Tatsuya Oku

Friction-Maintained Dynamic Stability

Mechanical systems relying on Coulomb friction to maintain dynamic stability may suffer a dynamic instability if exposed to an initial displacement exceeding a system-specific threshold. In fluid systems, even small values of negative damping are sufficient to drive the dynamic instability with sufficiently large initial displacement. The Tainter gate failures at the Folsom dam in 1995 and at the Wachi dam in 1967 are two well-known failures. To aid in preventing a recurrence, the authors engaged in a decade long research program that provided evidence that both gates failed due to an essential dynamic instability mechanism that all Tainter-gates may possess. This paper presents measurements suggesting “friction-maintained dynamic stability” of a full-scale 50-ton Tainter gate. Accompanying gate model studies showed that the gate can fail when exposed to an initial displacement exceeding a threshold value. The present study should serve to alert gate designers, owners and operators that many Tainter gates which have not yet failed may, nonetheless, have a high susceptibility to failure if and when they are exposed to a sudden input of energy resulting in an initial displacement exceeding the gate-specific threshold displacement.

Measurement of Instantaneous Flow-Rate Coefficients and FIV Characteristics of Tainter Gates at Large Openings

Tainter gates become dynamically stable at large gate openings, even though they usually exhibit intense dynamic instability at small gate openings. To assess which factors govern this stability at large openings, model investigations were conducted. The present study focuses on discharge characteristics and hydrodynamic pressures at large gate openings. The model tests were conducted using a 1/27-scaled 2-dimensional model of the Folsom Dam Tainter gate. The instantaneous flow-rate coefficient was obtained from LDV measurements of flow velocity accompanying periodic changes in the gate opening. Measurements of the hydrodynamic pressure associated with the flow-rate variation were also used to determine instantaneous flow-rate coefficients. Comparison of the directly measured instantaneous flow-rate coefficient form LDV measurements with that obtained from pressure measurements suggests that the flow field is less receptive to disturbances due to gate motion at larger gate openings. With increased gate openings, the fluctuating fluid force acting on the gate decreases, making the gate dynamically stable.Copyright

Coupled-Mode Dynamic Instability of Tainter Gates With Parallel Bending Vibration of the Skinplate

As part of the investigation of the dynamic instability of the gate closely related to the Folsom Dam Tainter-gate failure, and in order to assure the dynamic stability of the gate, the field vibration tests on three full-scale operational Tainter-gates were conducted. From these tests, the possible existence of another coupled-mode self excited vibration mechanism, which involves the dangerous dynamic coupling of the whole gate rigid-body rotational vibration with a “parallel” bending vibration of the skinplate was suggested. This paper presents the mechanism of the suggested coupled-mode self-excited vibration, theoretical analysis for the suggested dynamic instability, and 2-dimensional laboratory model tests results. Further, the need for retrofit countermeasures for Tainter gates which are currently installed in both Japan and the USA and susceptible to this dangerous coupled-mode dynamic instability is emphasized.Copyright