Although the response of anchored tanks is totally different from that of unanchored tanks, the development of numerous analyses for the dynamic behavior of the former under horizontal and vertical excitations has greatly affected the understanding of the seismic response of unanchored tanks. Early developments of the seismic response of liquid storage tanks considered the tank to be rigid, and focused attention on the dynamic response of the contained liquid, such as the work performed by Jacobsen [129], Graham and Rodriguez [71], and Housner [112]. Later, the 1964 Alaska earthquake caused large scale damages to tanks of modern design [79] and profoundly influenced research into vibrational characteristics of flexible tanks. Different solution techniques and simplified models were employed to obtain the seismic response of flexible anchored liquid storage tanks.