Joshua Cohen is a Principal at Fat Pencil Studio
Last night I walked down two flights of stairs into "the pit" of the Morrison Bridge. As part of a tour organized by WTS Portland, I got to see the current state of construction first hand, including the counterweight mechanism below the bridge. It's not the first time I've been "down below". Back in 2014, Fat Pencil Studio assisted Multnomah County in litigation related to the defective installation of a fiber reinforced plastic (FRP) bridge deck. Three years later, they are nearing completion of a new deck comprised of steel and lightweight concrete. The Eastern lift span deck is mostly complete, though the concrete is a bit rough pending installation of a thin driving surface to match the surrounding roadway. On the Western lift span, open steel mesh is awaiting a concrete pour. The last remaining FRP panels can be seen on the South side, with bolts and glue holding them in place.
The new concrete and steel bridge deck is more than twice as heavy as the open steel grate or FRP system that was installed previously. As a result, major changes had to be made to the counterweight and lift mechanism. Hundreds of steel plates were hung off the back of the already massive counterweight, and thousands of concrete blocks were carried inside by hand. The entire lift span is balanced on a trunnion pin, which also had to be bulked up to carry the added load.
All this engineering gets a live test this weekend when the new East span opens for the first time. If all goes according to plan, prime contractor Hamilton Construction will move ahead with pouring the West side deck in the coming weeks, and be on track to open the full bridge toward the end of October.