4A6(1) Disaster Exercise
Quebec Bridge Collapses
October 2011
Group 9
Mark Conlan
Roseanna Doolin
Stephen Maguire
2
Introduction:
The Quebec Bridge is a cantilever span bridge that crosses the lower part of the St Lawrence
River to connect Quebec City and Lévis, Quebec in Canada. It was built to improve
transportation between these two cities and thus improve Quebec’s trade competitiveness. The
construction of this bridge had been considered since as far back as 1852 but was only completed
in 1917 due to a number of significant setbacks along the way.
The bridge collapsed twice during its 8 year construction, once in August 1907 and again in
September 1916. The collapse in 1907 was attributed to the miscalculation of dead load carried
by the bridge. The collapse only lasted for a mere 15 seconds but it destroyed the southern arm of
the bridge, part of the central span and cost 75 people their lives. This collapse was much more
severe than the 1916 collapse in which the central span fell into the river while being raised into
position. A further 13 people were killed in this collapse which brought the total number of lives
lost on the bridge to 89.
The Quebec Bridge Company was granted the contract for the Quebec Bridge and work began on
its construction in 1900. A well respected engineer named Theodore Cooper was the project’s
first head engineer as the Quebec Bridge Company’s head engineer Edward Hoare had never
worked on a bridge longer than 90 meters before. The Bridge was completed in August 1917 at a
cost of $25 million. Its centre span of 549 meters (1800 ft) remains the longest cantilevered
bridge span in the world.
The Design:
Since the St. Lawrence River was a shipping
lane, and prone to difficult weather
conditions, the bridge had a number of
additional specifications that needed to be
designed for. Firstly, it was required to be
20.5m wide to accommodate 2 railway
tracks, 2 street car tracks and 2 roadways.
Secondly, it needed to provide enough
clearance for the shipping vessels to pass
under it safely. These specifications, along
with the width of the river itself, called for a
cantilever structure
Figure - Elements of a Cantilever Structure
The proposed cantilever structure for the Quebec Bridge was designed to provide a 549m wide
span. The initial suggestion was for the span to be 488m but this was increased by Cooper who
stated that the larger span would eliminate the problems with constructing piers in deep water,
lessen the effects of ice, and shorten the construction time for the piers. This change meant that
the bridge would be the longest cantilever structure ever to be attempted.
The completed piers were to stand about 8m above water level. They were made of huge granite
facing stones with concrete backing. The top 5.8m of each pier was made of solid granite. Each
pier rested on a concrete filled caisson that was 14.9m wide, 7.6m high, and 45.7m long and
weighed 16.2MN. The anchor pier towers would withstand a load of 34.8MN. The tension
eyebars were designed to withstand a maximum stress of 108.5MPa and were about 12.8m long.
The floor system was designed to carry a live load of well over 2.79MN.
Group 9: Mark Conlan, Roseanna Doolin, Stephen Maguire
Quebec Bridge Collapses
3
The Main Technical Factors that Lead to Failure:
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To keep down the increased cost of steel in the superstructure due to the increased span
length, the bridges specifications were modified to allow for higher unit stresses.
Construction went ahead using proposal drawings only. These were approximations and the
design was as yet incomplete.
The main factor responsible for the disaster was that no recalculation of the weight of the
bridge under the new revised specifications was conducted. The first realisation that the
bridge was considerably over weight came after a notable amount of the works had been
completed. The bridges estimated weight had increased by 18%, from 28 to 33 million kg.
This correlated to an increase in the already high member stresses of 7-10%. He allowed 145
MPa for normal loading and 165 MPa under extreme loading conditions [2]. The top chords
for the anchor and cantilever arms in the Quebec Bridge were slightly curved, for aesthetic
reasons. This added difficulty to the fabrication and also increased the secondary stresses on
the members, reducing their buckling capacity considerably.
Due to the considerable increased dead load, difficulties began to arise in construction.
Notable deflections were found in some of the chords, initially when the workers tried to
rivet the joints between these chords, the pre-drilled holes did not line up. Two 75-ton jacks
had to be used to partly able to straighten out these splices. The problems simply got worse,
and the deflections began to grow. In the space of under 2 weeks, deflections in one of the
lower compressive chords grew from 19 mm to 57 mm [2]. The members under the highest
compressive loads were actually buckling under their own self weight, and as they deflected
higher stresses were placed on the latticing as well as the rivets attaching the lattices to the
main compression members. The Engineers on site and Cooper, who was in a design office
in New York, initially attributed this to either a pre-existing condition from the
manufacturing process or to the cord being accidentally struck whilst being assembled.
