Haifax-Dartmouth Bridge Commission

[OldDartmouthMark]

Originally Posted by someone123

The idea that buildings have a lifespan and approach end of life is something I haven't heard nearly as much outside of a NS context. There are buildings with engineering flaws or components that wear out and discussions about whether it's worth maintaining them. Europe has many 800 year old wooden buildings.

Originally Posted by Offshore1

A short lifespan like is being discussed strikes me as odd. The Golden Gate bridge in San Francisco was completed in 1937. It had a retrofit about 20 years later to prevent twisting in the wind and deck replacement (40% lighter) done over 4 years from 1982 - 1986. I haven't seen any mention of a design lifespan. https://en.wikipedia.org/wiki/Golden_Gate_Bridge

See CBC article linked below:

https://www.cbc.ca/news/canada/nova-scot...eplaced-or-refurbished-by-2040-1.7583263

The engineers are the ones discussing lifespan, but presumably they are taking all of the factors into consideration, such as material fatigue, corrosion, deck degradation, etc., plus the potential for increasing capacity for the future. Is lifespan more of a concept relating to the calculation of cost for new build vs refit/modification? Perhaps. Regardless, their conclusion seems to indicate that replacement is the best option.

[Keith P.]

I'm no engineer, but older bridges were generally built much more heavily due to the understanding at the time of the ability to carry loads which also included the use of poured reinforced (and heavy) concrete decks. The MacKay uses an orthotropic deck which conceptually is similar to a deck built off the back of a house with supports and beams, on top of which in this case a steel deck is attached, usually by welding to make it a single unit. That ends up being lighter and hence requires less strength in the towers and cables. But as we have apparently learned, it may not be as long-lived as other types.

Bridge designs have changed over the years, from the heavy types used in the 18th and early 19th century like the Brooklyn Bridge, to suspension bridges like the Golden Gate and those here, to cable-stay designs that were more recently de rigeur, to futuristic-looking arch designs that are now all the rage.

[OldDartmouthMark]

It’s been many years since I’ve really thought about it in depth, but off the top of my head… The more lightly a bridge structure is built, the more it has a chance to be affected, in terms of metal fatigue, by cyclical loading, such as that caused by driving vehicles over it. Each metal, and it varies by alloy, has a limit, as a percentage of its yield stress, an endurance limit. For most steel, if the cyclical stresses remain below its endurance limit, it can endure an infinite number of loading cycles without experiencing fatigue. If loading causes the steel to be functioning above its endurance limit, then it will have a finite number of cycles before it experiences failure due to fatigue.

Vehicles in 1970 were large and heavy, but not as heavy as they are now. I recall in the 1990s comparing the weight of a really big old car, a 1971 Chrysler New Yorker, IIRC, and its weight was around 4300 lbs. I was surprised to find out that a mid sized SUV at the time, an Acura MDX, weighed around the same. Now vehicles are even heavier than that, with large pickups like many drive now weighing in around 6000 - 7000 lbs. EVs, as are becoming more popular now, are even heavier in comparison to an equivalently sized ICEV. So cyclical stresses on the bridge will naturally be greater.

Another consideration for lighter construction, i.e. structural members with smaller cross sectional area, is corrosion. As a percentage of its mass, thinner members have more surface area, and thus experience more reduction in strength as their surface becomes corroded. Additionally surface imperfections caused by corrosion cause local stress risers which will also make them more susceptible to fatigue. This is why they are continually painting the bridges, but it’s not perfect and corrosion happens.

This is really simplified and in layman’s terms, but the key takeaway is that if a bridge is built less robustly, it will have a shorter lifespan, objectively, than one that is built more robustly, like the Macdonald, the Lion’s Gate, and apparently, the Golden Gate. The “science” supports it, and as is popular now… there’s data on that… lol

Someone who really knows bridges could explain it much better than I ever could, and as KP says, there are many aspects and many different bridge designs and philosophies involved. Also, in the “old days”, things were built to last… overbuilt in today’s terms, as structure failures still occurred and engineers applied huge safety factors and then some when they designed things. Over the years “we” became more conscious of being economically thrifty and are always looking for ways to cut costs through using less material and increasing efficiency, but this has other costs, such as a shorter service life and essentially kicking the costs down the road for future governments to deal with.

[OldDartmouthMark]

I should add to the above, that this is only general information of how building structures closer to the limits can affect durability. I am not privy to the specifics of the MacKay Bridge, and don’t consider myself qualified to provide reasonable analysis, even if I had the information. It’s mostly food for thought for those who might wonder why designing a structure robustly can result in a longer lifespan. I mean, anything can be fixed if you want to spend enough money and time to do so. You could even end up with a George Washington’s axe situation where everything is replaced or upgraded piecemeal until none of the original structure remains after a period… but then maybe that might not meet the requirements for increased capacity after all that. You’re certainly not hanging an LRT deck off of it in its current configuration.

