Dynamic Properties of San Bernardino Intake Tower.

Three series of forced vibration tests were conducted on the intake tower located in Silverwood Lake near San Bernardino, California. In the first test series the water level in the reservoir was just above the base of the tower so the effects of water on its behavior were negligible. In the second and third series the water submerged nearly two-thirds of the tower. In the second test series the inside of the tower was empty, whereas in the third series the tower was filled with water to the same level as the water in the reservoir. During these tests seven modes of vibration were excited. Three of the modes were transverse modes of the bridge connecting the tower to land. Three more modes involved longitudinal movement of the bridge deck which can occur only at amplitudes of motion smaller than the clearances in the bridge supports. Thus, of the seven modes excited during the forced vibration tests, only one mode would be excited with significant amplitudes during strong earthquake ground motion.