Development of a New Bridge Lift System for Washington State Ferries

Abstract

Washington State Ferries (WSF) provides a critical state highway across Puget Sound, with 20 terminals in Western Washington utilizing 29 vessels, more than 25 million passengers and 11 million vehicles are carried each year, see Figure 1. It is the largest ferry operator in North America. With several capital replacement projects planned for the near future, WSF decided to reevaluate how it transitioned between fixed structure and floating vessel. This became paramount after an accident in 2001, when, presumably, a cable failed, dropping both the bridge (transfer span) and its operator into the water. While lift bridges serving auto ferries in Puget Sound have been running successfully since the 1930s and by WSF since the 1950s, and has provided a vital link for the unrestricted movement of vehicular and pedestrian traffic in and around the Puget Sound area; for most of this time, the bridge lift mechanism has been a single winch mounted on the transfer span, operating a cross-reeved cable and featured cable supported counterweights. This design scheme provided many years of reliable and safe service for WSF. Emergent operational problems, however, initiated the search for an alternative lifting mechanism to increase both safety and effectiveness. Foremost among the project objectives was to increase safety for the span operators and for the public. Early on in the project, it was determined that what was primarily needed was a failsafe design. That is, a design that, in the event of a single component failure, or credible failure cascade, would fail in a safe and predictable manner and must not result in harm to vehicles or persons. The failsafe system would not necessarily result in an operable system after a single failure.