Seismic design and retrofit of cylindrical liquid storage tanks.

摘要

Observed damage to cylindrical liquid storage tanks in past earthquakes and the presence of a large number of such tanks in regions of high seismic risk are two of the main reasons necessitating the need to retrofit these tanks. Past seismic damage to tanks included the buckling of the shell wall, pull out and/or damage to the base anchor bolts, and damage to the tank roof. This research addresses the cause of these damages and recommends retrofit schemes to prevent similar damage in future earthquakes.;Buckling in cylindrical storage tanks often occurs in one of two main modes: "elephant" foot buckling or diamond buckling. Various analytical approaches to compute the buckling capacity of shells are discussed and compared to a quasi-static non-linear finite element approach. A wide range of dimensions of anchored tanks are used in such a comparison with the available analytical approaches. Further, example of an unanchored tank is also presented where a general contact approach with a rigid foundation is used. Circumferential ring stiffeners as well as vertical stiffeners are discussed as possible retrofit schemes to increase the buckling capacity of cylindrical tanks. Dramatic improvement of the buckling capacity can be achieved by the use of ring stiffeners in the bottom third of the tank shell wall. Appropriate spacing of such ring stiffeners is illustrated. Partial vertical stiffeners have been shown to cause local buckling and tearing of the shell wall if not used in conjection with ring stiffeners.;A more detailed approach for the computation of the overturning moment and base shear capacity of a cylindrical storage tank, including the effects of the anchors, is presented. This approach also provides a path to designing anchors. Appropriate design of the anchors and the anchor chairs is discussed. Retrofit of unanchored tanks against the base motion is also addressed. Two possibilities are examined, either anchoring the tank or providing preuplifting. Finite element studies are presented for these possible solutions. Both methods show promise in reducing the excessive uplift seen in unanchored tanks during large seismic events.;Finally, the impact of the sloshing liquid on the roof and its connection with the tank shell is examined. Simplified approaches are presented to calculate the impact hydrodynamic pressures exerted on the roof for design of the roof and its connection. Suggested retrofit schemes are cylindrical perforated baffles or annular baffles. Both reduce sloshing, and thus, minimize the impact pressure of the sloshing liquid on the roof.