The location of a replacement structure for the aging Magnolia Bridge has been chosen after a few starts and stops, but now the Seattle Department of Transportation is asking the public for its thoughts on design details.
But it didn't quite work out that way at a public meeting SDOT held last week in the Catharine Blaine School auditorium. "It was kind of interesting," said project manager Kirk Jones. "Most comments did not have anything to do with the structure."
Instead, among other concerns, Magnolians at the meeting wanted to know how SDOT could slow bridge traffic as it reaches the Magnolia Bluff. "It's an issue that was brought up early in the process," he said, noting, "It's not really a bridge problem."
Traffic noise was also a concern for some. "From what I recall, I quoted just a couple decibels' change by 2030," Jones said. That's based on a 1-percent growth rate in traffic each year, he added. "It wouldn't be very noticeable."
SDOT has proposed using one of several different column types and one of several different girder types for the new bridge, depending which section is involved. Costs for the different types of columns and girders for each section of the bridge were also included in the presentation.
The choices for column types includes one with a curved flare. At a cost of $6 million, it would give the bridge a "classic appearance," according to a matrix analysis handed out at the meeting. There was also an angular flare column, which would cost $5.8 million and have a "timeless architectural style." Both of those types could allow textured surfaces, but both might require minor repairs after an earthquake.
The third choice was a tapered column. At a cost of $5.6 million, it would be the cheapest option, and while it would have a clean appearance, there would be limited opportunities for textures and highlights, according to the SDOT analysis.
There are four choices for the types of girders that would rest on the columns. At a total cost of $47.9 million, pre-stressed concrete girders would be fairly easy to build, but they would be less aesthetically pleasing, and there are engineering limits on curved sections of the bridge.
A second type was haunched, cast-in-place concrete girders that would be used only at the 15th Avenue West overpass as well as at the section of the bridge near Magnolia Bluff. The haunched version has an arched shape at the bottom, Jones said, and they would be able to accommodate curved sections but take longer to build. The two sections would cost $22.9 million, according to SDOT figures.
A third option is the straight cast-in-place concrete box girder. At a total cost of $30.6 million, that type of girder could be used everywhere except on the approach to the Magnolia Bluff, and it could accommodate a curved alignment, according to SDOT.
At a total cost of $30.5 million, steel plate I-girders could be used everywhere except by the bluff; they could also accommodate curved alignments.
And finally, at a cost of $27.2 million, a haunched steel-box girder could be used on the approach to the bluff. However, it would be able to handle only lower seismic loads because of less weight and reduced foundations, according to SDOT.
Bicycle and pedestrian amenities will also be included in the design, but what they will be is still up in the air, according to Jones. "We don't have details, and we are not ready yet to make a decision," he said.
"This is one element of input," Jones said of the public meeting, "and we'll be running this in front of our DAG (design advisory group)." The group meets the first Monday of every month, he said.
Russ Zabel can be reached at (206) 461-1309 or by email at rzabel@nwlink.com.[[In-content Ad]]
