February 3, 2009
Thank you for writing the SR 520 Corridor Program regarding a suspension bridge alternative.
Early on during project planning, WSDOT considered a range of options for crossing Lake Washington, including replacing the SR 520 floating bridge with a suspension or truss bridge. WSDOT eliminated these bridge designs from consideration for several reasons, most notably lake conditions, cost, and size.
The physical characteristics of Lake Washington create challenges for supporting a suspension or truss bridge. These challenges include depth and a soft lake bed. The lake is very deep, at approximately 200 feet under most of the bridge. In addition, soft soils below the lake crossing make it extremely expensive and difficult to construct the large towers required to support a suspension or truss bridge.
Another cost prohibitive factor of building a suspension bridge across Lake Washington is the length. The bridge would have to span over 7,500 feet, making it the longest suspension bridge in the world. Today, the longest existing suspension bridge is the Akashi Kaikyo Bridge in Japan, spanning 6,527 feet. The Akashi Kaikyo Bridge took 10 years to build at a cost of $3.6 billion. The likely cost of a Lake Washington suspension or truss bridge would exceed $4 billion – the likely cost of the entire SR 520 corridor from Seattle to Bellevue Way.
Finally, the sheer size of the structure would have considerable impacts to views in the area. The towers supporting the main span would need to be about 900 feet tall, approximately 50% taller than the Space Needle. In addition, the two anchor piers required at the ends of the spans would be massive concrete supports.
The safer SR 520 bridges will be designed and built to:
• Have a 75-year design life;
• Withstand a 1000-year event earthquake; and
• Weather windstorms up to 92 miles per hour.
Please visit the project Web site for the latest news and information on the program.
Thank you again for your interest.
Julie Meredith, P.E.
SR 520 Corridor Program
You have confused main span and bridge length in your analysis. The main span of the Akashi Kaikyo Bridge is 6,500 feet. The length of the bridge which includes the approaches on each side of the main towers is 12,831 ft overall. An engineering marvel to be sure!
An Evergreen Point suspension bridge would be very comparable the 8,000' Firth of Forth Roadway bridge in Scottland (main span 3,300') . This was constructed at the same time as the Evergreen Point Bridge but with a 120 year design life. The SR 520 crossing is much much closer in scope to the Tacoma Narrows than the Akashi Kaikyo. Even doubling the cost of that project we come in very close to the current estimates for another concrete raft and have an iconic structure that will last well into the next century.
Check again on the tower height. The WSDOT website says they would need to be 600', not 900'. That would put it right in line with other suspension bridges of that size. However, all of those bridges span shipping lanes and have twice the clearance the Coast Guard has approved for Lake Washington.
Look at the charts for Lake Washinton. The eastern tower would be in approximately 100' of water. On the western end there is an area which is 200' deep but it's not clear if this is where one of the towers would need to be located. Even if it is we solved that problem seventy years ago:
From the WSDOT website
The Engineering Challenge in 1938
The Tacoma Narrows is a difficult place to build a bridge. The water is over 200 feet deep. Swift, treacherous tides moving at over 8.5 miles per hour (12.5 feet per second) sweep through the channel four times a day.
There is no option uglier than paving the lake. Which bridge graces more picture postcards, Golden Gate or SR 520? The new roadway on stilts design I saw for keeping waves from breaking over the sides is even uglier, especially when viewed from lake level. Besides being a visual blight the two floating bridges cut up the lake detracting not only from the view but recreation. The current plans to provide mast clearance only at the eastern end will add two miles to the trip for sailboats moored at Leshi to get to Lake Union and the locks.