Dear Editor,

Starting on December 9, the Port of Seattle will conduct live load testing for the Terminal 5 (T5) Bridge. The Port should be more concerned about the Swing Bridge.

Swing Bridge cracks first appeared in a June 2019 inspection. SDOT has reduced the speed limit to 25 mph (very few comply) to reduce cracking. The double-leaf swing bridge has two 240-foot cantilevers that meet over the river’s center, like two diving boards. Standing at the center of the bridge one can experience a few-inch bounce with every heavy vehicle crossing. Since its 1991 opening, 20 million heavy vehicle crossings have flexed these concrete cantilevers. In 2021 the opening of Terminal 5 will increase heavy vehicle crossings to over a million every year. Fully loaded articulated Metro buses weighing 35 tons may also return to the bridge.

Each flexure contributes to a potential fatigue failure, the progressive brittle cracking under cyclic stresses below normal load. There is no engineering formula to predict fatigue failure. Load cycles to failure can only be predicted by physical testing. The Swing Bridge is a large scale and long-term fatigue failure test.

Double-leaf swing bridges are very rare, and with one exception, all are built of steel. West Seattle has the only double-leaf concrete swing bridge in the world. 

If the Swing Bridge fails, the Port’s half-billion-dollar investment in Terminal 5 will be stranded in the west bank of the Duwamish.

The Port should work with SDOT and Sound Transit to evaluate an immersed tube tunnel as an alternative route across the Duwamish. A tunnel will allow trucks grade-level-entry for a quick dip under the river. Gothenburg, the largest port in Scandinavia, is opening just such a tunnel next week. Gothenburg's 1,000-foot six-lane tunnel cost $250 million. SDOT’s engineering consultant incorrectly estimate a 500-foot tunnel under the Duwamish will cost $1,992 million. This is an unbelievable eight times more than the Gothenburg tunnel, for a tunnel half as long. 

Please review attached the graphics and view the YouTube links to the Gothenburg Tunnel.

https://www.youtube.com/watch?v=BT9s2Pf9Wms&feature=youtu.be

https://www.youtube.com/watch?v=2kcAIBFCz8w&feature=youtu.be

Respectfully

Bob Ortblad MSCE, MBA