Civil Insight: A Technical Magazine Volume 3 | Page 38
Dongol N. et al.
Civil Insight (2019) 37-44
1.1) Soil-Structure Interaction
The process in which the response of soil influences the motion of structure and vice versa is termed as
soil-structure interaction (SSI). The flexibility of soil mass causes differential settlement and rotation of
footings under the application of load.
1.1.1) Kinematic Interaction
In a free field, an earthquake will cause soil displacement in vertical and horizontal directions, but if the
foundation embedded in the soil is very stiff, it cannot follow the free-field deformation pattern. This is
influenced by kinematic interaction. It results from the presence of stiff foundation elements on or in soil,
which causes motions at the foundation to deviate from free-field motions (Kramer, 2007).
1.1.2) Inertial Interaction
Soil displacements occur due to earthquake motion of the ground termed as free field motion. If the
foundation is fixed or planted into the soil, it will not follow the free-field motion. On the other hand, the
weight of the super structure transfers the inertial force to the soil, causing another deformation in the soil
termed as inertia interaction (Kramer, 2007).
2) Research Hypothesis
In the preliminary design process, equivalent static seismic forces are used to determine the design internal
forces of structural members using linear elastic analysis of structure. Dynamic analysis is the one that
considers mode shapes and modal mass participation of structures for different building frequencies. Every
building does not just have a single frequency, but has different frequencies of vibration. When there is an
earthquake, the response of the building is a combination of different natural frequencies of the building
(Kabtamu, Peng, & Chen, 2018).
Pushover Analysis (PA) is a type of procedure that uses simplified non-linear Static Analysis (SA).
Pushover analysis is employed for assessing the seismic vulnerability of existing structures. PA is non-
linear; the gravity loads and the lateral load cases are applied sequentially in a single analysis. For a building
in its design phase, PA results help scrutinize and finely tune the seismic design based on SA, which is
slowly becoming a standard procedure for large critical structures (Leslie, 2013).
In substructure modeling, spring stiffness is used to account for frequency dependency of interaction. The
top 30m of surface soil stratum is considered a key influence on the structure and its respective ground
motions (Kiku et al., 2001). It is the simplest way to consider the SSI effects. In direct modeling, the
equations of motion are solved directly in their coupled form and in a single step.
The advantage of these methods is their versatility to cope with complex geometries and material properties,
but the data preparation and complexity of modeling makes it difficult to implement in everyday
engineering practice.
3) Structure Modeling and Analysis
While doing modeling and analysis using SAP2000 (SAP2000, 2015), the parameters considered for the
residential building shown in Fig. 1 are presented in Table 1. The ground floor plan and the south elevation
of the residential building are presented in Fig. 1.
The Indian Standard codes IS 1893 (Part 1) and IS 456 were used for the dynamic analysis, whereas ATC-
40 was referred to for the pushover analysis.
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