One of the more important aspects of a floating bridge is that it stays afloat.

That seems like a simple enough design request, and the existing SR 520 bridge over Lake Washington has met that vital criteria for more than 50 years.

But the time has finally arrived for a more weather, earthquake and time-resistant structure to take over the task of connecting Seattle and the Eastside, home to tech giants such as Microsoft, Concur and Expedia. And while the new floating bridge will stand more firmly against wind and seismic activity, it will also carry more vehicles and people in more efficient ways— and just plain look better doing it.

Construction of the new 7,710-foot bridge started in January 2012, and this weekend, the Washington State Department of Transportation will throw a grand opening party to allow the public to walk across, bike on and gawk at the multi-billion-dollar project. It’s an engineering and technological marvel, and GeekWire got a behind-the-scenes look into the innovation behind the world’s longest floating bridge. We also previewed the monitoring devices and maintenance facilities that make this floating span — connecting one of the key high-tech corridors in the country — a vast improvement over its 53-year-old predecessor.

Maintenance building

On the eastern side of the lake, beneath the approach bridge that connects the floating structure to land, a three-story brick building sits in the shadows, facing west toward Seattle.

The building houses technology that controls and monitors various aspects of the bridge and roadway including lights, traffic cameras, fire suppression systems and more. A large backup generator sits on the ground level to power all systems in the event of a power loss. A spacious maintenance shop is also on site for building parts and making repairs to bridge components.

At the shoreline, a dock — which even allows natural light to stream through for migrating fish — provides access to boats which can ferry workers and equipment across the navigation channel and out to the bridge.

Perhaps most importantly, a small touch-screen panel in a nondescript corner of the building illustrates the bridge-control system. Similar panels are also in the anchor galleries of some of the bridge’s 77 concrete pontoons.

Chris MacDonald, an assistant project engineer, demonstrated how the system can relay vital information, such as whether one of the concrete pontoons is taking on water.

“Every pontoon, and what’s here in the maintenance facility, all looks the same and can give you all the same information,” MacDonald said. “You don’t have to be in pontoon A to see what’s going on in pontoon A.”

MacDonald said the system can trigger 23 different kinds of alarms, setting in motion a series of calls to WSDOT personnel.

“There’s also a fair amount of control we can do,” MacDonald said. “It’s not just reporting information in.”

Data from all of the various systems will not only feed into the 520 facility, but will be relayed to WSDOT’s traffic management center in Shoreline, Wash., which is staffed 24-7.

The entire monitoring and control operation is obviously an improvement over what the old floating bridge offered, because retrofitting just didn’t make sense, said Dave Becher, director of construction for the 520 project.

“If you went to I-90, which is [23 years old], it was state of the art at the time, but it can’t compete with this,” Becher said. “Hood Canal is probably better than that, but what you see is that technology increases and is improving.”

Pontoons

Descending from the top to the very bottom of the bridge’s pontoons happens in two stages, both times through a hole that’s about 2 or 3-feet wide. The first ladder accesses an anchor gallery which is the main area maintenance workers would need to work in. It is here that electrical systems feed a monitoring panel that’s the same as the one back on shore at the maintenance building.

“This is the brain of the pontoon,” Becher said. And while the old bridge made it about 50 years without this type of technology, the hope for the new bridge is a much longer lifespan.

“You want 75 years,” Becher said. “But the intent is that it can last indefinitely if you can maintain it. It’s all about maintenance. A lot of this is to avoid a catastrophic incident.”

This part of the pontoon also houses the 3 1/8-inch diameter anchor cable which extends out into the lake to tie into one of three types of anchors depending on the depth of the lake and location of the pontoon. The cables and anchors — affixed to the lake bottom — are what provide stability to the floating bridge.

Another ladder descends into the lower gallery of the 28-foot-high pontoon. The floor is about 21 feet below the surface of Lake Washington, and the interior, with metal doors connecting various cells, looks like a cross between a prison and a battleship.

You don’t really feel like you’re this far underwater, though claustrophobes may not want to experience this submerged adventure.

The most vital piece of technology in this space is a simple-looking leak detection system that is reminiscent of the ball and chain in your toilet tank.

“If for some reason they were taking water, it trips an alarm so you’re aware of which pontoon exactly it is,” Becher said of the device, which has a float switch that sits about 3 inches off the floor of the pontoon. In the event that a pontoon is breached, a pump line can be dropped into the lower gallery and controlled at the deck above.

