Liquid which are subjected to dynamic loads. Therefore, it

Liquid storage tanks are used in
many fields such as water, oil, and gas industries. Many cases of tanks damages
have been observed due to past earthquakes. In order to sure the safety of
these storage systems and avoid fires, explosions, and environmental pollution, the seismic behavior of these systems
should be taken into consideration. Generally, the seismic response of a
structure has flexible base may differ from the same structure supported on the
rigid base. This difference occurs due to
effects of the soil-structure interaction (SSI). Soil-structure and fluid-structure interaction are important for tanks in the estimation of the dynamic behavior of elevated tanks which are subjected to dynamic
loads. Therefore, it is important to consider the effects of interaction
correctly for elevated tanks subjected to dynamic loads like earthquakes. Soil-
structure interaction influences many aspects in the design of a structure like
safety, serviceability, and cost. In addition, the soil-structure interaction
influences on the response of the structure during seismic events. Soil
influences should be taken into
consideration for different soil properties as substructure method to consider
soil effects 1. Generally, elevated tanks are assumed to be fixed at the
base, hence, the attention is focused on the dynamic behavior of the fluid. On
the other hand, the effects of soil on the dynamic
behavior of elevated tanks are also quite important. Many studies have been
made to investigate the effects of soil-structure and fluid-structure
interaction. Haroun and Ellaithy (1985) developed a model (elevated rigid tank)
considering liquid sloshing modes. Resheidat and Sunna (1986) modeled a
rectangular elevated tank and studied its dynamic behavior during earthquakes
considering soil-structure interaction. Resheidat and Sunna did not take into
consideration the sloshing effects on the seismic behavior of the elevated
tank. Haroun and Temraz (1992) studied models consist of two-dimensional X-braced
elevated tanks supported on isolated footings to determine the effects of
dynamic interaction between the tank and the supporting soil foundation. They
neglected the sloshing effects as well. It can be seen a few studies have been
made to understand the effects of interactions on elevated tanks. In this
study, a finite element elevated reinforced concrete tank is modeled and analyzed considering soil-structure
interaction. Fluid-structure interaction is out the scope therefore, it is not
considered in this review. The structural data of elevated tank are described
in Table II. The elevated tank models were put on three different cases; soft,
medium, and hard soil. Soil-structure interaction is represented by two types
of models. The first model is equivalent
spring model. There are different methods to evaluate the dynamic stiffness and
the effective input motion of a foundation. The
first method is modifying the fixed base of a structural system (Veletsos
and Meek 1974). Springs and dashpots represent the elastic and viscoelastic
properties, respectively. The elastic and viscoelastic properties depend on the
frequency of excitation. The second
method is the direct method. This method consists of a finite element and boundary element methods or a mixture of them in the time or frequency domain
(Wolf and Song 1996a, 1996b, Wolf 2003). Radiation damping occurs in an unbounded soil consists of a homogeneous
half-space (Wolf 2002), which changes the unbonded soil function into a complex
function. The third method is the substructure method. This method considers
the frequency dependent or independent dynamic stiffness and the damping of the
soil foundation system. A cone model proposed by Wolf and Meek (1992,1993)
represents an example of substructure method to calculate the dynamic stiffness
and the effective input motion of a foundation. In this method the structure is
assumed to be supported on springs. These springs represent the vertical,
horizontal, rocking, and torsion stiffness of the soil. The stiffness of the
spring is governed by the modulus of grade reaction of soil. The most common
model is the Winkler’s model. Springs are assumed independent hence, the effect
of the externally applied load becomes localized to the subgrade only to the point of applied loads. The raft is modeled as
a thick plate and elastic modulus is provided to raft material. Sap2000
software is used to model the water tank and provide springs in different
directions at the base. The static stiffness values of rigid circular
foundation are provided in form of spring as given in Table III, G: shear
modulus, r0: radius of circular foundation, v: Poisson ratio. Second model is equivalent
elastic solid model. In this model, soil mass is modeled as solid elements,
each solid element has eight nods with three degrees of freedom. Size of
elements should be between 1-1.5. Different type of soil properties is applied
on solid elements, especially modulus of elasticity and Poisson ratio. The width and depth od
soil is taken as 5 and 3 times of the water tank base dimension, respectively. At
the base of soil model fixed supports are provided and at periphery vertical rollers
are provided. Both models are analyzed and discussed as well as results are compared
by using Sap2000 structural software.  In
conclusion, it can be seen from the results that method of modeling depends on soil
configurations. It means, if there is hard soil under the structure, it’s useful
to use spring model instead of solid element model because solid element model consumes
lots of time for modeling and analysis. But, for medium or soft soils, it’s favorable
to use solid element model.