Maureen Sullivan & Julie Meredith,
I've lived in the Seattle Tacoma area since the mid 60's and for as long as I can remember local lore has it that floating bridges were built across Lake Washington because the depth and length of span make any alternate construction technique impossible. I think it's been repeated so many times that people living here except this without question as a God given truth.
The average depth of the lake is only 110 feet, less than the Tacoma Narrows. And, while the length of the Evergreen Point bridge is longer that the Tacoma Narrows it wouldn't even make the top 20 list for longest suspension bridges in the world. Ironically, the 20th longest bridge at 8,000' spanning the Firth of Forth in Scottland was constructed at the same time as the Evergreen Point Bridge.
The other argument floated for this construction technique is that it's cheaper. This may be true in the short term but our long experiment dating back to the original I90 span (1940-1990 R.I.P.) certainly has proven otherwise. With estimates for replacement at $1.5 to $3.4 billion one has to question even this fundamental truth. The new Tacoma Narrows bridge came in on budget and ahead of schedule at $744 million.
The Evergreen Point Bridge at 46 years of age is the Grand Dame of our fleet (her sisters have all sunk). Somehow the Homer Hadley bridge stressed beyond it's design limits by light rail is supposed to last 100 years yet experience has proven 50 is as long as anyone can hope for a concrete structure floating in the water to last. Why can't we move past the old wives tale of a floating bridge being the only possible method of construction and build a bridge that will span