May 2008

Online Series

The metallurgical history of Montreal bridges

By H.J. McQueen

The Quebec Bridge (1917) — record cantilever in nickel steel

The complete bridge viewed from the North Shore, emphasizing the straight lower girder chords (compression) and upper forged eye-bars (tension). The K members in each bay minimized distortion and facilitated erection.

To link rail lines from Montreal’s South Shore, the Maritimes and the United States with Quebec City and with potential lines going northward, a business consortium engaged Phoenix Bridge Co. of Pennsylvania to design a bridge across the St. Lawrence. For a long and high crossing of the main shipping channel, a cantilever bridge was selected because of recent success with the similar span Forth Bridge in Scotland. The design of the bridge progressed only sporadically because of financial difficulties. The consulting engineer, T. Cooper (a prominent American railway bridge engineer in semi-retirement), chose a slender design and ordered it not to be changed, even though the main chords had not been re-dimensioned to support additional bracing.

In line with normal practices, large components of the bridge were shop-fabricated and shipped to the bridge site. The anchor span to Quebec’s South Shore was built on a trestle and the main span was gradually extended outwards from the tower. During the process of inserting rivets into holes that were being brought into alignment by gravity loading, it was noticed that some rivets were popping out and that some compression components appeared distorted. None of the engineers on the site had sufficient experience to decisively interpret these changes and T. Cooper did not respond promptly to telegrams. On August 29, 1907, the chords buckled near the pier and the bridge collapsed, killing 75 men.

The Canadian government launched a truly comprehensive investigation that uncovered extensive plastic deformation of girders  and shearing of rivets, but no brittle fracture. Calculation of the strength of the chords and crushing of a one-third scale model chord confirmed an under design of 25 per cent. World expert bridge designers made extensive recommendations for the design of a new bridge. A consortium comprised of the Dominion Bridge Co. (Montreal) and the Canadian Bridge Co. (Windsor) developed a novel K bracing for each panel that permitted the insertion of all rivets as each bay was added.

To diminish the dead load of the heavy girders, nickel steel with 40 per cent greater strength than normal bridge steel was employed for 70 per cent of the structure, thereby raising the cost by a factor of 2.5. The addition of 3 per cent nickel shifted the eutectoid point, so that the pearlite was finer and higher in volume than would be a similar carbon steel. Modern nickel steels with small additions of copper and molybdenum have double the strength of carbon steel.

At the time of construction of the Quebec Bridge in 1917, the Canadian steel industry that began operations in 1901 did not yet have the capacity to produce heavy plate and forgings; consequently, these were produced by Carnegie and Bethlehem Steels through an open hearth technique. In the final construction stage in 1916, a cast saddle fractured in the jacking up of the centre truss and dropped it into the river, killing 16 men. However, the cantilevers suffered no damage, so the centre truss was rebuilt and positioned the following year.

Because the railway traffic never grew to the expected extent, the Quebec Bridge now carries trains on a single track and a three lane roadway. The highly effective K design has held the worldwide cantilever record of 547 metres for over 90 years; this design was employed for the construction of Montreal’s Jacques Cartier Bridge in 1929 and of the third longest cantilever in Calcutta in 1945. In 1970, Canada’s longest suspension bridge was built along side the Quebec Bridge to serve as a six-lane autoroute.

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