Field Vibration Tests and Proposition of Safety Measures for Dynamic Instabillity of Folsom-Dam Tainter-Gate

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Tainter gates (radial gates) are frequently used as a crest gate of dam for a flood release. A huge Tainter gate installed on Folsom dam in California, induced vibrations and failed, early in the morning of July 17,1995. A possibility that vibration participated in its failure was pointed out from just after the accident, and some failure analysis studies have been performed by applicants.In this study, verification studies of theoretical analyses developed for a possible coupled-mode flow-induced vibration of Tainter gates were made by conducting vibration tests for 45-ton Tainter gate and 3-D 1-13-sclaed model gate. In addition, failure analyses of Folsom dam Tainter gate were made and suggestions for dynamic stability were presented. Research results are summarized as follows :1.Field vibration tests of Folsom-dam Tainter-gate which was being planned to conduct at the beginning of this research project was not able to be conducted because of severe cautions to terrorism in USA. In t … More his study, thereupon, field vibration tests were conducted for a 45-ton full-scale gate under practical uses in Japan, to determine its in-air and in-water natural vibration characteristics, where steel bar breaking tests were first adopted to excite the huge gate to vibrate. Detailed measurements of responding vibration acceleration were made to identify the frequencies and modes of natural vibrations. In in-water vibration tests, hydrodynamic pressure also was measures, where a small pressure hole was made directly through the skinplate. The measured data were incorporated into the theoretical analyses to calculate the in-water vibration frequency and the dynamic instability curve. As a result, the theoretical analyses were well verified for its validity.2.Three-dimensional 1/13-scaled model of Folsom dam Tainter-gate was made, where the vibration modes and its in-water vibration frequencies were carefully designed to be exactly same as those of failed Folsom Tainter-gate. The 3-D model gate was installed in a large flume of water circulation type. As a result, the 3-D model gate definitely re-realized the same type of violent coupled-mode self-excited vibration that must have caused the Folsom gate failure. The theoretical analyses were well verified for its validity by the 3-D model gate tests also.3.Folsom gate Failure was made by applying the developed theoretical analyses, where the dynamic instability of the Tainter-gate is theoretically examined and the major characteristics of this instability were presented, and an FEM analysis of the structure was incorporated to reveals that the small vibration amplitude of at most 11.9mm at the spanwise center and bottom end of the skinplate induced the excessive hydrodynamic pressure load of about 1.6 times the static load, thus causing incipient failure of 4 bolts connecting a diagonal member to the main struts. Less