A consultant hired by the Halifax Dartmouth Bridge Commission (or Halifax Harbour Bridges, HHB) recommends that the MacKay Bridge be replaced with a new six-lane, cable-stayed bridge with a 500-metre long main span, costing $1.05 billion.
The consultant is Darryl Matson, a senior vice-president with COWI North America Ltd., a bridge and tunnel engineering firm. COWI was hired to “support HHB’s application to the Nova Scotia Utility and Review Board (UARB) for a toll rate increase due to the significant spending requirements anticipated in the next ten years (2021-2030).”
The proposed toll increase would see cash fares for cars go from the existing $1 to $1.25, and the MacPass fare for cars from 80 cents to $1.
The toll increase application seeks to cover over a quarter-billion dollars in capital, rehabilitation, and maintenance costs that HHB says it will incur over the next 10 years, as shown in this chart:
Those figures have not been examined by any third party, nor by the UARB. As of Friday, the UARB has not scheduled hearings on the toll increase proposal.
Matson notes that the 10-year budget related to the Macdonald Bridge “is needed to complete the major rehabilitation for the bridge elements that were not replaced during the Big Lift Project. This includes steel and concrete repairs of the approaches and painting of the portions of the bridge that were not replaced during the Big Lift, as well as regular resurfacing of the bridge deck.”
In contrast, the 10-year budget for the MacKay Bridge “is primarily needed to maintain the ageing bridge. This includes deck repairs for the approach spans, resurfacing, main cable dehumidification, coatings, and repairs of anticipated fatigue cracks in the main span deck.”
Further, Matson writes that “performing a major rehabilitation of the MacKay Bridge is not practicable. Therefore, while the bridge remains operationally and structurally safe, it is nearing the end of its useful life, and the ongoing maintenance and rehabilitation costs are expected to increase until it is replaced.” He says that should happen by the year 2040.
Preliminary scoping work for the new bridge is proposed to be conducted during the 2021-25 period, at a cost of $1.06 million. And early design work, land acquisition, and environmental assessments are proposed for the years 2026-30, at a cost of $12.42 million. That total of $13.48 million is included in the above table, and therefore in the application to the UARB for a toll increase; the bulk of the $1.05 billion price tag for the new bridge, however, is not included in the toll increase proposal, as those costs will be incurred after 2030. The report does not say if further toll increases are anticipated to cover those costs.
What’s wrong with the existing bridge?
There are plenty of suspension bridges around the world that are far older than the MacKay Bridge and that do not need replacing. The Golden Gate Bridge was opened in 1937, for example, and is still going strong. But suspension bridges need constant, if relatively inexpensive maintenance, and there have been a handful of spectacular failures.
A COWI-authored MacKay Bridge Feasibility Study completed in November 2020 is included in the UARB application.
“The MacKay Bridge opened to traffic in 1970 and is approaching the end of its service life,” explains the introduction to the study:
Details used in the original design, particularly those in the orthotropic deck, have led to earlier than anticipated fatigue issues that will not be easily repaired. HHB expects the MacKay Bridge would require significant rehabilitation work between 2030 and 2040 if it were to continue safely carry traffic, primarily because of the flexible deck system and associated fatigue vulnerabilities, and the amount of work required is expected to grow every year the bridge remains open beyond 2033.
