Construction milestone: New 520 bridge pontoons reach midspan

Nov 17, 2014, 5:23 AM | Updated: 5:28 am
Drivers crossing the lake will notice an apparent milestone in the construction of a new floating b...
Drivers crossing the lake will notice an apparent milestone in the construction of a new floating bridge. (WSDOT Photo via Flickr)
(WSDOT Photo via Flickr)

Drivers crossing Lake Washington on SR 520 will notice an apparent milestone in the construction of a new floating bridge. But actually driving on what your tax and toll money is paying for is still more than a year away.

Take a peek to the north as you pass mid-span. Those gigantic, connecting pontoons that will support the new bridge have reached the draw span of the existing bridge.

“Does that mean we’re halfway across the lake?” asked project spokesman Ian Sterling, with the Washington State Department of Transportation. “Well not necessarily, because you have these approach bridges that roll up to the actual floating portion of the bridge.”

Okay, but the floating section is half-way across the lake.

“Half of the pontoons needed to cross Lake Washington are now in place, in their permanent home, anchored and bolted together,” said Sterling. “Sixty-six of the 77 total pontoons need to build the bridge are complete and all of those are on Lake Washington.”

Every one of the concrete pontoons fabricated for the 520 project will be finished by early next year. That’s behind schedule because of cracks discovered in the first batch of pontoons due to a design flaw. The fix added $200 million to the cost of the project, according to Sterling.

Drivers should notice an obvious design change; the height of the bridge deck.

“We have road deck that’s going in on top of the pontoons, unlike the current bridge where you drive directly on top of the pontoons, essentially, (with) the new bridge pontoons, the road deck is suspended above the pontoons themselves,” Sterling explained.

The decking will sit 20 feet above the pontoons, keeping wave action off the roadway. The new bridge is designed for winds of 89 miles per hour and to withstand a 1,000-year earthquake.

One of the most common questions about the new bridge construction is how will drivers transition between the new six-lane bridge and the existing, four-lane roadway on the west side? Just like you’d think. Merging. New, west side connections are on a later construction schedule or not even funded yet.

“As we bring these improvements online, they go on one at a time, two at a time as soon as we complete them, we open them up,” said Sterling. “It’s not the perfect solution at this point in time, we’re just slowly marching along.”

The new floating bridge – originally scheduled to open next month (Dec. 2014) – is now due to open in April 2016, barring new delays.

“Of course, it’s construction,” Sterling cautioned. “Just like building a house, there is a chance, always, that things will change but at this point in time, it looks like 2016 (the bridge will open.)”

The new bridge will feature transit/ HOV lanes, wider shoulders and a 14-foot-wide bicycle and pedestrian path.

Funding for the ongoing construction comes from a 2003 gas tax increase, a 2005 transportation tax package and tolling. The overall budget for the new 520 corridor from I-5 to I-405 is $4.3 billion. Of that, $1.4 billion remains unfunded.

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Construction milestone: New 520 bridge pontoons reach midspan