The Main Human Factors that Lead to Failure:
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The consulting Engineer to the Quebec Bridge Company, Theodore Cooper was a highly
accomplished engineer at the time. He was a former director of the American Society of
Civil Engineers. The Quebec Bridge was to be his greatest and final accomplishment before
he retired, and due to his impressive credentials, he was rarely contradicted.
The chief engineer on site, Edward A. Hoare, had never worked on a bridge with a span
longer than about 90 meters [1], and was quite inexperienced. He kept correspondence with
Cooper, who was in a design office in New York via post and telegram. Cooper himself was
in relatively poor health, and only visited the construction site of what was to be the longest
spanning cantilever bridge in the world 3 times during the entire project, none of which
whilst actual construction work was underway.
There was also a break-down of communication, after some alarming deflections of the main
compressive members was discovered, Cooper was under the impression that work had
halted, whereas in reality they continued, and vital information relating to the unacceptable
deformations of key structural elements was sent via post instead of telegram to Cooper, who
was over 800 km away.
Due to a serious lack of action taken, in spite of the fact some of the construction workers
hadn’t reported for work in several days due to safety fears, one of the bottom compression
chords, which was already bent, gave way under the increasing weight of the bridge. The
Group 9: Mark Conlan, Roseanna Doolin, Stephen Maguire
Quebec Bridge Collapses
4
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load transferred to the opposite chord that also buckled, and the 19,000 ton structure of steel
fell into the St. Lawrence River.
The project suffered a second collapse in 1916, when a casting in the lifting apparatus broke,
causing the 5,000 ton centre span to fall into the water. A further 13 workers lost their lives
in this accident.
Lessons Learnt from the Disaster:
There is more to engineering than calculating loads and stresses. A project of the scale of The
Quebec Bridge highlights the importance of good project management. The ability to delegate
responsibility to other qualified persons is critical to the success of any large project.
Communication is also important in ensuring that the right information is shared between parties
so that accurate decisions can be made. The large distance between Cooper and the site hampered
this ability to make informed decisions on a day to day basis.
The ability of a person to recognize his or her own limitations is another important personality
characteristic that was evidently lacking in the makeup of Theodore Cooper. This ability to selfcriticise allows an individual to surround him or herself with people who if empowered will have
the right and the ability to question decisions made for the good of the project. Such was the
belief in his own ability that Mr Cooper chose only to surround himself with unqualified
individuals who had not the authority or the experience to question his own judgement. This lack
of humility is a characteristic which can have serious consequences if left unchecked.
The ability to listen to and acknowledge the opinions of others is also vital to a successful project.
Labourers on the bridge had questioned the bending of some of the members but were ignored
due to their perceived lack of expertise. This was despite some having worked on steel structures
for decades.
In the case of the Quebec Bridge the precarious financial situation of the client impacted on the
quality of the decisions regarding the construction of the bridge. When the recalculation of the
loads was eventually undertaken the cost of starting construction over again was deemed to be too
prohibitive. This was to have fatal consequences for the 75 workers on the bridge.
The Quebec Bridge disaster hastened the advent of engineering associations which have created a
greater pool of peer mentoring and consultation. This was deemed necessary due to the
consequences of relying on the reputation on individual engineers such as Theodore Cooper. A
tradition now exists whereby the Canadian Engineering Institute hand out an “Iron Ring” which
symbolises the bringing together of all members of the engineering profession and highlights the
importance of high standards and integrity in the profession.
Conclusion:
Although improvements in material quality and construction techniques have reduced the number
and severity of bridge collapses the main underlying cause has not changed. Human error has and
always will remain one of the biggest contributory factors in construction disasters.
Acknowledgement and acceptance of this fact will allow for greater systems of checks and
balances.
References:
1) civeng.carleton.ca/ECL/reports/ECL270/Introduction.html
2) matdl.org/failurecases/Bridge_Collapse_Cases/Quebec_Bridge
Group 9: Mark Conlan, Roseanna Doolin, Stephen Maguire
Quebec Bridge Collapses