الفهرس 
INVESTIGATION OF STRUCTURE’S SEISMICOnly 14 pages are availabe for public view
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Abstract
The seismic design of the mid-rise building requires determining the expected base shear,
lateral drift at each story level and internal forces of the structural elements. In the
analysis, it is common for the structural engineers to consider a fixed base structure which
means that the foundations and the underlying soil strata are assumed to be infinitely
rigid. This assumption is not proper since the underlying soil in the near field often
consists of soft soil layers that possess different properties and may behave nonlinearly
leading to drastic variation of the seismic motion before hitting the structure foundation.
In addition, the mutual interaction between the structure, its foundation and the
underlying soil during the vibrations can substantially alter the structure response. This
response variation depends on the structure characteristics, the soil properties and the
nature of the seismic excitation. Consequently, more realistic assessment of inertial forces
and displacements in structures requires a rational treatment of soil structure interaction
(SSI) effects.
In this research, comprehensive numerical study is carried out to investigate the seismic
response of mid-rise buildings subjected to different seismic excitations assuming full
nonlinear SSI employing PLAXIS V8.2 software. Three types of two dimensional midrise
moment resisting building frames consisting of five story (S5), ten story (S10) and
fifteen story (S15) are analyzed. Each building is considered to be founded on three
different types of soil representing firm soil (class A), medium soil (class C), and loose
soil (class D) conditions with shear wave velocity (Vs) of 1000, 270 and 90 m/s,
respectively. To avoid the reflection of propagating waves and truncated boundaries,
viscous boundaries are adopted. In addition, rigid boundary condition is assigned at the
bedrock to assign six different input ground motions having different levels of amplitudes
and frequency contents. Moreover, each building intermediate frame has been analyzed
with different base boundaries assuming: (i) fixed base; (ii) equivalent soil springs; (iii)
flexible base considering full SSI. The investigated results include amplification ratios of
acceleration and displacement at the top of building when compared with input motion.
Also, contain comparison between different boundaries response, and comparing the
results of flexible base with provisions of ECP-2012 in terms of inter-story drifts and
internal forces.
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According to the numerical analysis and comparison of results, considering SSI can
strongly alter seismic effects on the structure to different extents. This amplification
depends on many factors including soil types and properties, building heights and
characteristics, and frequency contents of seismic excitations. During the excitations, the
loose soil exhibited different strain levels leading to different levels of nonlinear
hysteresis that resulted in different energy dissipation levels and damping ratios. At high
strain level due to high frequency contents, the energy consumption and damping reached
a high level. Therefore, the resulted surface acceleration amplitude is suppressed
possessing the highest reduction.
The analysis of results showed that shorter period soil-structure systems (5-story building
over firm soil) possessed the highest amplification ratio of acceleration for high frequency
contents and lowest for low frequency contents earthquake. Longer period soil-structure
system (15 story building over loose soil) got the lowest amplification ratio of
acceleration for high frequency contents earthquake and highest for low frequency
contents earthquake. Also, the results showed that it is essential to consider full SSI
effects in the procedures of the seismic design of concrete mid-rise moment-resisting
frames. Generally, decreasing the dynamic stiffness of the subsoil (by decreasing Vs and
shear modulus G) the base shear ratios decrease drastically while lateral displacements of
the frames relatively increase. Moreover, assuming fixed base can lead to high
overestimation of the structure design forces and seismic response.
The displacement distribution along the building height revealed that almost the simple
cantilever mode was excited for all buildings when considered with fixed bases or
supported by firm soil (class A). However, the cases of buildings resting on soil class C
and class D resulted in different distribution indicating that higher modes of vibrations
were also excited.
Finally, the structural engineers can account for the effects of soil-structure interaction on
structural elements by either considering simplified models or performing full numerical
dynamic analysis with considering SSI.