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SCHNEIDEREIT: Getting the complete lowdown on the Big Lift

Slow down. Bump. Speed up. Slow down. Bump. Speed up. Slow down. Bump. Bouncing over a series of temporary speed bumps has become a daily routine for me and thousands of other commuters crossing the Macdonald bridge during Halifax Harbour Bridges’ $207-million “Big Lift” project.
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When the city goes to sleep, the Macdonald bridge comes alive with an impressive sound and light show. About 70 workers will be plugging away at the $207-million Big Lift project for another year and a half. (TIM KROCHAK / Local Xpress)

Columnist Paul Schneidereit and photojournalist Tim Krochak recently got a close-up look at the Big Lift. You can find the rest of this in-depth series here.

Slow down. Bump. Speed up. Slow down. Bump. Speed up. Slow down. Bump.

Bouncing over a series of temporary speed bumps has become a daily routine for me and thousands of other commuters crossing the Angus L. Macdonald Bridge during Halifax Harbour Bridges’ $207-million Big Lift project.

The bumps are the traffic transition points covering gaps or reducing sharp angles left in the roadway as the Macdonald’s 61-year-old deck — the roadway you drive on — is completely swapped out this year.

If the obstacles make for a somewhat more herky jerky trip across the harbour, it remains remarkable they’re about the only physical manifestation — other than the difference in smoothness of the old and new surfaces — commuters are feeling as a busy bridge in a medium-sized city is replaced under their wheels.

SEE ALSO:

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Halifax Harbour Bridges' photo gallery

 

A video posted by tim krochak (@timbophoto) on

To explain how it’s being done, it helps to first understand how suspension bridges function.

The Macdonald and many other suspension bridges work in a way very similar to a clothesline, says Jon Eppell, HHB’s chief bridge engineer and Big Lift project lead.

The “clotheslines” are the two massive orange cables (one on each side of the roadway) strung in parallel — and looping over top of the two bridge towers — between Halifax Harbour’s opposite shores.

Using the same analogy, the “clothespins” are the vertical hangers (two smaller cables) that drop down, every 10 metres, from those main cables. The hangers, in turn, hold the “wash,” the roadway itself.

Other than the two towers, the entire weight of the span, including traffic, is borne by the two main cables.

“The deck is the clothes. The clothesline is the orange cable. Everything is hung from the clothesline,” Eppell, a friendly, patient man, explained at his offices beside the Macdonald bridge when we met in late April.

So when sections of the bridge deck are completely cut away from the rest of the roadway, they should still just hang there.

Of course, added Eppell, “we have to be pretty controlled about it because we’re dealing with some pretty heavy clothes.”

Just how much weight do the main cables have to bear?

The old deck, from approach to approach, weighed 5,700 tonnes. The main cable itself — the weight of which must also be carried — and vertical hangers add another 800 tonnes. When the bridge is crawling with three lanes of traffic, that’s another 800 tonnes. Altogether, the load could reach 7,300 tonnes — or more than 16 million pounds.

The new deck will be somewhat lighter, “only” 5,320 tonnes, a difference of slightly more than 835,000 pounds.

Either way, those clotheslines carry a heck of a load. So, as you’d expect, they’re well anchored.

A main cable, 343 millimetres or 13.5 inches in diameter, is actually made up of 61 separate galvanized steel strands — each 37 millimetres (an inch and a half) thick — held together by tightly wound wire. At each cable’s terminus, its individual strands are splayed apart and cemented, six metres deep, into the bedrock.

The main cables disappear from drivers’ view at short towers called cable bents where the approach transitions to the span itself.

OK, so how are parts of the span actually cut away and replaced?

The old roadway connects in three ways. There’s the deck itself. Underneath, adding strength, are small, parallel steel I-beams running in the same direction as the bridge. Finally, there are the stiffening trusses, the big steel structures painted green — the large side walls of girders on either side of the deck — that play a key role in spreading load. Without them, heavy objects like buses could make the deck sag in one spot.

When a segment of span is to be removed, workers with the contractor, American Bridge Canada Co., first cut the deck. (The workers are from Ironworkers Local 752). That doesn’t affect the roadway’s ability to carry traffic, so the deck is also cut ahead on several future segments to be replaced.

Next, the lifting gantry — that’s the large yellow frame structure commuters see clamped to the hangers just above the deck — is positioned over a segment to be replaced.

