Damping enhancement of seismic isolated cylindrical liquid storage tanks using baffles

The effect of baffles in reducing earthquake responses of seismically isolated cylindrical liquid storage tanks is investigated in this study. Seismic isolation is a well-known approach to reduce the earthquake effects on structures by lengthening their fundamental natural periods at the expense of larger displacements in the structural system. To reduce such effects in a system a higher damping ratio is required. In moving liquid containers, baffles play an important role in damping the liquid motion. Thus to study the effects of using baffles in seismically isolated tanks, in the first instance the velocity contours in a cylindrical tank are analysed to determine the most effective shape of baffle. Next, the damping coefficients are analytically determined for horizontal ring shape and vertical blade shape baffles. To estimate the sloshing height level and the damping ratio, a methodology, based on Tank Body Spectra, is developed in which the higher sloshing amplitude and the relative fluid velocity with respect to baffles in base isolated tanks are taken into consideration. A computer program is developed to put all these together and investigate the effect of baffles for different tank dimensions under the effect of four different earthquakes. The results show that the average damping ratio of sloshing mode due to ring baffle increases with a decrease in liquid height and highest damping may be achieved for height to radius ratios of between 1.0 and 1.5. In addition, for reasonable ring baffle dimensions, an average reduction of 6% in base displacement of base isolated tanks and an average reduction of more than 30% in the sloshing height of base isolated and fixed base tanks may be achieved. To study the effect of baffles on the distribution of hydrodynamic and tank body forces with height, a simple dynamic model is proposed. The results of analyses using this model indicate a constant reduction in sloshing forces and different reductions in moment and shear forces for different heights. This happens because contribution of the sloshing force to the total hydrodynamic force varies with height.