Introduction:
On the 29th of August 1907 at approximately 5:30pm, a large groan erupted from two
compression chords situated on the anchor arm of the Quebec Bridge as it collapsed.
While work had been suspended the previous day due to concerns about the safety of
the bridge this did not follow through to the day of collapse and as such, 74 men
dropped to their death. The bridge was intended to be the longest cantilever bridge in
the world; however, human error proved that the 1907 bridge would not claim this
engineering feat.
Design:
Without the bridge, Quebec City would remain outside of the main transportation link
between Maine and the province of Quebec.
The peculiarities of the site made the design of the bridge difficult. Because St
Lawrence was a shipping lane, the 2800 foot bridge had to have an 1800 foot single
span approximately 150 feet above the water to allow ocean going vessels to pass
under. The bridge had to be multifunctional and was required to be 67 feet wide to
accommodate 2 railways tracking, two street car tracks and two roadways. In the case
of any cantilever bridge the weight of the central span is the key to the design .Steel
plate cylinders were to be used as the main compression members.
What happened?
The Quebec Bridge Company (QBC) was granted the contract for the Quebec as far
back as 1882. However, little advance was made on this project until 1897 when
Theodore Cooper – a famous aging engineer eager to leave his mark – was
approached by the QBC to overlook the ambitious project. Naturally, he agreed.
Another two years passed before QBC officials met with Cooper in New York
to review the bids for the project. In the end, the QBC-preferred bid from the Phoenix
Bridge Company (PBC) of Phoenixville, Pennsylvania won the project.
On May 1 1900, Cooper lengthened the design of the cantilever bridge from
1600 feet to 1800 feet. This change was planned to cut the construction time by at
least one year, as well as saving money on materials as the piers would be located
nearer to the shore, and thus less vulnerable. He also recommended some modified
specifications that would allow for higher unit stresses.
Between 1900 and 1903 there was ample time for detailed calculations of the
new design to be carried out, however, nothing of note transpired until the Canadian
government issued a $6.7 million bond to pay for the work. Suddenly, the
construction process began on the back of the theoretical estimates of weight that the
PBC had provided.
Due to poor health, Cooper was to visit the site only four times between 1900
and 1903, with his visit in May 1903 being his last. However, this did not stop him
from travelling from his home in New York to Ottawa to confront Robert Douglas, an
engineer who had criticised Cooper’s high unit stresses in the new design. This
criticism fell on deaf ears, due to the unwavering belief by others in the brilliance of
Theodore Cooper.
In 1905, Cooper demanded that a recently graduated engineer be in charge of
the overlooking the project and relaying information back to Cooper. His name was
Norman McLure.
By 1907 the imbalance between the theoretical dead load and the actual dead
load of the structure began to show up on the structure itself. On August 12 McLure
reported to Cooper that some buckling was occurring in chords 8-L and 9-L having
earlier wired Cooper detailing how the lower chords 7-L and 8-L were bent. However,
this obvious problem became muddied by the fact that Chief Engineer Deans insisted
that chords 7-L and 8-L were bent when they left the shop, to which McLure differed.
On the 27th of August 1907, chord 9-L was measured to be two and one
quarter inches out of line, and increase in deflection by one and a half inches in a
week. Work was stopped the following day, due to fear. However, work was restarted on the 29th of August possibly due to pressure from Chief Engineering Hoare.
McLure was dispatched to meet Cooper on the 29th of August. Cooper sent a
telegraph to Chief Engineering Deans for him not to add any more load to the bridge –
however, Deans ignored this. The bridge collapsed at approximately 5:30pm.
Factors contributing to the disaster:
With poor health keeping Cooper in New York, he turned the management of the
work crews to his deputy, Peter Szlapka. But Szlapka had no experience supervising
construction. The only person in touch with the workers was a young engineering
graduate named McLure. McLure began to notice problems. He noticed the bridges
bearing plates were buckling.
Early on in the design the chief engineer, Cooper, recommended that the span be
increased to 1800 feet to speed up construction and prevent piers being subject to
heavy ice flows during winter. This decision coupled with modified specifications
allowed for higher unit stresses. They were almost ignored in design considerations.
During the construction phase inadequate riveting of some of the bridges key weight
bearing lower chord members in the south anchor left some key elements unstable.
The main reason for the bridge collapse was an incorrect initial calculation of the
weight of the bridge. The bridges actual weight was almost 8 million pounds greater
than the design value.
Also there was a complete lack of action taken when deformations far exceeded
expected values. The claims that the beams were bent before leaving the workshop
caused confusion, and detracted from the gravity of the situation. In addition, the
decision to send McLure to New York to see Cooper personally caused a fatal delay.
The decision to continue work was also a key factor in the failure, as it was believed
at the time, that without additional loading the bridge may have survived long enough
for remedial action.
Conclusion:
Essentially, the failure of the bridge was due to a series of human errors. At many
points during planning, design, and construction of the bridge, action could have been
taken to avert the disaster which claimed so many lives.
Many lessons can be learned from this disaster. Not least, it showed the folly of
dependence on one person’s expertise. The inexperience of the team led to many of
the mistakes which caused the bridges failure.
The Royal Commission of Inquiry wrote in their 1908 report on the collapse, "We are
satisfied that no one connected with the work was expecting immediate disaster, and
we believe that in the case of Mr. Cooper his opinion was justified. He understood
that erection was not proceeding; and without additional load the bridge might have
held out for days."
Chief Engineer John Deans was rebuked for his poor judgment during the final days
of the crisis, and the Quebec Bridge Company was criticised for appointing the
unqualified Edward Hoare as the responsible engineer at the site.
The commissioners decided that "the failure cannot be attributed directly to any cause
other than errors in judgment on the part of these two engineers [Theodore Cooper
and Peter Szlapka] ...A grave error was made in assuming the dead load for the
calculations at too low a value...This error was of sufficient magnitude to have
required the condemnation of the bridge, even if the details of the lower chords had
been of sufficient strength."
The “Iron Ring” tradition of the Engineering Institute of Canada arose from this
disaster. The ring is worn as a reminder of the possible consequences of an engineer’s
actions. It serves as a visible reminder of the lessons to be learned from the fateful
bridge.