Right now, segments are being cut 20 metres long. Later, when work reaches the Halifax-side span over land, cuts will only be 10 metres. That’s because the segments will no longer be dropped and lifted using a seagoing barge, as now, but will have to be trucked on and off the bridge.

The lifting gantry is the workhorse. Weighing 60 tonnes, it uses heavy-lifting hydraulic devices called strand jacks that drop multiple cables that connect to lifting points on each deck segment about to be cut.

The strand jacks take up the weight of that segment. The vertical hangers are disconnected. Now, before anything more is cut, a critical step — achieving “zero forces” — is paramount. By raising or lowering their cables, the jacks adjust the forces pushing or pulling the segment in various directions. The goal is equilibrium.

“That means there’s no tension or compression, so that if I go and cut it, it won’t spring apart,” said Eppell.

When that’s done, workers first cut the stringers, the small I-beams beneath the deck. Then they cut the stiffening trusses, first the top, then the diagonal, then the bottom.

“That is the point of no return, basically,” he said.

Completely cut away, an old deck segment is lowered to the waiting barge. A new section is soon raised.

Which brings up the question many commuters no doubt ponder — how securely are new sections connected to the span?

Very, said Eppell.

After a new section is lifted into place, one end is bolted — with exactly 536 heavy-duty bolts to fasten decking and the new stiffening trusses (underneath the roadway on new segments) together — to the previous new roadway section installed. (The sidewalks and bike lanes will be bolted together later).

On the other end, where new span meets old span, temporary deck connections are used.

Temporary deck connections have two components, a nosing piece on the new segment and a frame secured to the old deck. The two pieces fit together — nosing piece sliding into frame — and are locked in place with “really big cotter pins.”

The vertical hangers (or clothespins) are attached to the replacement segment and lifting gantry’s strand jack cables disconnected. The clotheslines — a.k.a. the main cables — now carry the new roadway.

Two self-propelled mobile transports are then driven underneath the lifting gantry and take up its massive weight, allowing the lifting gantry to be unclamped from the hangers. The self-propelled mobile transports then move the lifting gantry further along the span, to the next section to be cut away, where the gantry is again clamped to the hangers.

The process of replacing a deck segment can then be repeated.

Now let’s get back to those bumps.

The temporary deck connection, which itself weighs 30 tonnes, is the reason for one of the bumps drivers encounter.

A thick traffic plate covers a gap of several feet created due to the temporary deck connection, said Eppell.

“The gap has to be there because, as we’re bringing this (new) deck up, we have to be sure we don’t smack into that one (the end of the old span). That’s one thing. That nosing I talked about, it takes up space, so I have to have it sticking out from this (new) deck. When I go to lower this (old) deck (later, in the next segment replacement), I need to have clearance so that I don’t hit the new deck I just installed.”

As work continues across the span, the location of that bump — the traffic plate covering the temporary deck connection gap — will also keep shifting.

What about the other two bumps I mentioned at the beginning?

The bridge is not, despite its sturdy appearance, an inanimate object. Temperature change, varying loads, wind conditions — all can affect the span, causing it to expand or contract slightly.

To allow for such movement, the Macdonald usually has four expansion joints — underneath the transitions from approaches to the span and at the two bridge towers.

Right now, however, two expansion joints are temporarily missing — Eppell says the integrity of the roadway is unaffected — due to the project.

A small traffic plate covers the missing expansion joint at the Dartmouth-side bridge tower. That’s a small bump, hardly noticeable. But there’s a more unforgiving bump — if you don’t slow down — over the missing expansion joint at the Dartmouth approach transition. Due to more severe angles created by the bridge work early on, a thicker traffic plate was needed there.

“We’re actually able to (soon) go in and replace that plate. We’ll be going with a thinner plate that will have reinforcing underneath it. It’ll still be a bump, but it won’t be as severe and we won’t need the asphalt. We’re anticipating that will happen in the middle of May.”

The expansion joints at the Halifax-side bridge tower and Halifax-side approach transition will eventually also be removed. All will be replaced once the new span is raised and repaved, probably in June 2017, said Eppell.

Take notice, motorists. That’s at least another year of bump, bump, bumping.

The entire project, employing about 70 workers, is still on schedule to be completed by the fall of 2017. Oh, and in case you were wondering why it's been dubbed The Big Lift, here's one last detail: At some point, after all the decking is replaced, they're going to lift the bridge by 2.1 metres.

Paul Schneidereit is a columnist with Local Xpress.

@schneidereitp



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