SR 520 Bridge Replacement, United States of America

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. Officially known as 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 decided to be improved.

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

The floating bridge construction is scheduled to begin in 2012 and complete by December 2015.

Project

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. The program, which will improve 12.8 miles of the corridor, is estimated to cost $4.65bn. It is divided into three main components: I-5 to Medina, Medina to SR 202 and Pontoon construction project.

The I-5 to Medina section will witness 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 will improve the 8.8 mile HOV line from Evergreen Point Road in Medina to the SR 202 interchange in Redmond. It is being carried out under a project termed as the Eastside Transit and HOV Project.

The Pontoon construction involves building of large pontoon structures for use in the replacement of the floating bridge in the event of failure. The pontoons will be 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

The Lake Washington project includes 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 will be evaluated every two years till the completion of the construction.

The pontoons will be cast in a facility that is being built in a 55 acre site in Aberdeen. Construction of the facility began in February 2011. Pile driving and site draining works are currently in progress. The works include construction of 900ft-long and 200ft-wide casting basin, 33 pontoons and 10 smaller stability pontoons on the shores of Gray's Harbour. The pontoons will be constructed and floated towards Lake Washington and then be used for the construction of the bridge. The casting basin is expected to be ready by 2012 and the pontoon construction is expected be completed in the first quarter of 2014.

Design

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 is expected to have a width of 115ft and the new pontoons will be 28ft-tall, 75ft-wide and 360ft-long. The bridge height above the water level will be increased from the current 13ft to 20ft.

The new replacement bridge is designed to include a six-lane bi-directional highway that includes two lanes for general traffic and a new transit / high occupancy vehicle (HOV) lane in each direction.

A 14ft-wide bicycle and pedestrian path will be 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

Kiewit-General joint consortium won the $367m contract for the SR 520 pontoons construction.

The $600m contract for the floating bridge construction is expected to be awarded in 2011. The three shortlisted contractors for the construction of floating bridge are SR 520 Corridor Constructors, Flatiron-Skanska-Traylor Brothers and Keiwit-General-Mansion.

Finance

Of the total program cost, around $2.6bn has been secured through state and federal funding and tolls collected on the floating bridge. The funds will be sufficient for the Eastside HOV improvements, Pontoon Constructions and the SR 520 floating bridge.