SODO guideway construction

At last week's Sound Transit Board meeting, Sound Transit's West Seattle project manager Brad Owen showed the extend of the guideway from the SODO station along the Spokane viaduct, across Highway 99 to the Duwamish bridge. He explained that rather than pouring its concrete deck in place, they plan to use precast segments. That will speed up the process and reduce cost.

The rendering shows the enormity of the guideway, adding a 4th level to the existing 3 levels (at grade, Spokane viaduct, Highway 99). At least the underside would be a bit nicer than the i-beam girders Sound Transit has been using lately along I-5.

Sound Transit has been using precast box girder segments in the past such as in Tukwila:

Both i-beams and precast segments have originally been developed for highway construction.

While assembling segments is faster than in-place construction, in Malaysia they precast full segments between towers. Then you don't need a bridge between two towers for installation but a crane or two. That speeds up installation. Instead of a structural box underneath, they use two U-trough girder segments, one for each track. As the U wraps around the wheels, any rail noise is contained within the concrete trough. As the sides of the segment are at door level, during an evacuation from one train to another the center can help riders to get across to the other train without a major gap.

The latest guideways for the Grand Paris Express line use segmented box girders too. For roads the box must be under the deck, but for Line 17 they put the box above the deck to help with evacuations. This allows the bottom of the guideway to be totally flat. With slender towers it creates the most stunning elevated urban guideway design I have seen thus far.

If Sound Transit would build guideways that way, I bet people would be even more supportive of allowing elevated lines through the urban core.