By By Dr. Matthias Schueller, P.Eng.
Deh Cho Bridge and Prabhjeet Raj Singh, P.Eng. PE Infinity Engineering GroupTransportation Transportation Infrastructure
The Deh Cho Bridge will be the first permanent crossing of the Mackenzie River in Canada’s Arctic region. The $180-million bridge will replace the operations of the Merv Hardie Ferry and the Mackenzie River Ice Crossing, resulting in...
The Deh Cho Bridge will be the first permanent crossing of the Mackenzie River in Canada’s Arctic region. The $180-million bridge will replace the operations of the Merv Hardie Ferry and the Mackenzie River Ice Crossing, resulting in savings from the elimination of the ferry and ice bridge operations. There will also be toll revenues collected from commercial vehicles crossing the bridge.
The bridge’s remote location – approximately 300 kilometres southwest of Yellowknife in the Northwest Territories — and the region’s extreme winter temperatures of up to -40 degrees Celsius created challenges for both the design and erection of the bridge. When completed (estimated to be December 2012), the 1,045-metre long composite steel truss will be the longest joint-less superstructure in North America.
At the crossing near Fort Providence, the Mackenzie River is approximately 1,200 metres wide. The design criteria require a navigational clearance profile of 185 metres by 22.5 metres for the main span to allow vessels to pass through, while the superstructure piers are required to resist the impact of vessels and the pressure of ice. The deck accommodates two lanes of traffic and has provisions for a sidewalk that may be added later. The maximum slope of the approach ramps is limited to 3.5%.
The extreme weather conditions allowed only a relatively short window with reasonable conditions for construction, between June and December. During the ice break-up periods between April and May any works supported by temporary foundations in the river had to be fully removed. Also, the delivery of materials to the north shore depends on ferry or ice road service since no alternative route is available. For all these reasons the bridge erection stages had to be carefully planned and executed.
For complex bridges it is good design practice to investigate at least one feasible construction method as a part of the design. However, for major bridges with extraordinary site conditions such as the Deh Cho Bridge, an economical construction scheme is paramount and typically governs the design. Another critical parameter was to minimize field activities. In developing the structural design it was therefore decided to apply assembly line design, fabrication and construction principles. The approach accommodated the transportation and other restrictions of the site.
The continuous superstructure consists of a steel truss box girder with a lightweight, composite concrete deck. Steel components were prefabricated and trial assembled around the clock in sheltered facilities specially designed for this kind of work. Consequently the fieldwork was reduced to bolt splicing the major steel pieces. Quality control in the shop eliminated major errors and deficiencies and thus avoided time-consuming corrective actions in the field.
Taking a cue from a cost-efficient car assembly line approach, the major steel components were designed with constant cross-sections and repetitive details to allow fast tracked and reliable design, fabrication and assembly processes. On site, the superstructure was erected using the proven incremental launching method. It allowed for a quick and economical construction progress independent of the river restrictions. This approach also significantly reduced the contractor’s risk since it avoided the difficult assembly of steel above water and at exposed heights.
The concrete deck consists of precast panels, which were produced and inspected in specialized plants before being shipped to the site. Cast-in-place concrete fill-ins ensure continuity between the panels and provide a composite action of truss and deck for live loads. This approach (the deck dead load is carried by the truss non-compositely) simplifies the camber analysis and allows for panels to be replaced in the future if required.
Similar principles using prefabricated standardized components have been applied to the pylons, cables, bearings, expansion joints, and lock-up devices. These are delivered to site as complete bridge components preassembled as far as reasonable to minimize the risk of misfits and preventable field work. For instance, the cables are Galfan-coated locked-coil strands that are delivered as complete units including the corrosion protection system and anchorage hardware. This approach avoids time-consuming and weather dependent stranding operations commonly required for stays made of parallel mono-strands.
From a structural perspective the cable-supported superstructure can be classified as a hybrid extradosed truss bridge system. The significant bending stiffness of the truss requires no anchor-piers and anchor-cables as traditionally found in cable-stayed bridges.
This design philosophy keeps the need for geometry control during site assembly to an absolute minimum. In contrast to conventional cable-stayed bridge construction, the Deh Cho Bridge stay installation is solely force and not geometry controlled. Final stressing of all 12 stays connected to each pylon tip is achieved by one simple jacking operation with only one degree of freedom (lowering the superstructure at the pylon pier by approximately 800 mm). Final cable adjustments are possible but not anticipated due to the fact that critical components are progressively trial assembled, accurately surveyed and corrected in the shop before they are delivered to the site.
The Deh Cho Bridge is an excellent example of howproven and economical construction schemes in combination with optimized in-plant fabrication techniques will keep bridge projects that are in remote locations and exposed to harsh climate conditions on track.cce
Owner: Government of the Northwest Territories
Bridge design: Infinity Engineering Group, North Vancouver (Matthias Schueller, P.Eng., Prabhjeet Raj Singh, P.Eng., Morgan Trowland, P.Eng., Chad Amiel, EIT, Arndt Becker). Design coordinator: Sargent & Associates. Project management: Associated Engineering. Territorial advisors: BPTEC-DNW Engineering. Quality assurance: Levelton Consultants. Erection engineer: Buckland & Taylor. Contractor: Ruskin Construction
Total bridge length: 1,045 m
(continuous without joints)
Spans: 90 m – 3 x 112.5 m –
190 m – 3 x 112.5 m – 90 m
Superstructure: Warren truss with composite concrete
slab (depth: 4.75 m)
Deck width: 11.29 m
Pylons: 2 A-shaped,
height 33 m from top of pier
Stays: 24 locked-coil cables,
100 diameter each
Substructure: 8 piers (max
height 22 m), 2 abutments,
with storage chambers