Ballast rock can also be added to the lower gallery to weigh a pontoon down and control buoyancy.

There are 21 longitudinal pontoons which measure 360 feet in length and 75 feet across. WSDOT says a single one of these pontoons is a little more than 11,000 tons, or approximately 23 Boeing 747 jets. A single pontoon anchor can weigh 77 tons. Two cross pontoons and 54 supplemental stability pontoons make up the remainder of the floating brigade.

The old bridge had 31 total pontoons. WSDOT says more than 30,000 linear feet of cracks have been repaired in those pontoons since the Inaugural Day Storm of 1993. While the old bridge was designed to withstand winds up to 77 mph, the new one is designed to withstand 89 mph winds.

Roadway and bike/pedestrian path

The main attraction for commuters headed to and from Seattle and the Eastside is happening high above the lake and the pontoons. The new elevated bridge deck features two general-purpose lanes and one transit/HOV lane in each direction, as well as shoulders for disabled vehicles.

There is also a 14-foot-wide pedestrian/bike path on the north side of the bridge.

Construction manager Greg Meadows said a regional shared-use path is complete all the way back to Bellevue and is ready to hook into the new bridge and extend over to Seattle. The bridge path could also become a lane for cars if expansion of the roadway ever happens — to accomodate light rail down the center, for instance.

Along the path there is 8,500 feet of railing and curved viewpoints, or “belvederes,” feature benches for pedestrians or cyclists who stop for a rest. Signage will be installed to provide information on Native American history, geography of the lake and information about the bridge, Becher said.

At the east and west ends, the bridge’s most pronounced architectural details are decorative lighting “sentinels” to serve as the “gateway to the floating bridge.” The lights will be white, but there is already talk about whether special colors will be employed for a significant Seattle Seahawks moment, or maybe when the University of Washington takes on Washington State.

On the completed east end, the land side of the bridge meets the water side at a massive expansion joint. Down below the roadway, at that transition spot, there are slider plates that would keep the bridge intact in the event that the lake level dropped dramatically. Meadows said if the Ballard Locks failed, Lake Washington could drop 20 feet.

“Every year they raise and lower the elevation of the lake water 2 feet,” Meadows said. “The Army Corps of Engineers does that for Locks. So you have to be able to take a floating structure and give it some capability to move while maintaining a rigid structure on the other side.”

Noise-dampening measures have also been installed in this area as WSDOT says it wants to be a good neighbor to residents who live just a stone’s throw from the bridge.

Standing in the middle of the wide, new highway and looking south, the old bridge falls out of view. So does the threat of waves splashing over the sides causing potential closures — or dangerous slowdowns and accidents.

The future

In the midst of a technology boom that is bringing more and more people to the region, WSDOT is projecting a healthy increase in the number of people crossing 520 every day. The tech community is already split by the lake with companies such as Microsoft, T-Mobile, Expedia and Concur on the Eastside of Lake Washington, and companies such as Amazon, F5 Networks, Tableau Software, Zillow and others on the west side.

By 2030, approximately 130,000 vehicles carrying over 235,000 people are expected daily.

The $4.56-billion 520 project still has improvements to be completed on the west end, including:

A new Portage Bay bridge.

A new Montlake interchange and lid.

The south (eastbound) half of the new west approach bridge.

New ramp for transit and carpools to reach I-5 express lanes.

Extension of the bike and pedestrian path with connections to existing trails.

When the bridge opens to the public for the first time Saturday morning, and to cars later in April, it’s the hope of workers like MacDonald, the assistant project engineer, that people will positively react to a number of things: the fact that much of the project is done; the addition of extra lanes; the addition of a bike path; the creation of a beautiful roadway; and the removal of a significant risk from an aging structure.

“I’m also somebody who used to travel across this bridge every day. Any one of those things would be a great improvement,” MacDonald said. “Not only do we have a safer, bigger, faster, easier to travel bridge, we also have one that looks nicer.

“It’s not an iconic structure in the sense of the Empire State Building. But it is going to be something that when they compare it to the old bridge, it’s not just functional, it does have some aesthetic appeal. People 100 years from now are going to say, ‘It’s still a nice bridge.'”