[Keith P.]

Originally Posted by OldDartmouthMark

Vehicles in 1970 were large and heavy, but not as heavy as they are now. I recall in the 1990s comparing the weight of a really big old car, a 1971 Chrysler New Yorker, IIRC, and its weight was around 4300 lbs. I was surprised to find out that a mid sized SUV at the time, an Acura MDX, weighed around the same. Now vehicles are even heavier than that, with large pickups like many drive now weighing in around 6000 - 7000 lbs. EVs, as are becoming more popular now, are even heavier in comparison to an equivalently sized ICEV. So cyclical stresses on the bridge will naturally be greater.

One thing I saw somewhere recently is that the electric buses HRM purchased a while ago not only cost twice as much as the traditional diesel jobs, but also weigh twice as much as well. Yikes!

[OldDartmouthMark]

I don’t know the specifics of the buses that Halifax is getting, but yeah it sounds like the extra weight is not without its problems:

https://eaglepressnews.com/headlines/hea...aise-concerns-for-local-road-maintenance

[someone123]

Presumably the metal fatigue would mostly apply to the decking, and re-decking is common. It would be interesting to hear more specifics about the MacKay.

[OldDartmouthMark]

Originally Posted by someone123

Presumably the metal fatigue would mostly apply to the decking, and re-decking is common. It would be interesting to hear more specifics about the MacKay.

It would be nice to know more. Maybe someone reads this forum who actually knows stuff, and can chime in with some better information, rather than rudimentary speculation.

As far as re-decking is concerned, a quote from the old article I linked to above.

Though the study isn't done, Wright said it's clear upgrading the structure would be complex — the approach spans need work and the deck's thickness is well below the current Canadian standard.

"You would have to fortify, strengthen the main towers, you'd have to make sure the foundations can withstand that additional loading. You'd have to install a new main cable or supplementary main cable. and then you're ready to do your big lift, none of which had to happen on the Macdonald."

Anything can be rebuilt, depending upon your standards of value/cost, so I guess with that in mind the lifespan of every structure in normal circumstances is infinite. I probably should have just said that and saved some time.

[OldDartmouthMark]

On a completely different subject related to the HH bridges, the story of a mystery bell:

Bridge bell: A Halifax mystery still ringing

Tucked away in a dark HHB storage space sits a 126‑year‑old brass bell, weighing roughly 250 pounds. It bears only two markings:
Meneely Bell Foundry, Troy, New York — 1899.

The Meneely Foundry was one of the most respected bellmakers of its era, renowned for church bells but also known to produce bells for ships, factories, and businesses. The foundry closed in 1952, and surviving inventories make no reference to this bell.

There are no inscriptions to identify its original home. No church name. No dedication. Just a date—and a mystery.

What we do know raises as many questions as it answers.

Somewhere between 1899 and 1955, the bell disappears from the historical record. It may have rung from a church tower. It may have served aboard a vessel. It may even have been blown into Halifax Harbour during the 1917 explosion, sitting half‑buried in mud for decades.

Then, in 1955, the bell resurfaces—this time as a fixture on the Angus L. Macdonald Bridge.

Veteran bridge staff are confident it once hung beneath the span, roughly 10 metres above the water, and there is reasonable speculation that it served as a manual warning bell for ships passing below. No wiring or control panel remains, but those components may have been removed as technology evolved—from mechanical warnings to electric foghorns, and eventually to today’s digital systems.

For decades, the bell remained largely unnoticed—until a curious maintenance crew decided to ask questions.

Maintenance Supervisor David MacFadden, one of four workers who helped move the bell, remembers the effort clearly.

“It took some time and some muscle, but we were curious. It was just wasting away down there. We thought maybe it should be on display or part of a bridge exhibit.”

The bell was carefully hauled up to the bridge deck and later placed into storage. At one point, it was even marked for scrap—but survived.

Today, the bell remains safe, silent, and unresolved.

“It’s tough to put a 126‑year‑old bell on display when you really only know a fraction of the story,” says Steve Proctor, Communications Manager. “Maybe it’s a romantic view, but if we could find it came from one of the churches destroyed in the Explosion, we’d love to return it.”

Even if its origins are not ecclesiastical, uncovering the bell’s first 50 years would help restore a missing chapter of Halifax’s past.

“At the very least,” Proctor adds, “we’d have something meaningful to put on a card if we choose to display it.”

And so the questions remain:

Where did this bell ring first—and does anyone have any memory of ever hearing it ?

