Towers, cables in designs for Portage Bay stretch of 520 bridge
Three new design concepts, including two using tall cables, are being considered for the Portage Bay stretch of a future Highway 520 bridge. Skeptics call it political theater to create momentum for an underfunded project.
Seattle Times transportation reporter
About the conceptsConcrete box girder: Low profile without overhead structures; longest span 360 feet, from Capitol Hill shore into the bay.
Cable-stayed: Six towers 180 feet high, with high-tension cables holding up the road deck, including an 800-foot main span
Extradosed: Partly cable-supported, but needing a thicker road deck to add strength; six towers 65 feet high, and a 700-foot main span
Part of the new Highway 520 bridge might be tethered from high towers and cables above Portage Bay, if the state follows through with new concepts by a renowned bridge designer.
One idea shows "W"-shaped supports below the road deck, a nod to the nearby University of Washington.
Color drawings of a cable bridge, as conceived by architect Donald MacDonald of San Francisco, depict short towers that intermingle with the masts of sailboats, when glimpsed from water level.
The state Department of Transportation recently released graphics and tiny models for public consumption — even though the state remains $2.2 billion short of paying for the Seattle portion of the $4.65 billion Highway 520 replacement.
The three designs: a box-girder bridge similar to the West Seattle Bridge but shorter; and two options that include cables and six towers.
Cable bridges generally cost more than a generic, viaduct-type bridge like the current one, but they allow more space for boats to pass beneath, especially at the Queen City Yacht Club directly below the highway route.
A box-girder bridge allows for a shorter profile, uncluttered by overhead structures.
Discussions are early, to be followed by more public forums and review by the Seattle Design Commission.
The state DOT hopes to finish the new six-lane bridge by 2018, but the actual timeline is anybody's guess.
Construction is under way on the Eastside landings and the floating pontoons on Lake Washington, where that segment of the bridge is to be completed by the end of 2014.
This latest cycle of public process — after 14 years and $200 million spent on designs projectwide for the corridor from Interstate 5 to I-405 — surprised some neighborhood advocates.
Some have been fuming lately about ornamental bridge posts, known as sentinels, that the DOT plans on Lake Washington.
"There is a very definite feeling we don't want to stare at a bridge instead of nature," said Fran Conley, coordinator of the Coalition for a Sustainable 520, a group that is waging a court challenge against the DOT.
"We already have the significant landmarks, which are the lake and the mountains."
Animations and environmental documents last year depicted a generic, viaduct-style crossing. However, Program Manager Julie Meredith said the DOT made a commitment then to work further with the community on possible improvements, and that the new visions are part of that.
She says some people want a signature bridge. In previous years, citizen groups suggested a suspension bridge at Portage Bay, or a high bridge from Foster Island to Husky Stadium.
But Jonathan Dubman, a Montlake advocate involved in those discussions and a member of Conley's group, says he hears of no outcry for a landmark bridge nowadays.
• Concrete box-girder bridge. This design allows an elegant, linear look that gains favor with some architects and engineers.
The most prominent Seattle example is the 1984 West Seattle Bridge, supported by huge, lengthwise girders. Box-girder spans are about to be built on 520 where the bridge meets land on the Eastside.
• Cable-stayed bridge. This type of bridge usually becomes a city landmark. The Ed Hendler Bridge between Pasco and Kennewick, built in 1978, was the nation's first major cable bridge in the lower 48, following another cable bridge in Sitka, Alaska. Another is the Highway 509 bridge across the Thea Foss Waterway in Tacoma, opened in 1997.
• Extradosed bridge. This is only partly supported by cables, allowing for shorter towers, cables and spans. But, in turn, the concrete deck must be thicker. A nearby example is the Golden Ears toll bridge southeast of Vancouver, B.C., opened in 2009.
In all three of MacDonald's versions, the main span includes huge columns in the soil at Capitol Hill, as opposed to being centered in the bay. This allows the bridge to emerge from the hillside in a one-of-a-kind way, he said.
"We like something very light on the environment," he added. "If you go with lightness, you end up with a cable system."
The cable and extradosed versions show towers on either flank of the bridge, and a pair in the middle between the westbound and eastbound decks.
The half of the roadway near Montlake needs to keep a low profile, so the road deck can fit beneath a future lid there, DOT says.
Ted Lane, representing the historic Roanoke neighborhood on north Capitol Hill, said tall cables are a relatively new technology, and not compatible with the area.
"What's needed is a design that could have been seen in 1910 but is built with modern technology," Lane said.
One alternative might be an arch bridge, he mentioned.
MacDonald designed Portland's cable-stayed transit bridge, under construction over the Willamette River, and retrofits to protect the Golden Gate Bridge from wind and earthquakes.
He had an epiphany this spring while visiting the Seattle Yacht Club, on the Montlake shore. Looking beyond the masts toward Capitol Hill, he then turned east toward Mount Rainier, whose triangular form might be echoed by bridge cables in the foreground.
None of these designs is likely to happen, because of money, Dubman said.
"This is all such an elaborate fantasy that's unaffordable," he said, going on to call the outreach "a full employment program for bridge designers."
Now that tolls have greatly reduced 520 traffic, the state ought to consider a leaner, cheaper four-lane layout, Conley said.
Rep. Jamie Pedersen, D-Seattle, said that debate is over — but he added he believes shoulders should be reduced on whatever six-lane bridge is built. But lanes five and six will be needed for transit, he said.
New taxes or I-90 tolls are needed to fully fund the Seattle half of the crossing.
Kerry Pihlstrom, a DOT project manager, insists the design work is timely, "so we can have an informed design to move forward with, once the funding comes into place."
Meanwhile, another argument is brewing about whether to include a bicycle lane on the Portage Bay bridge, adding 14 feet of width.
Mike Lindblom: 206-515-5631
On Twitter @mikelindblom