Design process and prefabrication combine for cost-efficient construction

By Richard Mulder

The new $180-million Third Crossing bridge, known as “Waaban Crossing”, over the Cataraqui River at Kingston, Ontario was built to provide additional crossing capacity, a connection with the east and west approaches to the city, improved emergency services, and new transit, cycling and walking pathways.

Kingston is the first city in Canada to use an Integrated Project Delivery (IPD) model for building a bridge. The IPD model establishes mutual goals between the design consultants, the contractor team, and the client team – a collaboration which shares responsibility and the financial risk or reward. The structure has three parts: the West Bridge, the Main Bridge, and the East Bridge which together support a two-lane highway and a 4-metre (13 feet) wide Multi-Use Path (MUP) along the southern side. Each part has its own structural configuration.

The West Bridge contains 17 spans of precast prestressed concrete girders (85 girders in total); the Main Bridge contains three spans of steel sections; and the East Bridge contains two spans of precast prestressed concrete girders (10 girders in total). Each precast concrete span has three NU 2400 (refers to depth of 2,400 mm) girders below the two-lane highway portion, and two NU 2425 girders below the MUP portion. The bridge spans approximately 1.2 kilometres (0.75 miles): 900 metres (2,953 feet) spanned by the precast girders, 815 metres (2,674 feet) for the West Bridge, and 85 metres (280 feet) for East Bridge. Boat traffic will use the navigation channel which passes under the Main Bridge span.

The bridge was designed for a minimum 100-year design life, exceeding the 75-year design life requirement of the Canadian Highway Bridge Design Code. To achieve a longer design life, two concrete mixes were used for the girders: a standard mix for the interior girders, and a custom mix with a corrosion-inhibiting admixture for the exterior girders. The NU 2425 girders are a custom section and therefore required custom formwork. The West Bridge girders are among some of the longest and heaviest in Ontario, and the NU 2425 girders are likely the tallest.

A drainage pipe system travels the full length of the bridge, located between the NU 2425 girders under the MUP. Brackets cast into the web of the exterior NU 2425 girders support the drainage pipe. As a safety measure additional hardware was cast into the webs of all girders to allow the installation of temporary wood bracing on site. Some measures of scale include: the total mass of girders at approximately 7,500,000 kilograms (7,500 tonnes equivalent to 1,250 elephants); a total volume of concrete of 3,100 cubic metres (109,475 cubic feet), enough to fill one and a quarter Olympic-sized swimming pools); and a total of 312 kilometres (194 miles) of 15.2 mm diameter strands were used. The total mass of the 1,695 precast prestressed concrete deck panels is about 3,500,000 kilograms (3,500 tonnes), representing a total volume of concrete of 1,400 cubic metres (49,440 cubic feet) and incorporating 115 kilometres (71 miles) of 9.5 mm (0.4 in.) diameter strand.

THE CONSTRUCTION
To support the bridge, caissons were cored into bedrock to depths of over 40 metres (131 feet). The river was found to be too shallow to accommodate a construction barge but too deep to install a full trestle economically. Instead, the design and construction team came up with a combination of rock causeway and steel trestle, which served as a working platform during the build.

Large projects usually come with complications. The Third Crossing Bridge included the following points:

• Because of high transfer strengths, additional quality control monitoring and testing were required. Concrete maturity curves for the mix were developed and additional cylinders were cured to match actual girder temperatures and to establish exactly when the transfer strengths were reached so that no time was wasted.
• The size, length, and mass of the girders made handling, transporting, and erection a challenge. The total mass of the NU 2425 girders required some plant upgrades and additional engineering to ensure safe lifting. Cranes and crane ropes were upgraded, and special lifting devices (lifting triangles) were designed to handle the 83-tonne mass. Windy conditions over the Cataraqui River led to difficulties in erecting the girders, however, only a few erection days were postponed due to wind.

The following were some of the difficulties that our team had to overcome:

• Difficult geometry and cut-outs for the deck panels led to custom formwork, more labour required, and longer days.
• High congestion of steel required at the ends to handle high shear forces made concrete consolidation at these points a challenge during pouring.
• All the various extra cast-in hardware increased labour time.

Finally, as the Cataraqui River is part of the Rideau Canal system which is protected as a National Historic Site of Canada, a Canadian Heritage River, and a UNESCO World Heritage Site, environmental protection was particularly important. In this regard, five ‘eco-passages’ were built into the temporary construction causeway to ensure the safe passage of fish, turtles, and other wildlife. The high degree of collaboration realized through the IPD process resulted in a timely and cost-efficient design, prefabrication and construction schedule beginning April 2020 and concluding December, 2022 when the bridge was officially opened.

THE NU GIRDERS
Developed at the University of Nebraska in the 1990s, NU Girders are prestressed concrete I-girders optimized for performance in two-span bridges and with full-length post-tensioning. They can be pretensioned girders, post-tensioned girders or a combination of both techniques. In a pretensioned girder, the top flange is reinforced with standard reinforcing steel comprising a basic grid of transverse and longitudinal bars, and four straight prestressing strands. A maximum of 72 strands can be accommodated, of which up to 26 are located within the web. These 26 strands can be deviated as necessary for the design.

The bottom flange is highly reinforced with straight prestressing strands. NU Girders can be fabricated in depths ranging from 1,200 mm to 2,800 mm, in increments of 400 mm. A 2,000 mm deep NU Girder is referred to as an NU 2000. (Courtesy NU Girder Bridge Design and Detailing Manual, 2018 by Alberta Transportation.)

RICHARD MULDER IS WITH DECAST LTD.
Photos Aerosnapper (smugmug.com)