The Victoria Tubular Bridge (1859) — Wrought Iron
For four decades after 1859, the Victoria Tubular Bridge had been a great success, both in terms of carrying rising rail traffic and fostering industrial activity in Montreal. The new double-track bridge was built with a steel truss that provided carriageways — a novelty for Montreal. Truss design had been greatly improved and made more economical by the availability of large, thick plates and angles. Furthermore, after 1865, steel production by Bessemer converters provided ductile and 50 per cent stronger metal at much lower cost.
For truss technology, as for the sheet and rib box girder, rivets were essential features in an age without heavy rolled I-beams, welding or high-strength bolts. Rivets were installed by hot-heading the straight end in the joint. The iron (or, in later years, the steel) was heated to between 900 and 1,000ºC, primarily to facilitate hammering the rivets tightly into the holes. In subsequent cooling, the contraction of the rivets developed sufficient clinching force to create water-tight joints. When building trusses, each joint was designed with a sufficient number of rivets to become as rigid as the plates and angles. Hot riveting, which required teams of four men to install 270 rivets per shift, was eventually rendered obsolete by welding or by uniformly tightened bolts.
Although steel had been made for cutting tools, its production volume increased rapidly after 1865 with Bessemer converters and after 1895, by means of open hearths. The cast steel ingots were much larger than big bundles of wrought iron muck bars and much less homogeneous. Nevertheless, most steels, such as austenite, could be hot worked as easily as wrought iron; hot working was conducted in the range 1,100 to 800ºC, below which pearlite formation caused hardening. The hot rolling technology for wrought iron was transferred easily to steel. Transition to steel in any application progressed according to the advantages it conferred and was completed by 1890 for ships with lighter hulls and for rails with much longer life. The first all-steel bridges were constructed in 1878 and in 1884 for Canada. In Britain, from 1850 to 1900, puddled wrought iron production fell from three to 0.5 million tons per year, while steel rose from 0.06 to 4.9 million tons per year. In Canada, the first mills in Sydney, Nova Scotia, and Sault Ste. Marie, Ontario, were inaugurated in 1901, thus a little late for the truss bridge.
The steel Pratt trusses on the Victoria Truss Bridge were 40 metres high by eight metres wide and were able to accommodate continuous rail traffic in both directions, as well as roadways on each side. Relative to the old tube, the trusses had four times the carrying capacity and weighed only 2.5 times more, at a cost of $2 million on the same piers (compared to $7 million for the tubular bridge, including piers). To reduce any traffic interruptions to less than two hours, the old tube was not dismantled until the new bridge was built around it; a framework was advanced span by span, rolling on the old bridge. Under chief engineer J. Hobson, the north half of the new bridge was built by Dominion Bridge and the south half by Detroit Bridge, with completion in 1898.
Because of its significance to trans-Canada rail freight, the bridge underwent another significant change to accommodate the St. Lawrence Seaway at its south end. One lift bridge replaced the first span and a second one was located at the opposite end of the lock. A spur was built in the bridge by replacing two spans with under-track girders without stopping traffic; the spur had six spans welded from rolled sections. The road traffic, accommodated at the spur by a fly-over, continues to be heavy despite the building of three road bridges.