Today’s afternoon commute was excruciating with every bridge in town feeling the effects of a blast of winter weather. The new $2.46 billion Port Mann Bridge was hit the hardest thanks to a glaring design flaw that saw huge chunks of ice falling from its supporting cables onto the bridge deck, damaging vehicles and injuring motorists. Having just opened a few short weeks ago this is the first hiccup (and a major one at that) in the young bridges life.
The problem comes from the design of the bridge. The main support towers of the cable stayed section are located in the centre of the bridge between the east and westbound lanes. The suspension cables run from these towers to the bridge deck in 4 groups, 2 of which connect to the bridge deck along the inner lanes directly under the towers. The other two groups attach to the outer edges of the bridge crossing over all lanes of vehicle traffic as you can see in the picture above. This means that in winter weather if there’s ice build up on the cables it will almost always fall onto the bridge deck as it starts to melt. A pretty major design flaw in a part of the world that experiences winter weather.
There are other cable stayed (suspension) bridges in the Lower Mainland and they are also susceptible to cable icing and this has resulted in ice falling onto their bridge decks but they all have a design that makes it a very rare occurrence. In the photo of the Alex Fraser bridge at the top of this paragraph you can see that the cables go from outer towers straight down to the sides of the bridge never crossing any lanes of traffic. The Golden Ears bridge and Pitt River Bridge have a similar design. The Pattullo and old Port Mann bridge are steel arch truss bridges which means their supporting structures are significantly lower so any ice fall would be at a much lower velocity. Their arched design also provides a gentle slope for ice and water to run off of during ice melt.
I’m not an engineer, I’m a bridge and architecture enthusiast and as such the design flaw of the new Port Mann didn’t occur to me until ice was crushing peoples windshields today. That being said there are some very well paid, highly educated people who designed this bridge who missed this problem completely. It’s not as bad as the Tacoma Narrows bridge design flaw (see video below) but it is one that will rear its ugly head whenever the weather turns cold and icy.
What can be done about it? That’s a good question. Heating the cables will undoubtedly shorten their lifespan but I wouldn’t be surprised to hear that idea released to the public in an effort to calm peoples nerves and this option was dismissed by the designers due to cost. The best answer to that question is one that is never going to happen. The bridge should’ve been designed for ice build up, it wasn’t, it’s too late to change it. I guess someone should’ve told Kiewit Corporation, based in Omaha, Nebraska (which gets winter weather) that it can get cold here on the west coast.