A rigid base is assumed in most seismic studies of liquid storage tanks. Additionally, recent studies have shown that permitting uplift can significantly reduce the damage to tanks. Uplift is the transient partial separation of the tank base from the supporting foundation. However, uplift of storage tanks is still not well studied and a rigid base is assumed in numerical studies. In this work, the simultaneous effect of uplift and a soil foundation are utilised to evaluate experimentally the effect of these two factors on the acceleration in the tank wall. A low-density polyethylene tank and a shake table were employed. Stochastically generated seismic excitation is employed based on the New Zealand design standard, NZS 1170.5, for medium soil, classification B. Three base conditions were considered: 1) fixed-base tank on a rigid support, 2) freebase tank on a rigid support and 3) free-base tank on a flexible support. To simulate the rigid support condition, the tank is resting on a steel plate rigidly attached to the shake table. A laminar box infilled with sand is utilised to represent a flexible soil condition. Results revealed that the highest acceleration at the tank top occurred when the tank is resting freely on a rigid supporting base. Furthermore, for this case the maximum uplift occurred at the same time as the maximum acceleration. However, for the case of a flexible support, the maximum acceleration occurred at the tank base and did not coincide with the time of maximum uplift. The maximum uplift occurred for a flexible supporting base and is not related to the maximum acceleration at the tank top. Thus, the distribution of the maximum acceleration, and hence uplift, depends significantly on the support condition.
This research addresses the influence of the load characteristics, that is, frequency content and maximum acceleration, on the wall stresses of an anchored water storage tank. A low-density polyethylene tank with a range of six different aspect ratios (water height to tank radius) was tested using a shake table. Eight sine excitations that cover the lowest free vibration frequency of the tank-water system were applied. Additionally, two sets of five Ricker wavelet excitations were utilized. Each set represents potential earthquakes with a bandwidth between a low and a high dominant frequency. The experimentally determined maximum stresses and those obtained from calculations using a common spring-mass model employed for seismic analysis of tanks were compared. The results reveal that the relationship between the excitation frequency and the wall stresses strongly depends on the sloshing behavior, especially when the frequency of loading is in the vicinity of the lowest freevibration frequency of the tank-water system. When the frequencies are dissimilar, there is a proportional relationship between stress and the maximum acceleration of the excitation. The spring-mass model was found to underestimate both the maximum hoop stress (for aspect ratios greater than two) and axial stress (for aspect ratios equal to 0.5). This occurs because is the spring-mass model cannot capture, in all cases, the contribution of chaotic sloshing to wall stress.
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