The Evergreen Point Floating Bridge, spanning Lake Washington, has been a testament to engineering innovation since its original construction in 1963. The bridge's design and subsequent replacement in 2016 presented numerous engineering challenges that required creative solutions. This article delves into the complexities faced by engineers in both the original and replacement bridges.
The Original Bridge's Design
The original Evergreen Point Floating Bridge was a pioneering structure,
utilizing a pontoon design to float atop Lake Washington. This design was necessary due to the lake's depth and muddy bottom, which made traditional bridge supports impractical. The bridge's floating section was the longest in the world at the time, measuring 7,578 feet.
One of the key engineering features was the drawspan, which allowed the bridge to open for tall boats. This was achieved by raising two 100-foot-long steel grids and moving an adjacent pontoon beneath them. However, this design also posed risks, as demonstrated in 1989 when an electrical fault caused the drawspan to open during rush hour, resulting in one death and five injuries.
Challenges Leading to Replacement
Over time, the original bridge faced increasing challenges. It was not designed to handle the volume of traffic it eventually carried, leading to significant congestion. Additionally, the bridge's structural integrity was compromised by its inability to withstand major windstorms or earthquakes, as it was built before modern seismic standards were implemented.
The bridge's pontoons and anchor cables were vulnerable to vibrations from storm surges and strong winds, which could lead to structural failure. Despite a seismic retrofit in 1999, the bridge remained at risk, necessitating closures during high winds. These issues highlighted the need for a replacement that could meet contemporary safety and capacity requirements.
The Replacement Bridge
The construction of the new Evergreen Point Floating Bridge began in 2011, addressing the shortcomings of its predecessor. The new bridge, completed in 2016, is the longest and widest floating bridge in the world, with a length of 7,710 feet and a width of 116 feet. It features six lanes of traffic and a multi-use trail, accommodating modern transportation needs.
The new bridge's design includes improved pontoons and anchor systems to withstand seismic events and adverse weather conditions. Despite challenges during construction, such as issues with pontoon quality, the bridge was successfully completed and opened to traffic in April 2016. The old bridge was dismantled by spring 2017, marking a significant achievement in engineering and infrastructure development.
