Transcript Quebec Bridge.ppt

The Quebec Bridge Failures
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Design considerations
History: What happened?
The day of the collapse
Factors contributing to the failure
Conclusion
The Iron Ring – a consequence of the
collapse
Design Considerations
• The bridge was necessary in order to connect
Quebec City to the main transportation link
between Maine and the province of Quebec
• The 2800 foot bridge was required to
 Have an 1800 foot single span
 Have 150 feet of clearance
 Be 67 feet wide to accommodate 2 railway lines,
2 street car tracks and 2 roadways
• Steel plate cylinders were to be used as the
main compression members
What happened?
• 1882: The Quebec Bridge Company
(QBC) was awarded the contract
• 1897: Theodore Cooper was contracted to
overlook the ambitious project
• 1899: QBC officials met Cooper to discuss
bids for the project. The Phoenix Bridge
company (PBC) won the project.
What happened? Contd.
• 1900, May 1: Cooper lengthened the span of the
cantilevered bridge from 1600 to 1800 feet
• Cooper also recommended some modified
specifications that would allow for higher unit
stresses caused by the longer span
• 1900 – 1903: The project remained stagnant
until the Canadian government issued a $6.7
million bond to pay for the work
What happened? Contd.
• Despite no revision of the original
calculations, construction commenced
almost immediately
• Cooper visited the site only 4 times
between 1900 and 1903.After May 1903,
he never visited the site again, citing poor
health requiring him to remain in New York
What happened? Contd.
• Robert Douglas criticised the high unit stresses
in the new design
• Belief in Cooper was such that these criticisms
were ignored entirely
• 1905: Norman McLure, a recently graduated
engineer, was placed in charge of the project on
site, as Coopers right hand man
What happened? Contd.
• 1907: McLure wired Cooper regarding bending
of the lower chords, 7-L and 8-L
• 1907,August 7: McLure reported to Cooper that
some buckling was occurring in chords 8-L and
9-L
• Confusion was created by Chief Engineer
Deans, who declared the chords were bent
when leaving the workshop
What happened? Contd.
• 1907, August 27: An increase of deflection
of a further 1.5 inches was measured
• A letter was sent to Cooper outlining the
increase of deflections in just 7 days
• Work was halted due to safety fears
August 29th: The Collapse
• McLure was dispatched to New York to consult
with Cooper
• McLure reached Cooper at the same time as the
letter which had been dispatched 2 days
previously
• Cooper believed the bridge would stand long
enough for a study of the deflections if it was not
put under any further loading
August 29th: The Collapse contd.
• A wire was written by Cooper, instructing
Chief Engineer Deans to ensure no further
loading was placed on the bridge
• Unbeknownst to Cooper and McLure,
construction had recommenced that
morning
August 29th: The Collapse contd.
• In his rush to meet his train, McLure did
not send the wire to the site
• A meeting was set up for that evening to
discuss the deflections
• Almost exactly as the meeting broke up,at
5:30pm, the bridge collapsed
August 29th: The Collapse contd.
• 85 workers were on the bridge when it
collapsed
• Only 11 survived
Factors contributing to the collapse
• Cooper turned the management of the work
crews to Peter Szlapka, but Szlapka had no
experience of supervising construction
• McLure was the only team member in contact
with the workers
• Though McLure noticed the problems with the
bridge, he was too young and inexperienced to
be able to take action to correct these problems
without permission from his superiors
Factors contributing to the collapse
• The 200 foot increase in span length was almost
entirely ignored in design considerations
• The estimations of self weight were out by
8million pounds (~3630 tonnes)
• Inadequate riveting of some members due to
deformations left some key elements unstable
Factors contributing to the collapse
• No actions were taken when deformations
exceeded expectations
• Delays in communications between the
site and Cooper led to action being taken
too late
• The decision to continue working led to
overloading of the unstable structure
Conclusion
• Human error led to the collapse of the bridge
• Royal Commission of Inquiry report of 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."
Conclusion contd.
Chief Engineer Deans was rebuked for his
poor judgement in returning to work
without proper analysis of the situation
The Quebec Bridge Company was
criticised for appointing unqualified
supervisors
Conclusion contd.
• “…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
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.