e-mosty June 2018 American Bridges American Bridges | Page 50

The design of this structure over the Ottawa River
presented several major challenges, including
environmental constraints, the proximity of the
existing bridge and the presence of a geological fault
at the bridge.
During the design phase, the option to reduce the
number of piles in relation to the existing bridge was
chosen by the owner as this limited the
environmental impacts of the project.
Optimization of the concept led to the choice of a
variable-inertia steel caisson bridge, continuous over
three spans, including a main span of 10 meters and
two 85-meter bank spans.
Considering the location of the bridge and the tight
construction schedule, a steel structure had the
double advantage of being able to be quickly built in
fabricator plants and then transported by truck for
assembly during all year periods.
The use of beams with variable inertia, in addition to
allowing the optimization of the use of the material,
gave a slender and harmonious aspect to this large
structure.
The presence of a geological fault at the right of the
structure represented an important stake during the
design.
As a result of multimodal modeling and seismic
analysis of the bridge, caisson piles with rock socket
were retained for the pile foundations. Seismic
isolators were incorporated to reduce the impact of
an earthquake on the structure.
In addition, special control measures had to be taken
to ensure that the sockets of the caisson piles had the
necessary length to withstand the forces while
remaining in the sound rock zone, without
encroaching on the shear zone of the fault
characterized by a highly fractured rock.
The main beams were erected using a combination of
conventional methods and atypical methods: the edge
spans were erected from temporary piers, using
mobile cranes and temporary piers, while the
assembly the central section of 90 meters in length
was made by jacking the two box beams
simultaneously from a barge.
Since the structure is continuous, the beams had to
be lifted about 1.3 meters to the abutments to
achieve geometric compatibility allowing the bolting
of construction joints.
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The required erection method involved the use of a
400-metric-ton crawler crane, temporary supports,
several jacks, and a 1.3-meter abutment lift to
connect the splices in the center section.
The use of a steel frame was the large contributor to
the success of the project, which was completed in
less than 18 months, the main assembly of the
structure was done in the heart of winter, thereby
saving precious months, without stopping work.
Once the bridge was fabricated, the team entered a
time trial race with the general contractor to
complete the assembly before the spring breakup.
Given the bathymetry conditions of the Ottawa River
at this location, a temporary barge bridge was put in
place, making assembly of the structure more
complex.
To respect the schedule, it was decided in the
preliminary draft not to fabricate the structure in a
linear way as usual, but rather to manufacture the box
girders according to the assembly order desired by
the general contractor.
This required particular logistics during the fabrication
of the components, which made it possible to deliver
the beams in time. The design team was also forced
to adapt to the singularity of this project by
partitioning the shipment as soon as the drawings
were approved to allow for a rapid production.
The weight of some beams reached more than 90
tons, exceeding the lifting capacity with the
fabricator’s plant, therefore Engineers were asked to
find a solution for moving these parts.
The team of experts therefore set up a system
consisting of hydraulic cylinders and rollers that made
it possible to slide each of the boxes out of the factory
without having to use the cranes. Beams were
shipped to the site, as soon as the fabrication was
complete, so as not to delay assembly.
Challenges were even more complex than anticipated,
as some restrictions issued by the Department of
Transport, such as height, weight, or certain traffic
barriers due to construction, were not known at the
outset. All challenges were met and the project was
successfully completed.