There is a
popular approach in the structural field
this approach assumes the base of the structure
as a fixed base but in reality, the soil under the structure deforms
because the soil has the ability to
deform. In the conventional seismic
design of buildings frame, the same
approach is also assumed. Because of the soil response,
the seismic response of the structure gets changed. The soil response may
decrease the stiffness of the structure and increase natural periods
of the structure. In this paper, the
effect of soil flexibility on the performance
of medium-high RC framed structure (5bay
by 5bay 6 story) with various plan irregularities resting on different types of
soil (Hard soil, Medium soil, soft soil) is taken into consideration. Two types
of model approaches are used in order to represent the soil- structure
interaction(flexibility). The first
approach is Winkler model which is
developed by using spring stiffness equations. Every spring has six degrees of
freedom. Along these six degrees of freedom,
the stiffness is determined by using spring stiffness equations. The second approach is elastic continuum which
is developed by finite element method using Sap2000. The idealization of elastic continuum depends on a common
experience which says deflections in soil media do not happen directly under
the loading area but also in certain
limited zones outside the loading area. According
to on the idealization of the elastic continuum,
a soil block of 75m x 75m (1.5 times the width of the structure) is assumed.
Soil block is modeled by FEM. Beams and columns are modeled as frame element
with 2 nodes having 6 degrees of freedom at each node, the foundation is modeled as 8 nodes concrete
element and soil mass beneath the foundation is modeled as 8 nodes solid
element with 2 degrees of freedom at each node. In conclusion, when the soil
flexibility increases, the response of the structure, beam moment, column
moment, and base shear also increase. Irregularity does not have clear effects
on the response of the framed structure.
Soil-structure interaction has effects on moments of columns and beams. For
instance, has been found that the models with soil-structure interaction have
high values of moments in columns and beams when compared with models without
soil-structure interaction. The results from FEM method are more accurate than Winkler method. FEM method is suggested for
analysis of structure resting on soft soil. Finally, when the structure is
resting on soft soil, it’s important to consider soil-structure interaction

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