johnwood1946

New Brunswick History and Other Stuff

The Great Saint John Steel Cantilever Bridge

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By JohnWood1946@hotmail.com

The Saint John River is old, geologically speaking. It has had ample time to carve a wide flat plane serving a large drainage area and to be edged by levees and bayou swamps, for example. The mouth of the river at Saint John is very different, however. Glacial deposits cut off the river’s natural course and forced it through a narrow gorge at the Reversing Falls. The Reversing Falls feature is therefore young, geologically speaking.

The Reversing Falls was a logical place to build a bridge because the river was narrower there than at other locations. On the other hand, the deep rapids required that such a bridge have a long span and that there be no piers in the river. Thus it was a considerable time after settlement before a bridge was built.

The first successful crossing was completed in 1853 when the first wire cable suspension bridge in Canada was built to carry a road over the Falls. This soon became insufficient, however, as the ‘great mechanical blessing of steam’ was changing transportation. By the 1880s railways were being built at a feverish pace and, as far as that industry was concerned, the Reversing Falls remained an encumbrance to commerce. On the east side of the Saint John River was the Intercolonial Railway which ran to Shediac; and on the west side was the New Brunswick Railway extending to Vanceboro with connections to U.S. destinations. Both of these lines were built by the European and North American Railroad between the 1860s and 1871. A rail crossing of the Reversing Falls was needed.

The St. John Bridge and Railway Company was incorporated on 25 March 1881 to build a rail bridge across the Falls. This was opposed by some legislators but was supported by Premier William Elder and others. The St. John Bridge and Railway Extension Company was also incorporated in 1881 to build the rail line from Fairville across the bridge to the Intercolonial station on Mill Street. The President of these companies was Thomas R. Jones. James Murray Kay was Treasurer and T.B. Robinson was Secretary. Frederick E. Barker, Payson Tucker, John H. Parks, Robert Robinson, and Arthur Sewal were also recognized as principal organizers on a plaque commemorating the completed bridge. Peter Archibald, Chief Engineer of the Intercolonial Railway, was charged with arranging for design and construction.

 

Hon. William Elder, Premier 

Peter Archibald wrote a specification for the new bridge. The specification was dated June 20, 1883, wherein Archibald envisaged four truss spans over the river. The main span was to be 414 feet long and the other three were to be 50, 75 and 100 feet in length. The tender allowed bridge companies to submit alternative proposals, however, and the Dominion Bridge bid was for a double cantilever truss with a suspended span. The 414 foot span would have required falsework support during construction and this was not possible over the rapids.

Peter Archibald’s specification was competent for its day, though antique by later standards. It had been the practice for a long time to leave details of design to bridge builders who, in earlier days, had been artisans rather than engineers. Early general specifications were written in 1871 by Clarke, Reeves and Company of Pennsylvania and in 1873 by George S. Morison of the Erie Railroad. Peter Archibald’s specification was therefore up to then-modern standards by taking responsibility to specify minimum technical requirements.

Archibald’s design loading was not up to current standards, however. He indicated that the train load was to be 2,500. lbs./ft. with two locomotives of obscure description superimposed on top of this. Dominion Bridge substituted a more modern and heavier configuration with locomotive axle loads, followed by a uniform load of 4,000 lbs./ft.

The final design was by John Abbott, President of Dominion Bridge, and his Assistant Engineer W. Bell.

Double Cantilever Bridge with a Suspended Span

Note that West is on the right.

The main part of the bridge was a double cantilever truss with a suspended span, consisting of three separate but interconnected structures. The truss at each end was supported at its center point and at one end only, so that the other end cantilevered over the abyss. The cantilever ends then supported another truss over the Falls, and this truss was the ‘suspended span’. The cantilevers were prevented from overturning by large anchors at their landward ends which kept them from lifting upward.

It was determined late in the design that one of the piers needed to be relocated for better rock support. The design was then revised to be non-symmetric with the westward cantilever being 380 ft. 4 ins. long while the eastward cantilever remained 285 ft. 3 ins. long. The suspended span was 142 ft. 7-1/2 ins. long so that the whole arrangement was just over 808 feet in length. The overall length of the bridge was just over 1,203 feet (367 metres) with the addition of the eight-span approach trestle at the west end.

The West Approach Trestle

The trusses were also ‘pin connected’ since their parts were connected together by large-diameter pins rather than by rivets. All of the tension members were steel bars 8, 9, and 10 inches wide by over an inch thick; while the compression members were riveted together using steel channels and plates to form box sections. The members were arranged to maximize the number of tension members and to minimize the number of compression members. This made fabrication and erection simpler. The connecting pins were around four to six inches in diameter.

Steel for the bridge was fabricated at the Dominion Bridge plant in Lachine which had opened in 1882. The Reversing Falls job was their first major project and was followed by construction of the C.P.R. bridge over the Saint Lawrence River in Lachine in 1886. Steel came mostly from the Steel Company of Scotland, with lesser amounts from the Aachener Works in Germany. The Masonry substructures were built by M.J. Hogan of Quebec. Steel erection began on April 9, 1885 and was completed on July 9, 1885. The first train crossed the bridge on July 20, 1885 and the bridge was tested on July 31st. The official opening was on October 1, 1885.

Dominion Bridge’s design loading of locomotive axle loads followed by a uniform load of 4,000 lbs./ft. was typical for its time. However, locomotives were becoming larger at a rapid pace and this design loading soon became inadequate. Cooper’s ‘E-loading’ is a method of comparing different loadings, and the Dominion Bridge design was equivalent to between E26 and E34 for different parts of the bridge. Many railway bridges were actually carrying E40 or more by 1920, and the industry standard for design had become E50.

Another indication that the bridge was light was the test result of July 31, 1885 when a deflection of four inches was recorded at the center of the suspended span. This is only an indirect indication of strength, but was about twice what would be permitted today.

It is therefore not surprising that the bridge required upgrading in around 1905 and was replaced by the present railroad bridge in 1921. Very many bridges across the continent also needed rebuilding as locomotive weights increased faster than anyone had imagined possible. The present bridge is also a double cantilever truss with a suspended span.

Four significant bridges have been built over the Reversing Falls at Saint John and this double cantilever with a suspended span was the second of these. It was a significant accomplishment and contributed to the development of the city and the province. The Engineers’ Club of Philadelphia dubbed it “The great Saint John steel cantilever bridge”.

 References:

McFarlane, H.W. and K.H. Lawson, The Reversing Falls Bridges, possibly in draft form, imprint 1987.

Nason, David, Railways of New Brunswick, New Ireland Press, 1992.

Hannay, James, History of New Brunswick, Volume 2, Saint John, N.B., 1909, p. 339, 40.

C.P.R. Engineering Department history files and drawings (various), Calgary, Alberta.

The Engineers’ Club of Philadelphia, Proceedings, Volume 5, pages 181, 82.

Wood, John, Actual Loads Applied to Railroad Bridges and Bridge Design Loads, 1830-1905, American Railway Engineering and Maintenance of Way Association, 2009.

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Written by johnwood1946

July 13, 2011 at 3:02 PM

Posted in Uncategorized

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