Large-capacity ground-supported cylindrical tanks are essential for storing various liquids, including potable water, petroleum, chemicals, and liquefied natural gas. Ensuring their structural integrity during strong ground shaking is crucial for modern facilities. History has shown that inadequately designed or detailed tanks have suffered extensive damage in past earthquakes, leading to severe economic and environmental consequences. This article explores the seismic behavior of such storage tanks, analyzing different forms of earthquake-induced damage and insights from analytical and experimental studies.
Types of Earthquake Damage in Storage Tanks
During an earthquake, steel storage tanks can sustain various forms of damage due to different stress mechanisms. The most common types of earthquake-induced damage include:
- Elephant-foot buckling: Large axial compressive stresses due to beam-like bending of the tank wall can cause localized buckling at the base, resembling an elephant’s foot.
- Sloshing damage: The movement of the liquid inside the tank can generate significant wave action, which can damage the roof and the upper portions of the tank wall.
- Base anchor failures: Poorly detailed base anchors can experience excessive stress, leading to ruptures in the tank wall.
- Sliding and base shear: If the seismic forces overcome frictional resistance at the base, the tank may slide, leading to further structural damage.
- Base uplifting effects: In unanchored or partially anchored tanks, the uplifting of the base can cause damage to piping connections, excessive joint stress at the plate-shell junction, and uneven settlement of the foundation.
Early Analytical Studies on Hydrodynamics
Initial analytical studies focused on understanding the hydrodynamic response of liquids in rigid tanks placed on rigid foundations. It was found that the liquid inside the tank behaves in two distinct ways:
- Impulsive liquid: A portion of the liquid moves rigidly with the tank wall and experiences the same acceleration as the ground, contributing predominantly to base shear and overturning moment.
- Convective (sloshing) liquid: The remaining liquid moves in a long-period sloshing motion, which determines the height of the free-surface waves and affects the required freeboard.
Further studies indicated that the flexibility of the tank wall can amplify the acceleration of the impulsive liquid beyond the peak ground acceleration, making the assumption of a rigid tank nonconservative.
Effects of Foundation Flexibility
Tanks supported on flexible foundations with rigid base mats experience additional effects such as base translation and rocking. These factors result in longer impulsive periods and generally greater effective damping, which significantly influences the tank’s impulsive response. However, the convective (sloshing) response remains largely unaffected by both the tank wall and the foundation flexibility due to its inherently long oscillation period.
Seismic Response of Unanchored and Partially Anchored Tanks
In practical applications, full base anchorage is not always feasible or economical. Many storage tanks are either completely unanchored or only partially anchored at their base. Studies on the seismic response of such tanks revealed that:
- Base uplifting can reduce hydrodynamic forces within the tank but significantly increase axial compressive stress in the tank wall.
- When unanchored tanks are placed on flexible soil foundations, the increase in axial stress is less pronounced. However, foundation penetration can become severe, leading to cycles of large plastic rotations at the plate boundary.
- While flexibly supported unanchored tanks are less prone to elephant-foot buckling, they are more susceptible to uneven foundation settlement and fatigue rupture at the plate-shell junction.
Additional Experimental and Numerical Studies
Over the years, numerous experimental and numerical studies have provided valuable insights into the seismic behavior of storage tanks. While this paper focuses on the elastic analysis of fully anchored, rigidly supported tanks, studies involving foundation flexibility and base uplifting effects are available elsewhere for further reference.