|Two conceptual views of an "unbalanced" Burnside Bridge show the cable supported option, above, and the tied arch. (HDR)|
Unbalanced. Asymmetrical. Funky?
An advisory discussion by members of the Portland Design Commission and the Historical Landmarks Commission suggests that a new Burnside Bridge should be divided into three parts, and look unlike any other bridge spanning the Willamette River.
The 650 feet at the east end likely will be supported by two towers with cable supports or a tied arch. The center of the bridge would have a bascule mechanisms based within two piers extending deep into the river bed. The western portion of the bridge would consist of a truss structure positioned under the surface deck, thus keeping views unobstructed for west-bound travelers heading into the Skidmore-Old Town Historic District.
Members of the two commissions reached a tentative consensus on a three-part bridge based on unstable soils on the east side and the disparate urban environments on the river’s two sides.
“You are entering two different worlds,” said Don Vallaster, an architect on the design commission. While the west side has a largely 19th Century feel with its old historic buildings, the east side has become modern, angular, tall and glassy.
The Burnside Bridge was identified by regional officials in 1996 as the primary emergency route connecting 19 miles of city streets in case of a major earthquake. Several other bridges are expected to suffer serious damage or route obstructions in a magnitude-9 quake. The existing Burnside Bridge is expected to collapse at that magnitude.
Yet for bridge designers, the bridge needs to be more than a passage for vehicles, bicycles and pedestrians. They want it to represent the heart of Portland and the vital dividing line among four city quadrants. And they would be pleased if it can be attractive enough to draw favorable attention as a landmark and even be a draw for tourists.