Question: Norm McDonell of Des Moines wants to know why the state is preparing to build another floating bridge on Highway 520 over Lake Washington instead of a more traditional “over the water” bridge.

“It seems to me that the life expectancy of floating bridges, based on past experience, is about 50 years,” he wrote in an e-mail to Traffic Q&A. “The floating bridges seem to be high maintenance, unusable in high winds, with short life spans compared to conventional bridges, and like boats they can sink. If past experience with floating bridges is the measuring stick, the proposed 520 floating bridge may have to be replaced before it is paid for.”

Answer: The 1.5-mile Evergreen Point Bridge and its approaches, built in the 1960s and lacking today’s design standards, are vulnerable to earthquakes and windstorms and need to be replaced, according to the state Department of Transportation.

Broch Bender, DOT spokeswoman for the Highway 520 project, said engineers determined that a suspension bridge would not work for several reasons:

 • Suspension bridges need to travel in a fairly straight line, which was not possible within this curved corridor.

 • A connection could not be made to the proposed Pacific Street interchange over Marsh Island.

 • The size of the three to four support towers for a suspension bridge, at approximately 630 feet in height, would have been nearly the height of the Space Needle and out of character with the surroundings.

 • Conventional fixed bridges, such as the new suspension bridge over the Tacoma Narrows, are expensive to build in deep waters with soft beds.

In addition to a suspension bridge, the Montlake community asked the state to explore the possibility of a cable-stayed bridge. The state ruled it out because the bridge would be so high that noise would reach a larger group of neighborhoods, and because it would require 500-foot support towers.

The state estimates it will cost between $3.9 billion and $4.38 billion to replace the bridge that carries between 155,000 and 160,000 vehicles a day. Construction would last from 2013 to 2020.

The state claims some expertise in this area. DOT officials call Washington the “floating bridge capital of the world,” with the four longest and heaviest floating bridges. They are the Highway 520 bridge, the Interstate 90 Murrow Bridge, the I-90 Hadley bridge and the Highway 104 Hood Canal Bridge.

The state DOT has a Web page devoted to the Highway 520 project. Go to www.wsdot.wa.gov and click on “SR 520 Bridge Replacement” under the projects section.