The Earthquake Engineering Online Archive

A laboratory study of the fluid-structure interaction of submerged tanks and caissons in earthquakes

Byrd, Robert Clark

UCB/EERC-78/08, Earthquake Engineering Research Center, University of California, Berkeley, 1978-05, 167 pages (540/B93/1978)

An experimental study comparing the results of measurements of forces on a submerged tank model resulting from earthquake excitation is presented. The experimental results are compared with analytical solutions for the case where the model is submerged in water of a depth equal to 2.5 times the tank height and for the case where the depth exactly equals the height. Details are presented for the design of a 1- to 100-scale model of a circular cylindrical structure which is 34 m in height with a mass of approximately 250,000 tons. The model includes a foundation system which simulates elastic halfspace soil stiffness in three degrees-of-freedom. The experimental results are presented in the form of inertia coefficients measured in harmonic motion at varying amplitudes and over a frequency range of 0.3 Hz to 2 Hz in prototype-scale. Coefficients are presented for horizontal, vertical, rotational, and horizontal-rotational coupling. The relationship between these coefficients and the physics of the fluid-structure interaction are discussed in detail. The study leads to the following conclusions concerning earthquake-induced forces on large submerged, gravity-type structures: (1) Available analytical techniques provide good estimates of hydrodynamic inertia force coefficients for submerged structures of simple form. (2) A correct estimate of foundation dampening is likely to be the most critical point in calculating the hydrodynamic forces on a submerged gravity structure. (3) Foundation stiffness only influences the hydrodynamic force by changing the resonant frequency. (4) Frequency dependence in the inertia coefficients is not likely to be an important consideration. (5) Coupling in the hydrodynamic inertia forces between the horizontal and rotational modes is not likely to be an important consideration in structural design. (6) Hydrodynamic dampening will not be an important factor for deeply submerged structures but may be significant in near-surface and surface-piercing structures.

Available online: http://nisee.berkeley.edu/documents/EERC/EERC-78-08.pdf (38 MB)