Continuity Strengthens South Fork Hoh River Bridge Replacement

Abstract

T South Fork Hoh River Bridge (see Fig. 1) is a 142 ft 3-Y4 in. (43.4 m) long, three-span continuous prestressed concrete structure made up of pretensioned decked bulb tee girders. The bridge replaces an old deteriorated timber structure using the existing middle piers. To increase the load capacity of the bridge, the girders were post-tensioned for continuity. The bridge is designed for off-highway logging trucks and overload crossings of large tower and yarder units. This article describes the preliminary studies, design and economic considerations, advantages of continuity, post-tensioning layout and joint details, construction aspects and post-tensioning operations used to construct this unique bridge. The South Fork of the Hoh River is Iocated on the Olympic Peninsula, southeast of Forks, Washington (see Fig. 2). This region has the highest average annual rainfall in the continental United States, averaging 140 in. (3556 mm) per year. Storms have been known to bring up to 6 in. (152 mm) of rain in a 24-hour period and this, when combined with melting snow, can cause the river to rise quickly, sending logs and debris rushing downstream. In this constantly moist environment, decay is rapid and omnipresent, thus presenting formidable obstacles to bridge construction and longevity. The South Fork Hoh River Bridge, also known as the Marsh Bridge, was originally built in 1960 for HS20 loading using log stringers and timber decking over two interior reinforced concrete support piers on footings embedded in rock. Twenty-two years later, the log stringers were decaying, threatening structural integrity. After the original bridge was built, the