Snow and ice falling off your brand-new $2.5-billion cable-stay bridge, the widest such bridge in the world, and damaging cars below isn’t good news for anyone. It requires an immediate fix. As noted in December, a mix of rain, snow, freezing rain and temperatures that hovered around freezing in the greater Vancouver, B.C., area showed the unhappy result of the new Port Mann Bridge, which opened eight of 10 lanes on Dec. 1: falling chunks of snow and ice.
At the time, B.C. transportation officials weren’t sure if the December weather event was a freak mix of odd weather patterns or a design flaw. The same falling ice problem beset the 20-plus-year-old Alex Fraser Bridge, just a handful of miles downstream on the Fraser River, although at a smaller rate. But falling ice hadn’t been a problem there for the last few decades. Still, B.C. couldn’t take chances the falling ice would stick around on the Port Mann all winter, every winter, as it did on Dec. 20.
At the expense of the contractor, Kieweit-Flatiron, crews custom-designed a cable sweeper to fit around the bridge cables and remove snow and ice before it can accumulate. In addition, engineers have identified highly water repellent, or hydrophobic, coatings and de-icing sprays to apply to the 147,000 feet of cables. Officials were planning on testing a mix of four coatings—both on the bridge and in a specialized laboratory—to see which performed best in Vancouver-area weather. The coatings are in addition to de-icing applications, similar to what is used on aircraft.
Each sweeper fits around the outside of a bridge cable and is lowered and raised along its length using a specially designed winch, all while scrapers and brushes on the sweeper remove the snow and ice buildup. Each of the 152 cables over the roadway—the bridge has a total of 288 cables—received a sweeper in late January.
At approximately 2 ft long, an individual contraption contains four scrapers set one centimeter off the cable to avoid scratching the cable sheath. Four thick-bristled brushes wrap the cable behind the scrapers and rest directly against the surface of the cable sheath. Each scraper moves on eight cater wheels, four on the top and four on the bottom, ensuring a smooth glide while counter weights keep the steel-framed sweeper running evenly. A winch-and-cable system runs each sweeper at up to 200 ft per minute. The cables now installed with sweepers range from 230 ft to 820 ft in length.
“We must finish real-world testing, but we believe that the cable sweepers, coatings and de-icing sprays will be effective enhancements to the bridge,” wrote Scott Cassels, president of Kiewit Infrastructure Group Inc., in a statement. That real-world testing may have to wait until next winter, as the Vancouver area has seen above freezing temperatures much of 2013.
Tim Newcomb is Engineering News-Record’s Pacific Northwest contributor. He has also written for TIME, Popular Mechanics, Popular Science and more. You can follow him on Twitter at @tdnewcomb or visit his website here.