Crossing the Narrows

Abstract

This article describes the process by which the new Tacoma Narrows Bridge was sited, bid, designed and built on a location directly next to the existing bridge. It is one of the first suspension bridges in North America to be built with the design-build method and the first in the world to have its design tested by two complete, side-by-side bridge models in a wind tunnel. The location is best known for being the site of the bridge that collapsed in July 1940, four moths after it opened as a result of wind-induced instability. The collapse became an engineering case study for generations of students. The first replacement for that bridge was built on the site in 1950. The new Tacoma Narrows Bridge will ease congestion on that span. The site conditions are unique and challenging, both in terms of wind and weather, rushing currents, and unstable geology and soil. Special adjustments had to be made for a number of reasons. Details are given about design specifics and descriptions of conditions at the site. Twice-daily tidal fluctuations, for example, exceed five meters across a 40,000 ha. area of the Puget Sound. This is the closest that two suspension bridges have been built to each other on such a large scale. The project included plans to rehabilitate the old bridge. The entire undertaking came in under budget and on time.