2024 #CCEAwards Showcase: A. Murray MacKay Bridge Deck Panel Replacement

Photo courtesy COWI.

Category: Transportation

Award of Excellence Winner: COWI

The A. Murray MacKay Suspension Bridge, which opened in 1970, is a critical four-lane arterial that carries 65,000 vehicles daily over the Halifax Harbour, between Halifax and Dartmouth, N.S. Recently, Halifax Harbour Bridges (HHB) engaged COWI to design modular replacement orthotropic steel plate deck (OSPD) panels, for installation during weekend bridge closures, to extend the bridge’s life, improve safety and prevent future long-term bridge closures, which could otherwise cause unprecedented disruption to the city.

Based on the complexity of replacing sections of the deck, COWI recommended HHB proactively prefabricate such panels to keep on-hand and to establish erection procedures for their installation.

In-kind replacement

COWI’s project scope included but was not limited to project management, detailed design, deck inspection during repaving, structural analysis of the bridge during erection and design services during construction.

The firm’s overall design approach was to replace each panel ‘in kind’ by matching its geometry and details for constructability and to prevent any additional weight for a structure already near its capacity limits.

COWI undertook an erection sequencing analysis before providing its suggested procedure and worked closely with the contractor to ensure successful panel removal and installation.

The exact dimensions of existing panels vary along the bridge. COWI designed and detailed the new ones to be modular, capable of replacing most of the older ones.

Each new OSPD panel is approximately 9.6 m long and 5.5 m wide and weighs 10,000 kg. The full construction project included fabrication of and corrosion protection for two such panels, the removal of two existing panels (and more than 2,000 bolts) and, finally, the installation of the two new panels.

The modular panels can be fabricated and installed at any location along the bridge with detailed connections, including oversized and slotted holes, allowing for prefabrication prior to knowing where they are required.

Addressing challenges

Replacing the deck panels of a suspension bridge is rare—and there were many challenges to doing so for this particular bridge, which the application of engineering principles needed to address:

1. Unknown location of the repairs

Modular design allows HHB to prefabricate replacement panels in advance and then install them in virtually any location, providing maximum agility in risk management.

2. Public use below bridge

As people use roadways, waterways and buildings below the bridge, an erection method was selected involving crane lifting from deck level with a below-deck access platform and protective measures.

3. Timing constraints

Work occurred during weekend bridge closures, from 7 pm. on Friday to 5:30 a.m. on Monday, i.e. 58.5 hours. The deadline pressed the need for hour-by-hour planning and extensive contingency assessments to ensure the bridge would be safe for use by the public upon reopening.

4. Extreme and unpredictable weather

Suspension bridges are vulnerable to wind, especially during rehabilitation, so structural integrity during construction was paramount in the design and in ensuring public safety throughout the project.

5. Atypical loading

To prepare for the potential of the bridge being open during a panel replacement, the suspended structure was assessed under live loads with an OSPD panel removed. Composite with the stiffening truss system, the OSPD transfers shear between the trusses as part of the lateral load resisting system.

Work was done at night and on weekends to mitigate impact on weekday commutes. Photo courtesy COWI.

A vital link

The MacKay Bridge is a vital link joining Dartmouth and Halifax. It provides more than $120 million of economic benefits to the Nova Scotia economy annually by enabling $73 million in employment income, supporting 1,145 jobs. The only other vehicular harbour crossing is the Macdonald Bridge, which cannot by itself accommodate all daytime traffic or heavy vehicles (excluding buses).

The project was executed with consideration to its impacts on Halifax Regional Municipality (HRM) stakeholders and events, including the Halifax Port Authority (regarding commercial and recreational shipping traffic that passes under the bridge), Halifax Transit (updating bus and shuttle routes) and the 2023 Parade of Lights. Local partners were major participants throughout the project and invaluable to its successful completion.

Further, HHB wanted to share the necessity and the engineering complexity of the project through the media to assist the public’s understanding. Photos, clips and dedicated social media posts helped achieve this objective by providing ample public notice and showing the work taking place on-site.

The most sustainable solution

For engineers and owners alike, the opportunity to extend the life of infrastructure assets should be top of mind when attempting to increase or restore capacity, prior to any consideration for a replacement and new design. There is a shared responsibility to mitigate carbon-intensive new construction.

The process of extending the capacity of an existing bridge will depend on its type, but the most sustainable solution is always to reuse the existing structure, making use of monitoring, inspection and testing. The ingenuity of engineers can develop schemes with a difference.

In the case of this project, an ambitious and rare scheme was performed to extend the life of an existing structure. Further, if the MacKay Bridge were ever closed for any period beyond a weekend, its associated carbon footprint would be significant, due to traffic idling and longer commutes.

Keeping ahead

Although the bridge was and remains in a safe and serviceable condition, the timely rectification of observed cracking has allowed HHB to reallocate resources to other maintenance matters. Eliminating the need for unexpected emergency repairs and sudden bridge closures has a far-reaching, positive impact on the residents of Halifax and Dartmouth.

Understanding the bridge was showing signs of its age, HRM residents adapted to the occasional nighttime and weekend closures for necessary maintenance, to stay ahead of the natural process of deterioration and to act promptly before it could become too severe.

While more full bridge closures will still likely be required for future work, HHB is continuing to improve the structure’s safety through careful planning and execution of rehabilitation programs, with COWI remaining a trusted advisor for these efforts.

The project has succeeded in extending the useful service life of the MacKay Bridge and validating a panel replacement program that HHB can resume at a future date as maintenance needs arise. And even though the work was performed on an in-service bridge, the impact on the public was minimized—and the project was completed on schedule.

Murray MacKay Bridge Deck Panel Replacement, Halifax and Dartmouth, N.S.

Award-winning firm (prime consultant, lead structural engineer and engineer of record): COWI, Halifax (Dillon Betts, PhD, P.Eng.; Jorge Perez Armino, P.Eng.; Claus Frederiksen, P.E.; Aaron Ferguson, P.Eng., Justin Thomas, EIT; Alex MacPherson; Sabine Wilkie, MASc, P.Eng.; Lily Xu, P.Eng.).

Owner: Halifax Harbour Bridges.

Other key players: Dexter Construction – Part of The Municipal Group of Companies (main contractor, asphalt resurfacing), Cherubini Metal Works (principal subcontractor, steel fabrication and erection), SOFiSTiK (construction software).