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Duckabush River Bridge

A concrete, through tied, ribbed arch was constructed on Highway 101 over the Duckabush River in 1934 by the West Coast Construction Company. The 168-foot bridge consists of a 110 foot concrete arch and two concrete girder spans. Unlike the flat truss or girder, the arch exerts a horizontal thrust on the skewbacks. In most arches, massive abutments and foundations are necessary to resist the horizontal thrust. However, in the tied arch, the horizontal thrust is resisted by longitudinal ties which extend between the hinged springing points.

In the Duckabush River Bridge, the deck slab itself, which is hung by suspenders from the pair of arch ribs, acts as a tie. Since the arch is in compression the deck slab is subject to a tensile stress. The double function of the deck slab was an economical solution, and it eliminated the need of massive abutments. The two arch ribs of the Duckabush River Bridge were connected by three struts to provide lateral rigidity against traffic and wind loads. The visual impact of this short-spanned concrete overhead arch is monumental, and demonstrates the simple organic power of the concrete form. The Duckabush River Bridge is one of five concrete tied arches within the State. It is not the longest of the five, but its rise is the greatest. Although there are examples of tied arches that were built throughout the 1920s and1930s, it is a rare concrete arch form.