__1963: __The world's longest floating bridge, the Evergreen Point bridge, opens. It connects Seattle with communities on the east side of Lake Washington.

Pontoon bridges have been around since ancient times. Lash some boats together side-by-side in a stream or river, put some planks across them, and you've got a serviceable bridge. Armies love 'em because they can be deployed quickly so troops and equipment can be deployed quickly.

For a large, permanent bridge, the concept is scalable, but not easily. However, if you need to bridge a deep body of water that has a soft bed, a more conventional design might not be feasible. That's what faced Washington state engineers who set out to bridge Lake Washington. And they'd done it before, with the shorter Lake Washington Floating Bridge, opened in 1940. (A few miles south of the Evergreen Point bridge, it now carries the eastbound lanes of I-90.)

Starting in August 1960, construction crews ashore built 33 hollow, concrete boxes, each 15- or 16-feet high and about the length of a football field. These huge pontoons were floated and then towed into position, where they were linked by thick steel cables to anchors to hold them in place. The 62 anchors, buried deep in the lake bed, weigh about 77 tons each. Building the bridge cost a relatively modest $21 million ($154 million in today's money).

The bridge has a retractable drawspan in the middle that is raised to protect the structure from strong winds. But at 7,578 feet, the floating portion is essentially a 1.42-mile barge with a road on top of it.

That road is state Route 520, which links Seattle with Bellevue and Redmond, where a somewhat well-known software company later made its headquarters.

Seattle's growth, of which the tech boom is no small part, has put a huge load on the bridge. Designed to carry 65,000 vehicles a day, it now carries 115,000. That wear and tear, coupled with storm damage, has led to costly repairs.

Crews have patched more than 30,000 linear feet of cracks in the concrete pontoons since a huge storm on the day President Clinton was inaugurated in 1993. The drawbridge section got stuck in the open position for a while in March 1999.

The Washington State Department of Transportation says if the bridge were to sink, the average commute between Seattle and Redmond would increase from its current 33 minutes to 55. WSDOT has determined that retrofitting the Evergreen Point Bridge to current seismic and safety standards would be more expensive than building a new one.

So, it plans to construct a new floating bridge just north of the current one, starting next year. The new Evergreen Point bridge would have six lanes (plus a bike and pedestrian path) instead of four, cost about $4 billion, and open in 2014.

Perhaps they'll call it Evergreen 2.0, or Evergreen 2-Pont-0.

Source: Various