George Massey replacement bridge to avoid ice bomb issues, says project head

The B.C. government says the lessons learned from the 'ice bomb' closures of the Alex Fraser and Port Mann bridges should ensure the replacement for the George Massey Tunnel won't suffer the same fate.

Norm Parkes, the executive director of highway operations says the most recent closure of the Alex Fraser Bridge on Monday was caused by a particular combination of weather and the colour of the cables.

"Overnight we had a fairly significant drop of snow. It was fairly still, and we anticipated the snow would stay on the cables," said Parkes.

"But then we got an inversion, the temperature rose ... and then the sun came out .... and the cables rose from –5 C to 5 C and caused it to shed all the snow."

One of the key factors was the black colour of the cables which allowed them to absorb the heat from the sun, he said.

"Our ice techs were telling us to watch out when the sun came out," said Parkes, who notes the Port Mann Bridge has white cables which don't heat up as quickly.

Another was a wind that pushed the falling ice onto the vehicle deck below.

Parkes says while the closures of the Alex Fraser bridge this winter are the first ones in the 30 years since it was built, it makes sense to have a look and to learn what happened and then perhaps make changes to the bridge.

No crossover cables

That's exactly what the team working on the replacement for the George Massey Tunnel is doing as well.

While the design and contractor for the proposed $3.5 billion bridge have yet to be selected, executive project director Geoff Freer says the specifications require the design prevents ice or snow from falling onto vehicles below.

One of the key factors is that no cables will be allowed to cross over the vehicle deck, which has proven to be troublesome on the Port Mann Bridge where chunks of falling snow and ice have smashed vehicles below.

"From the very beginning, our plan was to ask contractors not to have cables over traffic," said Freer. That means when snow or ice does accumulate, it is less likely to fall on the bridge deck.

The design would also have to ensure that any beams that might cross the deck also won't shed ice onto the deck either, which has been a concern during the recent closures of the Alex Fraser Bridge.

"The contractor will have to look at that and figure out a way to stop that from happening ... It all depends on what design you end up with," said Freer.

Snow removal system required

The second major requirement is a system like the one on the Port Mann Bridge that uses collars dropping down the cables to clear any snow and ice that accumulates.

Freer says the collar system on the Port Mann Bridge, which was installed after it was built, is actually considered to be one of the best solutions in the world to a problem that plagues cable-stayed bridges in snowy climates.

"Luckily, it doesn't happen here very often, compared to other parts of the world," says Freer. "A lot of places it happens fairly frequently and quite often they close the bridges to traffic."

"We've learned at other bridges around the world and from Port Mann, and they are doing the same thing. They are coming here to look at Port Mann because it has been the most successful system in the world."

One example is one of the largest cable-stayed bridges in the world, the Öresund Bridge that connects Sweden and Denmark, which was closed in 2011 and 2013 after chunks of ice began falling from the cables.

"There are people around the world looking for improvements, but right now, the Port Mann system is the best that is out there," said Freer.

Weather plays a role

Freer notes that some of the factors will be out of the engineers control, such as how frequently weather patterns align to load up the cables with snow.

"We are lucky to be close to a marine environment, and it doesn't happen as often. Things have to be just right to get a snow that is going to stick."

"You do need to get those special conditions when you have high pressure in the Interior and a low pressure coming in from the ocean."

The Lower Mainland has two other cable-stayed bridges, the Pitt River and the Golden Ears bridges, but Parkes says neither of these has had problems with ice falling from cables.

That's in part because neither has cables over traffic, and in part, because local weather conditions haven't led to a buildup of snow, he says.

Why use cables at all?

Freer says while the design for the bridge has yet to be selected, it is likely to be a cable-stayed bridge because that's the best way to span the width without putting a tower in the Fraser River and creating environmental concerns.

"It doesn't have to be a cable supported bridge, but given the distance across the river, the seismic and soil conditions, and we have a commitment to no piers in the river ... we anticipate there will be some sort of cable design."

The Lions Gate Bridge also has overhead cables, but Freer says weather conditions generally prevent snow from building up on them, something that will also factor into the George Massey design.

"It is right at the ocean, so we get less accumulation there when it does snow. It is generally warmer.  We anticipate similar conditions at the George Massey Tunnel. We are right at the ocean there."