The SR 520 floating bridge is a two-lane floating bridge that spans from interstate highway I-5 in Seattle to state route highway SR 202 in Redmond, Washington, US. Officially known as the Governor Albert D Rosellini Bridge, it was opened in 1966. It was earlier known as the Evergreen Point Bridge.

The existing floating bridge and the connecting Portage Bay Bridge and Union Bay Bridge are supported by hollow columns, which are vulnerable to severe storms and earthquakes. During a heavy storm or a 50mph wind, the waves of the lake beating against the southern wall of the bridge may lead to the breakage of the pontoons and anchor cables. It may further cause the bridge to sink or collapse. Moreover, the existing bridge does not meet the modern earthquake design standards.

The floating bridge has therefore been decided to be replaced and the connecting bridges improved in order to make them safer and more reliable. Approximately 115,000 vehicles pass through the floating bridge each day, which is more than the estimated 65,000 for which it was designed. The connecting bridges are also improved.

The new replacing bridge pontoons have ability to tolerate up to 92mph of wind. The replacement provides a new west approach to the bridge to withstand a 1,000-year earthquake event. In addition, the new bridge reduces travel times and makes the journey easier for people crossing the Lake Washington.

Construction of the floating bridge began in 2012 and the bridge was opened to traffic in April 2016.

Bridge replacement project details

The floating bridge replacement is part of the SR 520 Bridge Replacement and HOV Program which aims to replace the SR 520 floating bridge and improve the Portage Bay Bridge and approach roads in the corridor. Improving 12.8 miles of the corridor, the programme is estimated to cost $4.128bn. It is divided into three main components, namelyI-5 to Medina, Medina to SR 202 and Pontoon construction project.

The I-5 to Medina section witnessed replacement of the SR520 floating bridge, interchanges and roadway between I-5 and the eastern shore of Lake Washington.

The Medina to SR 202 section improved the 8.8-mile high-occupancy vehicle (HOV) line from Evergreen Point Road in Medina to the SR 202 interchange in Redmond. It was carried out under a project termed as the Eastside Transit and HOV Project.

"The floating bridge replacement is part of the SR 520 Bridge Replacement and HOV Program."

The pontoon construction involved building of large pontoon structures for use in the replacement of the floating bridge in the event of failure. The pontoons were built early as their construction takes a long time, which could delay the bridge restoration upon catastrophic failure.

The final environmental impact statement for the floating bridge replacement project was released in 2011.

Construction of the SR 520 Bridge replacement

The Lake Washington project included replacement of 15 of the 52 anchor cables on the Hadley floating bridge and replacement of 15 of the 58 cables on the SR 520. The cables help bridges withstand the storms and the waves. The unreplaced cables were evaluated every two years till the completion of the construction.

The pontoons were cast in a facility that was built in a 55 acre site in Aberdeen. Construction of the facility began in February 2011.. The works included construction of 900ft-long and 200ft-wide casting basin, 33 pontoons and ten smaller stability pontoons on the shores of Gray’s Harbour. The pontoons were floated towards Lake Washington and then used for the construction of the bridge.

A total of 12 pontoons were floated from Aberdeen and Tacoma in July 2012. As of September 2012, five new pontoons have reached Lake Washington. The pontoon construction was completed in the first quarter of 2014.

Design of the replacement bridge

The existing floating bridge has a width of 60ft. It is supported by 33 pontoons, which are 360ft-long and 16ft-tall. It is also supported by 62 anchors, which weigh around 77t each. The new SR 520 has a width of 115ft and the new pontoons are 28ft-tall, 75ft-wide and 360ft-long. The bridge height above the water level was increased from the current 13ft to 20ft.

The new replacement bridge was designed to include a six-lane bidirectional highway that includes two lanes for general traffic and a new transit / HOV lane in each direction.

A 14ft-wide bicycle and pedestrian path were built on the north side of the bridge. The shoulders are designed to be wider so that vehicles can be pulled over in the event of breakdown of the bridge.

Contractors

Khe Kiewit-General joint consortium won the $367.3m contract for the SR 520 pontoons construction.

The $586.6m contract for the floating bridge construction was awarded in August 2011 to Keiwit-General-Manson joint venture. The three contractors shortlisted for the construction of floating bridge were SR 520 Corridor Constructors, Flatiron-Skanska-Traylor Brothers and Keiwit-General-Manson.

Financing

Of the total programme cost, around $2.6bn were secured through state and federal funding and tolls collected on the floating bridge. The funds are sufficient for the Eastside HOV improvements, pontoon constructions and the SR 520 floating bridge.

In October 2012, the Washington State Department of Transportation (WSDOT) granted a $300m federal loan for the west side bridge between the floating bridge and Montlake.