Today’s Columbian features an article on how CRC engineers may place the transit component inside the bridge structure, reducing materials costs by not requiring wider spans or an additional span for transit.
If a Box Girder construction method is used, there could be sufficient room for transit lanes (and emergency evacuation for pedestrians) within the interior structure of the bridge itself, without creating clearance issues for ship traffic:
Placing mass transit in one of the main spans would reduce the number of bridges to two and reduce the number of pillars to 17 from 21. That, in turn, might save 60 to 100 feet of riverfront for other uses.
No examples can be found in North America, but the design has been used in Asia and Europe, including the Reichsbrucke bridge in Vienna, the Nuselsky Bridge in Prague, and the approach to the Tsing Ma bridge in Hong Kong.
Its place under the roadbed would bring transit closer to the ground and allow a stop as close to the Columbia River as Fifth Street instead of Sixth or Seventh under other options.
One downside to the design would be the limited view for mass transit passengers. It would be essentially a tunnel. The design could be open at the sides, but the view for passengers looking east, generally a grand sweep of the river and Mount Hood, would be obscured at least partially by the adjacent northbound span.
The article indicates that this engineering work is very preliminary. It remains to be seen how much money this might save, and whether the pros and cons of the idea will be acceptable to the various stakeholder groups.
Read the full article: Bridge planners think inside the box.
A little Googling around found this site with a couple of good diagrams showing a cross-section of the Tsing Ma Bridge with transit inside. (Scroll to the bottom.) It illustrates how vehicle lanes and transit can fit together in one span without significantly increasing the size of a box girder bridge.