A section labelled “Challenges Facing the Existing Bridge” explains how a 50-year-old bridge is falling apart:
• Suspended spans deck plates are significantly thinner than current codes allow, resulting in excessive deflections (strains) with truck passage and associated fatigue issues;
• Many of the stiffening truss elements in the suspended spans likely require strengthening to meet current standards: the truss diagonals and bottom chords are at their capacity, and the top truss chords would need to be carefully evaluated and strengthening is likely. Strengthening these elements would require additional weight be added to the bridge, for which there is no current capacity. This in turn can create a cycle of strengthening elements to support the strengthening components;
• When considering rehabilitation options, it is not practical or cost effective to strengthen the existing orthotropic deck system due to the details of the existing design, and therefore, rehabilitation options are only practical if the suspended span superstructure of the bridge is replaced. Since a replacement suspended span would be significantly heavier than the existing superstructure (due primarily to the added deck plate thickness and larger trusses), it is highly likely that the main cables would need to be supplemented with additional strands, and the towers and bents would also require significant strengthening;
• Previous inspections have found corrosion throughout the structure, both in the suspended spans and the approaches. These locations include the orthotropic deck plate top surface, main cable strands, and the top flanges of the approach box girders. The corrosion observed in some of these locations is significant, and significantly more than would typically be observed on a 50 year old bridge. Substantial maintenance would be needed in the future to extend the life of the bridge;
• Main cable inspections have confirmed the presence of moisture within the cable bundle, up to Stage 4 corrosion (NCHRP scale) on strand wires, and some broken wires. To limit future deterioration of the steel, cable dehumidification would be recommended to extend the life of the main cables if they are kept in service;
• Tower foundations would need to be evaluated for increased loads including possible impact by Panamax/Post-Panamax vessels;
• Raising the deck to achieve increased navigational channel clearance would be limited by the distance between the deck and main cables; and
• Deck widening to add additional traffic lanes would be difficult due to the configuration of the stiffening trusses and cable bents. Active Transportation (AT) lanes are feasible as they can deviate around the towers and cable bents.
Fix the old bridge or build a new one?
COWI examined the following options for the bridge:
Option 1 – Rehabilitate the Existing Bridge
• Option 1A – Rehabilitate – No added features – 4 lane
• Option 1B – Rehabilitate – Add two AT lanes – 4 lane + AT [Active Transportation — a bike and pedestrian lane]
• Option 1C – Rehabilitate and twin the existing bridge – 6 lane + AT
Option 2 – Replacement
• Option 2A – New 500 m main span – 6 lane + AT – cable-stayed bridge
• Option 2B – New 500 m main span – 6 lane + AT – suspension bridge
• Option 2C – New 800 m main span – 6 lane + AT – cable-stayed bridge
• Option 2D – New 800 m main span – 6 lane + AT – suspension bridge
• Option 2E – New 500 m main span – 4 lane – cable-stayed bridge
• Option 2F – New 500 m main span – 4 lane – suspension bridge
A detailed analysis found that the best option for rehabbing the bridge is Option 1A, at a cost of $840 million. That option has the lowest cost of the three rehabilitation options, and maintains the existing bridge alignment. It would entail a rebuilding of the bridge reminiscent of the redecking of the Macdonald Bridge, but would additionally require replacing the large suspension cables. The sequencing of construction would look like this:
• Strengthen foundations, towers and piers on suspended spans and approach spans
• Modify cable anchorages and install new tower saddles
• Install new or supplementary cables
• Replace suspended spans deck system
• Replace approach spans deck system
That deck of the rehabbed bridge would be raised one to four metres, allowing some, but not much, additional clearance for ships. It would have a design lifetime of 75 years. (Sea level is expected to rise two metres over that period.)
On the downside, during construction there would be “significant impact to travelling public due to lengthy road closures,” and there would be no ability to add active transportation lanes to the structure.
As COWI sees it, there’s much more bang for the buck for a new bridge, and Option 2A is the best of those. That option is costed at $1.05 billion.
The span would be built just to the north of the existing bridge, so there would be no disruption to traffic during construction.
This bridge would have a design lifetime of 100 years, and would be raised 10 metres above the existing bridge’s height. And, says the study, there are other advantages to this proposed bridge, most especially that it would have six lanes — two regular traffic lanes in each direction, plus one lane in each direction for emergency vehicles and buses, and potentially as high-occupancy vehicle lanes. As well, there would be two three-metre wide active transportation lanes — one for pedestrians and one for cyclists.
On the downside, Halifax Harbour Bridges would have to acquire more property, particularly on the Dartmouth side, at and around the Bedford Institute of Oceanography. In particular, the Canadian Food Inspection Agency building would have to be razed and replaced.
Additionally, an “S” curve in the roadway is required on the Halifax side of the harbour. (“On the Dartmouth side, a single curve to tie in north of the existing curve near the current toll plaza would be possible,” says the study.)
As well, the eastern tower would be in the water, and so therefore would have to be strengthened for potential ship collisions.