Performance-Based Design of Movable Bridge to Incorporate Sea-Level Rise

Abstract: Washington State Ferries (WSF) is a part of the Washington State Department of Transportation highway system. It is the largest ferry system in the United States with a fleet of 21 ferry vessels connecting a network of 20 ferry terminals. All of WSF’s ferry terminals consist of at least one movable bridge. The movable bridge connects the fixed trestle (pier) to the vessel deck during the loading and unloading of the vessel.

The movable bridge, approximately 90’ long, has the onshore end fixed. The offshore end raises and lowers to adapt to the changing tides and vessel freeboards. The fixed end of the bridge rests on a bridge seat and the movable end is raised and lowered with the help of mechanical systems supported by offshore towers. The design maximum and minimum gradients of the movable bridge are the gradients at which the bridge serves the design high tide and design low tide.

As sea levels rise, the likelihood of the tides exceeding the design high tide increases. Every time the tides exceed the design high tide, large trucks have difficulty navigating the slope changes between the vessel deck and the trestle. Delays in sailings result. The increased tide also tests the limits of the mechanical system that raises the span.

WSF is currently designing the replacement of two movable bridges at its Bremerton terminal. The bridges have a design life of 75 years. Sea-level at Bremerton terminal is predicted to rise between 0.5’ with a 99% likelihood of exceedance to 4.6’ with a 1% likelihood of exceedance by the year 2100. The trestle on the fixed ends of the bridges was constructed in the year 1989 and is not due for replacement until after the year 2050. As a result, the ability to raise the bridge seats to account for sea-level rise is limited. The length of the bridge span cannot be increased as it will require relocating several landing aids at a significant cost increase.

With such a large uncertainty in sea-level rise predictions and the site-specific constraints of the trestle and landing aids, WSF developed a performance-based design methodology, that relies on minimizing the risk of impacts to sailings. This methodology accounts for several sea-level rise likelihood scenarios and also the sailing schedules at Bremerton terminal to establish the design high and low tides. The bridge seats and towers are designed as adaptive. One bridge seat’s current elevation is 16.73’ (MLLW) and the design elevation for construction in 2027 will be 18.93’ (2.2’ higher). This increase accounts for sea-level rise until 2050 when the trestle will be replaced.

In 2050 when the trestle is replaced the bridge seat and tower can be raised an additional 3’. This adjustment will be based on the revised predicted sea-level rise at that time for the remaining movable bridge design life.

How does this project / topic contribute to the advancement of the industry and profession?: This project uses a risk-based approach to address sea-level rise. It is a novel approach to help extend the life of the structures even though there is much uncertainty in the future of sea level rise. Other ferry terminals and ports can use a similar approach to designing resilient structures.

Does the project / topic implement new and innovative techniques, materials, technologies, and delivery methods?: This project uses a risk-based approach to address sea-level rise. It is a novel approach to help extend the life of the structures even though there is much uncertainty in the future of sea level rise. Other ferry terminals and ports can use a similar approach to designing resilient structures.

What was the most challenging aspect of your project / topic and how did you handle it to ensure success?: Proposing a bridge seat elevation for construction in 2027 based on the existing trestle elevation and future sea level rise.

Who is the target audience for this paper?: Planners, Designers, and Engineers who design movable bridges.