For professional engineers in private practice JANUARY/FEBRUARY 2012 DEH CHO BRIDGE INNOVATION IN CANADA’S ARCTIC TUNNELLING BELOW NIAGARA FALLS TRUE TALES FROM BIM www.canadianconsultingengineer.com p01-02 CCE Feb12_3.indd 1 12-02-07 8:43 AM transportation Deh Cho Bridge A NEW CROSSING IS BEING BUILT OVER THE MACKENZIE RIVER ON A DIFFICULT AND REMOTE SITE IN CANADA’S ARCTIC REGIONS. THE BRIDGE WILL BE THE LONGEST JOINT-LESS SUPERSTRUCTURE IN NORTH AMERICA. BY DR. MATTHIAS SCHUELLER, P.ENG. AND PRABHJEET RAJ SINGH, P.ENG. PE INFINITY ENGINEERING GROUP 16 www.canadianconsultingengineer.com January/February 2012 p16-27 CCE Feb12 FEATURES_2.indd 16 12-02-07 1:42 PM transportation 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 Mack- enzie River Ice Crossing, resulting in savings from the elimination of the ferry and ice bridge operations. There will also be toll reve- nues collected from commercial vehicles crossing the bridge. The bridge’s remote location - approximately 300 kilometres southwest of Yellowknife in the Northwest Territories -- and the re- gion'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 super- structure 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 pa- Total bridge length: 1,045 m rameter was to minimize field activities. In developing the struc- tural design it was therefore decided to apply assembly line design, (continuous without joints) Deh Cho Bridge fabrication and construction principles. The approach accommo- Spans: 90 m – 3 x 112.5 m – dated the transportation and other restrictions of the site. 190 m – 3 x 112.5 m – 90 m The continuous superstructure consists of a steel truss box girder Superstructure: Warren truss with a lightweight, composite concrete deck. Steel components were with composite concrete prefabricated and trial assembled around the clock in sheltered fa- slab (depth: 4.75 m) cilities specially designed for this kind of work. Consequently the Deck width: 11.29 m fieldwork was reduced to bolt splicing the major steel pieces. Quality control in the shop eliminated major errors and deficiencies and Pylons: 2 A-shaped, thus avoided time-consuming corrective actions in the field. height 33 m from top of pier Taking a cue from a cost-efficient car assembly line approach, Stays: 24 locked-coil cables, the major steel components were designed with constant cross- 100 diameter each sections and repetitive details to allow fast tracked and reliable de- Substructure: 8 piers (max sign, fabrication and assembly processes. On site, the superstruc- height 22 m), 2 abutments, ture was erected using the proven incremental launching method. with storage chambers It allowed for a quick and economical construction progress inde- pendent of the river restrictions. This approach also significantly reduced the contractor’s risk since it avoided the difficult assembly Chad Amiel/Infinity Engineering Amiel/Infinity Chad continued on page 18 January/February 2012 Canadian Consulting Engineer 17 p16-27 CCE Feb12 FEATURES_2.indd 17 12-02-07 1:42 PM transportation continued from page 17 Dennis Hicks/Infinity Engineering Hicks/Infinity Dennis Above: south approach; the bridge was built using incremental launch- ing and with prefabricated components. Photo previous page: installing forestay cables on north tower. Right: cable stressing. of steel above water and at exposed heights. The concrete deck consists of precast panels, which were Engineering Amiel/Infinity Chad 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 ac- tion 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 geometry controlled. Final stressing of all 12 stays connect- replaced in the future if required. ed to each pylon tip is achieved by one simple jacking op- Similar principles using prefabricated standardized com- eration with only one degree of freedom (lowering the su- ponents have been applied to the pylons, cables, bearings, perstructure at the pylon pier by approximately 800 mm). expansion joints, and lock-up devices. These are delivered to Final cable adjustments are possible but not anticipated due site as complete bridge components preassembled as far as to the fact that critical components are progressively trial reasonable to minimize the risk of misfits and preventable assembled, accurately surveyed and corrected in the shop field work. For instance, the cables are Galfan-coated locked- before they are delivered to the site. coil strands that are delivered as complete units including The Deh Cho Bridge is an excellent example of how- the corrosion protection system and anchorage hardware. proven and economical construction schemes in combina- This approach avoids time-consuming and weather depen- tion with optimized in-plant fabrication techniques will dent stranding operations commonly required for stays keep bridge projects that are in remote locations and ex- made of parallel mono-strands. posed to harsh climate conditions on track. CCE From a structural perspective the cable-supported super- structure can be classified as a hybrid extradosed truss Owner: Government of the Northwest Territories bridge system. The significant bending stiffness of the truss Bridge design: Infinity Engineering Group, North Vancouver requires no anchor-piers and anchor-cables as traditionally (Matthias Schueller, P.Eng., Prabhjeet Raj Singh, P.Eng., Morgan found in cable-stayed bridges. Trowland, P.Eng., Chad Amiel, EIT, Arndt Becker). Design coor- This design philosophy keeps the need for geometry dinator: Sargent & Associates. Project management: Associated control during site assembly to an absolute minimum. In Engineering. Territorial advisors: BPTEC-DNW Engineering. contrast to conventional cable-stayed bridge construction, Quality assurance: Levelton Consultants. Erection engineer: the Deh Cho Bridge stay installation is solely force and not Buckland & Taylor. Contractor: Ruskin Construction 18 www.canadianconsultingengineer.com January/February 2012 p16-27 CCE Feb12 FEATURES_2.indd 18 12-02-07 1:42 PM.