Quebec Bridge over St. Lawrence River

(Historic Bridges)

3D Satellite

It is a cantilever bridge with a suspended span. As is typical now, the suspended span for the 1916 design was built on shore, floated out into position, and then jacked up into position. Unfortunately, in 1907 the south tower collapsed during construction. In 1916, the suspension span fell while being hoisted into position. In 1917 they successfully completed the bridge.

Popular Mechanics, Dec. 1917, Public Domain

[The following information, unless otherwise noted, is from TheCanadianEncyclopedia (TCE)]

In 1907 it would be the largest structure of its kind and the longest bridge in the world, outstripping the famous Firth of Forth Bridge in Scotland. It carried trains, cars, and pedestrians. (It is still the longest cantilever bridge. [Wikipedia])

The spans of the 1907 design by Theodore Cooper were increased from 490m to 550m to reduce the construction cost of the piers. A Canadian government engineer reviewed the design and criticized the very high stresses caused by the longer spans. But Cooper blew him off. And Cooper refused to supervise the construction on site.

A young engineer by the name of Norman McLure was the first to see the problem. On August 6 McLure reported to Cooper that the lower chords on the south arm were bent. Cooper wired back almost plaintively "How did that happen?" McLure reported two more bent chords on August 12 but Chief Engineer John Deans insisted that work continue. On August 27 McLure measured the bend again. The deflection had grown. He informed Cooper who wired the bridge company in Pennsylvania: "Place no more load on Quebec bridge until all facts considered." Cooper assumed that the work had stopped. Deans had read his wire but ignored it.

The bridge collapsed on Aug 29, 1907 killing 75 of the 86 workers.  "A group of sightseers looked back in horror when they heard the sound, for they had only left the bridge minutes before."

TCE

TCE

The Royal Commission of Inquiry investigating the calamity excoriated John Deans for his poor judgment in allowing work to continue when it was obvious that the bridge was in danger. The brunt of the blame, however, was placed on the shoulders of Theodore Cooper, who had committed grave errors in design and his calculation of loads. There was criticism of the bridge company for putting profit above safety and for engineers who neglected their professional and moral duties.

It took two years just to clean up the mess. The Canadian government assumed control and rebuilt it with much heavier (and uglier) cantilever arms. On Sept. 11, 1916, 13 men were killed when the span fell while being hoisted into position. It was finally completed in 1917.

[The end of information from TCE]

To keep the bridge cheap, Cooper had already used unusually high allowable stresses in his initial design. And he did not recalculate the stresses when the span was lengthened! [ASCE]

McCord Museum

This image indicates that the first design built the suspended span in place.

Cleveland Statue University
[This is a much more technical article if you want more detail about the first design and its failure.]

The upper cords are in tension while the lower cords are in compression. Steel is strong in tension. It is a challenge to design truss members to handle compression. That is why it was the lower cords that first buckled.

In contrast to the 1907 failure, the only info I can find on the 1916 failure is "a problem with the hoisting devices." [Wikipedia]

By historical archive, Link, cropped

These two Facebook videos were my motivation for writing this post: moving the suspension span and 1928 usage and maintenance of the bridge.

Historic Bridges says that we can look forward to a third collapse because the Canadian National Railroad considers painting to be an aesthetic consideration rather than needed maintenance! ("
demolition by neglect" ) Evidently they have allowed the rust to get rather thick. Now there is the expense of removing the rust before it can be painted. Government entities have even offered to pay half of the cost, but CN still refuses to maintain the bridge.

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