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Abstract:
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Frames are the most widely used structural system for multistorey
buildings. A building frame is a three dimensional discrete structure consisting of
a number of high rise bays in two directions at right angles to each other in the
vertical plane. Multistorey frames are a three dimensional lattice structure which
are statically indeterminate. Frames sustain gravity loads and resist lateral forces
acting on it.
India lies at the north westem end of the Indo-Australian tectonic plate and
is identified as an active tectonic area. Under horizontal shaking of the ground,
horizontal inertial forces are generated at the floor levels of a multistorey frame.
These lateral inertia forces are transferred by the floor slab to the beams,
subsequently to the columns and finally to the soil through the foundation system.
There are many parameters that affect the response of a structure to ground
excitations such as, shape, size and geometry of the structure, type of foundation,
soil characteristics etc. The Soil Structure Interaction (SS1) effects refer to the
influence of the supporting soil medium on the behavior of the structure when it
is subjected to different types of loads.
Interaction between the structure and its supporting foundation and soil,
which is a complete system, has been modeled with finite elements. Numerical
investigations have been carried out on a four bay, twelve storeyed regular
multistorey frame considering depth of fixity at ground level, at characteristic
depth of pile and at full depth. Soil structure interaction effects have been studied
by considering two models for soil viz., discrete and continuum. Linear static
analysis has been conducted to study the interaction effects under static load.
Free vibration analysis and further shock spectrum analysis has been conducted to
study the interaction effects under time dependent loads. The study has been
extended to four types of soil viz., laterite, sand, alluvium and layered.The structural responses evaluated in the finite element analysis are
bending moment, shear force and axial force for columns, and bending moment
and shear force for beams. These responses increase with increase in the founding
depth; however these responses show minimal increase beyond the characteristic
length of pile. When the soil structure interaction effects are incorporated in the
analysis, the aforesaid responses of the frame increases upto the characteristic
depth and decreases when the frame has been analysed for the full depth. It has
been observed that shock spectrum analysis gives wide variation of responses in
the frame compared to linear elastic analysis. Both increase and decrease in
responses have been observed in the interior storeys. The good congruence shown
by the two finite element models viz., discrete and continuum in linear static
analysis has been absent in shock spectrum analysis. |