Slip Phenomena by Occurrence of Air Film Between Tank and Basement

摘要

In this paper we deals with slip phenomena of oil tanks. Oil tanks whose bottoms are flat are set onrna concrete base and they are not anchored. So, when they are accelerated by an earthquake, they may bernrocked or slid. Actually the phenomena have sometimes been observed since Alaska earthquake in 1964. Ifrna tank moves and the pipes attached to the tank brake, the oil leaks out and it is in danger. As for the reasonrnwe have discussed whether the coefficient of friction is small or large. But some cases cannot be explainedrnon the viewpoint of the coefficient of friction. Therefore, we tested with a tank model on a flat shakingrntable. The diameter and the height of the acrylic model is 150mm and the wall thickness is 5mm. Thernbottom is made of vinyl chloride whose stiffness is lower than the acrylic one. The four pressurerntransducers are set on the bottom to measure the air pressure between the bottom and the shaking table.rnWater volume in the tank and the center of gravity of the tank can be changed. The acceleration and thernmovements of the tank were measured in sweeping a vibration acceleration of the shaking table at arnconstant frequency. As a floor material, acryl and rubber were chosen. As a result, the acceleration at whichrnthe tank begins to slip is less than the theoretical value calculated from the coefficient of friction. Anotherrncharacteristic is the fact that the tank slips more easily at the higher frequency. The reason is why the airrnfilm causes between the bottom and the shaking table atter rocking and the film makes it easier to slip. Thatrnis, the apparent coefficient of friction is less due to the air film than the real coefficient cf friction. Therneffect is more in case of the higher frequency and the higher coefficient of friction. The water in the tankrnmakes the slip acceleration more or less owing to sloshing.