Dynamic Behavior of Nagawado Arch Dam in the Event of 1984 Naganoken Seibu Earthquake

Nagawado dam, a slender arch dam of 155 m height and 355.5 m chord length, recorded maximum accelerations of 193 and 242 gal at the crown and the 1/4 point of the crest arch, respectively, in the September 14, 1984 Naganoken Seibu earthquake of magnitude 6.8. The maximum accelerations in the foundation rock were in the range of 21 to 31 gal. Fourier analysis and band-pass filtering of corrected accelerograms, and spatial and temporal diagrams of the computed displacements of the dam and the foundation indicate the significant structural modes of peak frequencies 3.3, 3.6, 4.7, and 5.5 Hz. These frequencies correspond to the antisymmetric first, the symmetric first, the symmetric second, and the antisymmetric second modes of the dam. The first three peak frequencies were in good agreement with the values of corresponding vibration modes obtained by a small-scale model test. The derived frequencies and vibration modes were found also compatible with the results of eigenvalue analysis of a preliminary 3-D finite-element model as a whole. Orthogonal sets of accelerograms recorded at the points across a treated fracture zone in the foundation rock did not record any significant relative motions.