by connecting together the half-arches on each side of the center pier, on the principle suggested by
Brunel.
The two halfarches would thereby counterbalance each other, and be prolonged until they met their counterparts, which would spring from the abutments on each side of the straits.
This plan was rejected by the
Admiralty, as it gave the specified hight at the
crowns only of the arches.
The idea of the
beam was then conceived, and the question arose how to build it. The round, or elliptical, form naturally presented itself, and, after the spans were determined, the three towers were commenced, and an expert board was appointed to determine the shape and structure.
The board consisted of
Messrs. Fairbairn,
Hodgkinson, and
Clark.
The points determined were, —
The tube must resist compression on its upper portion and extension on its lower portion.
The material should be principally accumulated above and below, and the middle should be hollow, whether it was of iron or wood.
Several evident reasons indicated iron as the material, and it was eventually determined to run the trains
through the hollow iron beam instead of
upon it.
Experiments determined that, while a cylindrical tube, with a given weight, was rent asunder beneath, and an elliptical tube was crushed at the top, a rectangular tube was more rigid than either.
Then followed the discovery that, instead of riveting together
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2647]
layers of plates, the same amount of material in a cellular form would give greater strength; so the layers were divided, and the spaces between them subdivided, by upright plates, into cells extending throughout the top and bottom of the tube.
The plate-iron sides and angle-irons formed ribs and skin to the double spine.
Chains for assistant support were contemplated, but eventually discarded.
The tube was calculated to bear a distributed weight of 4,000 tons (2,240 pounds each), or 2,000 tons in the center; being 9 times the calculated load it would have to bear.
The piers are 3 in number, and the land abutments 2; the tubes 4, in appearance (or rather they appear continuous), but they were made in pairs, one for
up trains, the other for
down trains, so that there are actually 8 tubes, forming parallel bridges of 4 tubes each.
The tubes next to the shore, on each side, were built upon scaffolding, and are each 230 feet long.
The two spans in the middle of the strait are each 472 feet in length, and rest upon 3 towers or piers; the outer ones have each a base of 55 feet × 32 feet, and are 198 feet high.
Each contains 210 tons of cast-iron beams and girders.
The central, or “Britannia,” pier is 460 feet distant from its fellows, and is founded upon a rock.
It is 230 feet high, and has a base 62 × 52 feet. It weighs 20,000 tons (
English). Its exterior has 148,625 cubic feet of granite, filled in with 144,625 cubic feet of sandstone.
387 tons of cast-iron beams, or girders, are worked into it.
The tubes are composed of wrought-iron plates, from 3/8 to 3/4 of an inch thick, the largest being about 12 feet in length, strongly united by rivets, and stiffened by angle irons, and vary in exterior hight, which is 30 feet at the center of the bridge, diminishing to 22 feet 9 inches at the abutments.
Their exterior width is 14 feet 8 inches, or 13 feet 8 inches in the clear, inside.
The tubes were built upon platforms on the Caernarvon shore, and floated on pontons into spaces between the piers.
One side of a tube was floated into a vertical recess in one side of the tower formed for its reception, and then the other was floated down into a similar recess in the other tower, a portion of the lower masonry being omitted to allow, it to pass in. These vertical recesses in the sides of the tower formed guides for the ends of the tubes while being raised to their elevated position.
800 men were employed in the removal of a tube, which was deposited on ledges of masonry on the piers.
The raising of each tube was separately performed by means of powerful hydraulic presses.
The larger press had a cylinder 11 inches thick, the piston 20 inches in diameter, and a lift of 6 feet. The cylinder weighed 16 tons, the machine 40 tons (
English, 2,240 pounds). See
Fig. 2627, page 1156.
Two smaller presses had rams 18 inches in diameter, and acted at one of the tubes, while the larger one operated at the other end. They stood on the piers, and lifted by chains.
The chains and lifting-frames added 400 tons to the weight to be lifted.
The power was derived from 2
steam-engines of 40 horsepower each, delivering water through a 1/2-inch pipe into the cylinder below the ram. As the tube rose it was secured beneath.
It was raised by successive lifts of 6 feet each, each occupying 30 minutes. The ends rest on rollers so as to allow the tube to expand and contract without grinding the coping of the pier.
The bridge contains 9,480 tons of wrought-iron, 1,988 tons of cast iron, and 1,500,000 cubic feet of masonry.
Its cost was about $3,000,000. It was first crossed by a railway train, March 1, 1850.
Some other data are given under bridge (which see).
The Victoria tubular bridge at
Montreal, forming a part of the Grand Trunk Railway of
Canada, was designed by
Stephenson, and built under his direction by
James Hodges of
Montreal.
It was completed in December, 1859, and opened for travel August 25, 1860.
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Broom-handle lathe. |
The total length of this immense structure is only 176 feet less than two miles; it is supported upon two abutments and 24 piers, having 25 spans of tubing, the center one of which is 330, and each of the others 242 feet long.
The hight of the central tube is 60 feet above the summer level of the water.
The bridge has a slight descent from the center toward each end. The two center piers are 24 feet wide, the others being 16 feet, and each presents a wedge-shaped cutwater up stream to break the ice, which on the
St. Lawrence frequently attains a thickness of 3 to 5 feet, often becoming piled to the hight of 50 feet or more on breaking up in the spring.
The tubing of this bridge differs principally from that of the
Britannia Bridge, before described, in not being cellular at top and bottom.
Each plate and piece of iron was punched in
England before being sent to
Canada, so that little, except putting them together and placing them in position, remained to be done on the spot.
This, however, was a work of great labor, requiring the erection of a rigid timber stage or trussing supported by massive temporary piers of wood, on which the tubes were built up plate by plate.
These trusses were removed when the work was completed, leaving the tubes resting at each end on the piers and abutments alone.
Expansion and contraction are provided for by rollers underneath the end of each span.
The total cost was £ 1,250,000. The first stone of this bridge was laid July 22, 1854; and on December 17, 1859, the first train passed over.
