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How to keep the world’s longest floating bridge…afloat

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If there was ever a good test of the effectiveness of your expansion joints, it would be to recruit 7,000 cyclists, 12,000 runners and 27,000 pedestrians onto the world’s longest floating bridge in its inaugural 24 hours.



And this is exactly what happened for the Governor Albert D. Rosellini Bridge, more commonly known as SR 520 Bridge, on April 2—the weekend of the project’s opening celebration.The party included a 10 mile run, 20 mile bike ride, and fun fair on the bridge complete with a ribbon cutting ceremony.



The original SR 520 Bridge was built in 1963 as a four-lane toll bridge and needed to be replaced due to increasing population and traffic, as well as general deterioration. Its replacement exceeded its length by 130 feet.



The SR 520 Bridge now spans 7,580 feet (2,310 meters) across, running along the state route from Medina to Seattle, and has 36 expansion joints from Mageba USA to keep it intact.



“Every bridge moves due to wind or from traffic, which introduces vibration onto the bridge. That vibration has to be absorbed by some element of connection,” said Rodrigo Miranda the SR 520 project manager from Mageba.



“For being a floating bridge, it has some vertical movement because of the water level rising. The design of the bridge makes it able to take a 20 foot drop based on the water level of the lake,” said Miranda.



In fact, the nine modular expansion joints (TENSA MODULAR LR-LS) that Mageba provided can handle longitudinal movements of over six feet each (2,000 millimeters). This, combined with 27 single gap joints (TENSA-GRIP RS) which can handle movements of up to three inches (80 millimeters), means that there is a lot of room for movement along the bridge’s length.



The joints all use Hamilton Kent Pavement Seals as a part of their design. The rubber seals provide plenty of elasticity within the expansion joint while keeping it watertight. Mageba, a global company with most of its manpower based in Turkey, decided on Hamilton Kent as their local supplier when they started doing business in the US in 2011.



“We’ve been working closely with Hamilton Kent for all of the joints that we have provided in the US so far,” said Miranda.

The largest joints in the SR 520 project have 16 gaps in them. This means up to four feet of longitudinal service movements, and five feet of extreme limit state movements (ELS). This requires a tremendous amount of flexibility and durability from the rubber HK seals.



“Each joint as a whole system is designed for maximum movement,” said Miranda, who noted that the bridge has more than 30 pontoon sections. “The movement gets distributed along the whole bridge.”



Over the summer, contractors will continue to disassemble the original bridge, which runs parallel to the new bridge. Meanwhile, pedestrians, cyclists and motorists are enjoying their record-breaking commutes across Lake Washington on the new bridge.