Rehabilitation study of Toutle River tied-arch bridges

The Toutle River tied-arch bridges (Br. No. 51140 E & W) are located on Interstate 5 in western Washington State and were open to traffic in 1969. Approximately 33,000 vehicles a day cross these bridges; 22 % are trucks. Large deflections result with the passage of these trucks, causing the bridges to vibrate in the first mode of vibration. A strengthening and stiffening study was conducted to minimize deflection, vibration, and fatigue cracking. Several stiffening methods were analyzed. The addition of diagonal tension members between each vertical hanger and addition of compression members placed near the quarter points of the bridges emerged as the most promising options. The bridges also experience fatigue cracking. The floor-beam webs have cracks at most tie-chord connections and at exterior longitudinal stringer connections. Stiffening the arches would decrease the fatigue stress range in the tie chord. Retrofitting the floor system to accommodate thermal movements would minimize distortion-induced fatigue in the floor beams. At the bridge bearings, extensive cracking is present at the fillet welds connecting the upper bearing blocks to the tie chord. A bearing replacement was recommended. Adding post-tensioning strands to the tie chord was also recommended as a safeguard against sudden failure. Estimated retrofit costs were also developed.