Sunshine Skyway Bridge Ship Impact Design of Low Level Approaches

Abstract

S afety was in everyone's mind when a new Sunshine Skyway Bridge an innovative cable stayed prestressed concrete structure — was authorized across Florida's lower Tampa Bay. In 1980 a main span of one of the older twin bridges had collapsed during a catastrophic ship impact. The risk of a similar impact to the new bridge was evident due to wayward vessels not only in the area of the central channel but all along the crossing, thus creating an essentially shore-to-shore ship impact risk over a 4.18 mile (6.73 km) length. During design, a decision was made that advanced today's trends toward incorporating safety, reliability, and ease of maintenance into design criteria. That decision was to extend ship impact criteria beyond the channel spans normally protected, to include also the lengthy low level approaches — a type of structure traditionally designed for its own live and dead loads, but not ship impacts. This made the design of the low level approaches a new type of challenge, and our firm, Parsons Brinckerhoff Quade & Douglas, Inc., was asked to take up the task. We discovered that protection could be provided, at only a modest cost increase over traditional design, by exploiting the elasticity of prestressed concrete to absorb and transfer impact loads. As a result, for apparently the first time in the United States, a major crossing (see Fig. 1) would be designed for a ship impact anywhere on its length —the first shore-to-shore protection. This article describes the design approach and construction of the low level approaches, and the application of prestressed concrete to solving the complex problem of ship impact requirements.