Rehabilitation of TDOT'S SR56 Deck Truss Bridge Crossing Center Hill Lake

Abstract

In the fall of 1949, a new bridge near Smithville, TN, carrying relocated SR56 over Caney Fork River was opened to traffic. Begun in fall 1947, the 1,786-foot-long structure rose 274 feet above the riverbed. The bridge construction and road relocation was necessary to accommodate closure of the newly constructed Center Hill Dam and subsequent 64-mile-long, 190-foot-deep impoundment of the river. The bridge, designed for AASHO H15-44 live load, was constructed by the Nashville District of the U. S. Army Corps of Engineers. It consists of two 18-panel, 771-foot-long, cantilevered deck trusses, each with 8-panel suspended deck truss spans and two 61-foot-long deck girder spans at each approach. The Warren type trusses are 24 feet curb-to-curb (c/)c and are 60 feet deep above the 215-foot-tall main piers. The bridge was originally constructed with a 24-foot, curb-to-curb roadway and 18-inch-wide safety curbs. In 1977, the bridge was widened to support a 44-foot, curb-to-curb roadway to improve sight distance at the entrance to the newly constructed Edgar Evins Appalachian Craft Center located near the east end of the bridge. Widening was accomplished by adding truss floorbeam cantilevers to support one new stringer line each side and by widening the approach piers and abutments to support one new line of beams each side. Analysis of the 1977 modification revealed the bridge could accommodate AASHTO H-15 at Inventory level and AASHTO HS-20 at Operating level. These live load capacities were acceptable considering the low volume of commercial traffic on the route at that time. In 2010, due to the Federal mandate to re-rate all deck truss bridges including gusset plate connections, the ravages of time on the condition of the bridge, and the 15 ton posting causing lengthy detours for heavy trucks and busses, Tennessee Department of Transportation (TDOT) decided to engage the firm of Modjeski and Masters, Inc. to re-analyze the bridge and provide strengthening recommendations to upgrade the capacity for AASHTO LRFD HL-93 live load. Extensive three-dimensional truss analyses were completed for design of strengthening to support the increased loading and to validate construction staging for minimal through traffic interruption. A thorough review of truss gusset plate capacities was also completed. Temporary support of several truss member webs was required during removal of lacing bars and attachment of new cover plates. Deficient gusset plate capacity at multiple locations required temporary support of truss panels while truss members were cut and spliced during installation of gusset doubler plates. Specific construction staging was detailed for removal and replacement of existing rivets and/or bolts with new high strength bolts prior to strengthening of the trusses. A supplemental support system was designed for installation on fracture critical pin-ended truss suspenders. External post tensioning was installed to supplement the capacity of the top strut of an approach span four-column pier bent. Using lightweight concrete for the deck slab and barriers, providing new stringers, reinforcing floorbeams, and with truss member and gusset plate strengthening the desired HL-93 live load capacity was achieved.