When the new SR 520 bridge opens up to traffic in phases on April 11 and 25, what will you get with the new bridge as you drive across it?
Not only is the new bridge a half century newer than the old 520 bridge, the technology is also upgraded. Heavier strong bolts hold the pontoons together, and tougher cables anchor the heavier bridge to the lake bed.
But let's start with size and design as it affects the practical aspects of driving.
The new bridge is a lot more resistant to storms. Cars on the old 520 bridge were a mere six feet above the surface of Lake Washington. Spray, even waves, would crash onto traffic. Sustained winds above 50 miles per hour forced the bridge to close. And even with winds in the 30s and 40s, driving across the bridge could be a white knuckle adventure.
The new bridge sits at least 20 feet in the air. There's an open space between the bridge deck where the cars drive and the pontoons below, which sit in the water and keep the bridge afloat. The pontoons are also wider, which makes the new bridge more stable during storms. The new 520 bridge is designed to withstand winds of up to 90 miles per hour.
Then there's the bridge deck itself, which is 56 feet wider overall. That creates a high occupancy vehicle lane in each direction to speed busses and carpools along, but also allows for a full-width shoulder. The shoulders on the old bridge are only two to four feet wide, meaning a disabled vehicle that even tried to pull over ended up blocking a lane of traffic. That issue is fixed on the new bridge.
On the elevated deck, the extra space also allows most of the maintenance to go on below traffic. On the old bridge, lanes had to be closed to allow access to the pontoon hatches and other parts of the bridge. Now all of that happens out of sight and out of the way.
A decade ago, growing concerns about the region's earthquake threats and storm hazards forced officials to rethink the survivability of the old 520 bridge. The hollow columns that hold up the fixed portion of the bridge at the high rises were considered vulnerable to earthquakes.
How long will it last? Since its opening in 1964, the old 520 bridge has served for just over half a century. The new bridge has what's called a "design life" of 75 years, and with proper maintenance, WSDOT engineers believe it could easily do 25 more, doubling the life of the old 520 bridge.
There is more work to do. The connection all the way to I-5 and replacement of the old trestle is still a few years off.
