The Quebec Bridge, spanning the St. Lawrence River in Canada, was the longest cantilever bridge during its time and faced two significant collapses during construction in 1907 and 1916 due to structural miscalculations and pressures to proceed. Following these tragedies, a new design was established, utilizing a stronger k-truss design, leading to its completion in 1917. The incidents prompted the creation of Canadian design specifications for bridged engineering and highlighted the need for a collaborative approach in major structural projects.

1. Quebec Bridge
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2. About
• Quebec bridge is a rail, road and pedestrian bridge situated on the St.
Lawrence River, Quebec, Canada.
• It was at time of construction the longest cantilever bridge in the world with
clear span of 549 m.
• The bridge construction was finally completed in 1917 after Two Failures or
tragedies to bridge the harsh icy waters of the St. Lawrence river.
• First collapse occurred on August 26, 1907. Second collapse occurred almost
after a decade on September 11, 1916
• Quebec bridge is a cantilever truss bridge with two spans cantilever from
both sides and simple span suspended in between.
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3. History
• The story of the bridge begins back in 1887 with the formation of
Quebec Bridge company which came up with the proposal to erect the
bridge over the river to increase business traffic in the city of Quebec
and places nearby.
• Various proposals for the bridge were submitted out of which The
Phoenix Bridge Company’s proposal was selected which called for a
cantilever bridge of 45 m in height above the high water mark.
• Theodore Cooper, a well-known American bridge designer, was selected
as the project’s consulting engineer by the Quebec Bridge company. He
endorsed the Phoenix design as the “best and the cheapest” of those
submitted, although he decided to lengthen the center span from 488 m
to 549 m in order to eliminate the uncertainty of constructing piers in
such deep water, lessen the effects of ice, and shorten the time of
construction of the piers.
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4. Concept Dimensions & Details
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5. First Collapse 1/5
• As soon as the south anchor arm was completed (in early 1906) the
assembly of the cantilever arm could begin (the falsework was then
also moved to the other side of the river for the construction of the
north anchor arm).
• Using the anchor arm as a counterweight – securely anchored at the
end for the uplift forces – the main span was being erected using the
cantilever technique (one half from either side of the river).
• At just about this point in the construction (in early 1906) it was
discovered that the amount of steel material delivered to the bridge
site considerably exceeded the originally estimated amount. It was
because no correction for self weight of the structure was made when
the span length was increased from 488 m to 549 m. Cooper however,
decided to forge ahead and Not introduce any changes.
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6. First Collapse 2/5
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7. First Collapse 3/5
• When the cantilever had been extended further out over the river,
increasing the stresses even more in the truss members – it was found
that the out of plane bending of chord A9-L had increased from some
20 mm to alarmingly 57 mm.
• Even though this deformation – representing an out-of-plane
deflection of L/305 (the length L of the member being 17.44 m) – is
hard to detect by the naked eye, it was a clear signal that something
was quite wrong.
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8. First Collapse 4/5
• Cooper, realizing the danger of the situation, wanting the
construction to come to an immediate halt, telegraphed to the
Phoenix Bridge Company: “Add no more load till after due
consideration of facts’’.
• However, the contractor (i.e. the Phoenix Bridge Company) was
under great pressure not to delay the construction, so the work
continued by unfortunately moving an erection crane one step
further out on the cantilever.
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9. First Collapse 5/5
• The A9L bottom
compression chord,
which was already bent,
gave way under the
increasing weight of the
bridge. The load
transferred to the
opposite A9R chord,
which also buckled. The
piers were the only part
of the structure that
survived.25-Jan-19 Majzoob Mohammed Arbab 9

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11. Second Collapse
• In spite of the enormity of the accident, it was decided that the project
must be completed. In 1908, a board of engineers was appointed that
arranged for and supervised the design and erection of a new bridge.
Except for the piers, nothing from the previous structure was recoverable.
• In April 1911, the contract was awarded to the St. Lawrence Bridge
Company of Montreal. As before, the design called for a cantilever-type
bridge, but it differed from the previous one with the lower chords of the
cantilever arms several times stronger. One important innovation that
added strength to the bridge was the K-truss design.
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13. • Construction of the bridge started in 1913, and eventually the two
approach spans, the anchor arms and cantilevers went up on either side
of the river. By 1916, the bridge was nearly completed.
• The workmen faced a difficult task in moving the span upstream, but all
went well with that part of the job. The span was carried on scows that
were guided by tugs. It was a slow process, but eventually the span was
maneuvered into position between the cantilever arms where huge
lifting hangers, attached to the ends of the arms, raised it by hydraulic
means off the scows. The span was to be lifted two feet at a time in a
repeat operation until it was in place between the two arms.
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14. • The southwest corner of the span tore away and sagged. A few seconds
later, the other ends pulled off their supports and the whole span came
loose and disappeared into the river. Thirteen men were carried to their
deaths and several others were injured.25-Jan-19 Majzoob Mohammed Arbab 14

15. The investigation by the board of engineers determined that
the span did not buckle as Larocque and others claimed.
Rather, the loss resulted from the failure of a casting in the
erection equipment that temporarily supported the southwest
corner of the span.
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16. Final Completion
• On Sept. 20, 1917, the suspended span was lifted into position and fastened to the
cantilever arms. At last, the world’s longest cantilever bridge was completed and the
first train crossed it in October. Two months later it was opened to regular trains,
vehicle and pedestrian traffic.
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17. Major Causes of Failure
• An underestimation of the self-weight of the structure
• Temporary splicing of the chords using bolts
• The load carrying capacity had been overestimated resulting
in weak cross-sections
• The stabilizing bars (the crossing diagonals and horizontals)
were not strong enough to resist out-of-plane buckling of the
compression chords
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18. Lessons Learned
• The collapse of the bridge finally led to the development of Canadian
design specifications for bridge structures.
• Engineering design is necessary for the means and methods of lifting
operations.
• The responsibility of execution and design of such an important
structure must Not be left in the hands of one person
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