If you have photographs, documents, family stories, or leads that might help trace the bell’s journey, HHB would love to hear from you at [email protected]

.Some histories are written in books.

Others are waiting—quietly—to be heard again.

And a video report from the CBC:

Halifax bridge commission needs help solving bell mystery

[HalifaxRetales]

I have a decent inside source

the current plan (not finalized) for the bridges is
is that a third crossing will be built near the MacKay ~5 years

a cable-stayed design (like the Zakim by the TD Garden in Boston )
the plan is 6-8 lanes with 2 of them being protected for transit

the MacKay would remain as long as it is safe to maintain

land etc still needs to be aquired

[OldDartmouthMark]

^ Very interesting. Thanks for sharing!

[Keith P.]

The preferred plan was always for a new "new bridge" to be built parallel to the MacKay. That is the only location that makes sense given approaches on either side.

I can see the old "new bridge" being limited to passenger vehicles and light trucks only, like the Macdonald is, to extend its life and reduce maintenance demands. Cable-stayed bridge designs are the current soup du jour in the engineering community.

[terrynorthend]

Originally Posted by Keith P.

The preferred plan was always for a new "new bridge" to be built parallel to the MacKay. That is the only location that makes sense given approaches on either side.

I can see the old "new bridge" being limited to passenger vehicles and light trucks only, like the Macdonald is, to extend its life and reduce maintenance demands. Cable-stayed bridge designs are the current soup du jour in the engineering community.

That sounds about right. The immediate drop in demand on the McKay by opening another bridge will also help extend it's life.

BTW I got a kick out of your naming. So that means we'll have an old new bridge, a new old bridge, and a new new bridge

[JET]

Originally Posted by terrynorthend

That sounds about right. The immediate drop in demand on the McKay by opening another bridge will also help extend it's life.

BTW I got a kick out of your naming. So that means we'll have an old new bridge, a new old bridge, and a new new bridge

Are you not leaving out the old old bridge?

[ArchAficionado]

Hopefully whatever eventually gets built (and it won't be 5 years. maybe 5 years until they start the design process, probably more like 15 until something is actually operational), they'll have the foresight to design the transit ROW to accomodate future conversion to light rail or trams. Big, expensive infrastructure like this needs to be built with future proofing in mind, because decades elapse before the opportunity to build the next one arises.

[someone123]

They could probably move somewhat quickly to start construction if the will is there. This sounds more or less like what they've been talking about for years.

I wonder if they will also fix the approaches, if it ties in correctly with the Windsor St Exchange, and if there will be further work to improve the 102-WSX connection.

It's unrelated but I also think a Northwest Arm bridge could be a good project, and the knee-jerk opposition has a lot to do with assumptions about any bridge being a freeway-like conduit for commuters. That's not what it has to be like. If you had a hypothetical bridge that carried electric buses and active transportation, for example, it could have a pretty mild impact. It can be attractively designed and complement the area as an amenity for residents, not just commuters. The discussion around it reminds me a lot of past discussion about how highrises are evil, all look like concrete boxes, and then the reactionary voices who said NIMBYs would forever block their construction (even years after HRM reformed its policies and a lot of construction already started).

[Keith P.]

The main issue with an Arm bridge regardless of traffic type is where it lands on the peninsula side and on the mainland side. Both areas are ritzy areas of HRM where such things would be very unwelcome. A less contentious (but less effective) option might be a short bridge (or even an overpass ramp) from the end of Herring Cove Rd to Quinpool at the Horseshoe Island area to eliminate the need for that traffic to enter/use the rotary.

[terrynorthend]

Originally Posted by JET

Are you not leaving out the old old bridge?

No, Macdonald is the new old bridge. It got that name after the big lift.
Old new is the McKay
New new is this proposed bridge

[someone123]

Originally Posted by Keith P.

The main issue with an Arm bridge regardless of traffic type is where it lands on the peninsula side and on the mainland side. Both areas are ritzy areas of HRM where such things would be very unwelcome. A less contentious (but less effective) option might be a short bridge (or even an overpass ramp) from the end of Herring Cove Rd to Quinpool at the Horseshoe Island area to eliminate the need for that traffic to enter/use the rotary.

It's not very helpful for cutting travel distance though. South St over to the park looks reasonable as long as it's not a wide bridge.

Realistically these areas should also be zoned for at least medium density. A lot of the monster homes basically are medium sized buildings, just organized as a single housing unit instead of 4 or 8.

[Keith P.]

Except South St is a narrow 2-lane sharply uphill that goes through very expensive neighborhoods adjacent to Dal. On the opposite side any access would cut through Sir Sanford Fleming Park. Not a chance. You might do better with a proposal to use a trebuchet.