It was commuter chaos once again yesterday, when the Alex Fraser Bridge was closed for the second time in as many weeks with so-called “ice bombs” raining from its cables.
The bridge is just 30 years old, and it’s never been a big problem before… so what exactly is going on? We spoke to an engineer to find out.
While CKNW was unable to speak with someone from the firm that designed the bridge, we did speak with Allan Russell, President & CEO at McElhanney Consulting Services to try and get a handle on what’s going wrong with the Alex Fraser.
Russell admits there’s some angst in the industry trying to figure it out.
On the surface level, Russell says it’s clear what’s going on.
“On a cable stay bridge, the cables are always in vibration, the cables are always moving. And when yiou get the ice buildup it changes the aerodynamics of the cable a little bit, and as such it will vibrate and the ice will eventually release,” he says.
But what is happening doesn’t explain why it’s happening – a particularly troubling question since the problem hasn’t cropped up in the prior three decades.
On that issue, Russell says experts are still stumped.
He says the ice bombs have caught the attention of the Post-Tensioning Institute, the professional industry association for bridge builders of this type.
“They are starting to look at this. It is not something that has traditionally been identified as a problem,” he says.
Russell says cable-stay bridges like the Alex Fraser have run into problems before; 20 years ago, some bridges even failed when hit by very specific rain storm conditions that caused an interaction between water running down vibrating cables.
He says the industry was able to design solutions at the time, and may be able to yet again.
“It’s probably going to be something similar, and there’s going to have to be significant research going on with regards to the ice buildup,” Russell says.
But he says that could be slowed down by the fact engineers aren’t working with much data. If the problem has cropped up on other bridges before, it’s not been reported to authorities and data hasn’t been collected – meaning there’s still a lack of understanding as to exactly what’s going on.