According to several studies on past earthquakes, Dynamic loading influences the seismic reaction of buildings. Sudden distress and failures in structures have been seen as a result of soil-structure interaction. Soil-structure interaction increases shear stresses in the structure\'s bottom storeys, making it less rigid and increasing lateral displacements such as storey drift, inter-storey drift, and roof displacements. Increased lateral displacements in high-rise buildings cause occupant discomfort. The interaction between soil and structure lengthens the building\'s natural time period. Three modelling approaches, fix foundation building, medium stiff base building, and low stiff base building, have been investigated and analysed using SAP2000 V14, taking into account soil-structure interaction (SSI). The results can be used to build a detailed performance-based seismic design approach that considers the influence of soil structure interaction.
Multi-story structures are increasing rapidly in metropolitan areas in developing countries like India to provide accommodation and workplace for the population. A highrise building's structural plan has an impact on its behaviour during a large earthquake. An irregular plan or vertical configuration is one of the most common reasons of failure during earthquakes. As a result, irregular constructions, particularly those in seismic zones, are a source of concern. In most cases, structures include a number of irregularities, and predicting seismic reaction based on only one irregularity may not be enough. The kind, degree, and location of irregularities in structural design are critical since they help to improve the structure's use and aesthetics. The impact of soil structure interaction on seismic efficiency has been studied in the past, and buildings with and without soil structure interaction have been built (SSI). These structures are studied, built, and evaluated using SAP2000 software under two different boundary conditions: fixed-base and considering nonlinear dynamic analysis soil-structure interaction . To idealise the soil, an elastic continuum technique is best suited method .
Interaction of soil and structure cannot be ignored while designing structures like buildings, bridges, nuclear power plants, liquid retaining structures, dams, etc., against the expected earthquake forces .
The objective of the study is to propose simplified and practical design approach which enables designers to consider effects of interaction of soil and structure in earthquake analysis and design of multi storey irregular building to assure the design safety and reliability. In recent years, most standard codes have treated low stiff soil in irregular buildings with recommendations to design a fixed base regular structure. It has been discovered by previous research that the seismic response of irregular buildings is a complex phenomenon. The response of a ten-story irregular building under soil structure interaction with various soil conditions was compared for seismic loads in this study.
II. MODELLING PARAMETERS
A. Building Modeling
A G+9 reinforced concrete building has been modelled in SAP2000 V14 with regular fixed base building and flexible base. For conventional building fix base is considered at lowermost storey. For soil structure interaction two medium stiff and low stiff soil has been considered as per IS code. Building is situated in Zone V as per IS 1893(Part I):2002. Building height is considered as 35 m with 3.5 m floor to floor height. Raft foundation is selected for the analysis with 700 mm thickness. The length of beam in transverse direction X and longitudinal direction Y is 5 m respectively.
Dead load is calculated as per the density of material and dimension of element used in modelling. Load of brick masonry on external beam is 15.40 kN/m and 7.70 kN/m on internal beams. Parapet beams has the load intensity of 6.60 kN/m. M25 grade of concrete is used RCC members.
B. Soil Modeling
Linear elastic continuum is the most suitable method to idealized soil response and behaviour of supporting soil medium. In Elastic Continuum or finite element model, the finite soil mass is considered based on convergence study, with boundary far beyond a region where structural loading has no effect. Indrajit Chowdhary et. al (2009), suggests expressions to calculate the mesh size of soil. Clay of low Compressibility (CL) and Silt of high Compressibility (MH) is selected for this analysis as per IS 1893. Soil is modelled by using solid element with a depth of which is about 2.5 times more than the width of the building. In this case soil depth is considered as 50 m. Interface element is provided between structure and soil. These elements are provided to avoid no tensile forces will be transmitted between soil and structure. Interface element is provided between soil-raft. For modelling of interface element spring is used and the stiffness is calculated. Stiffness of building is calculated using empirical formula developed by M.H Rayhani et. al.(2008). For Clay of low Compressibility (CL) and Silt of high Compressibility (MH) values of spring stiffness is 168611 kN/m and 35787 kN/m respectively.
III. RESULTS AND DISCUSSIONS
A. Time Period
Time-period is compared for irregular fixed base building and building with medium and stiff base. Result is shown figure 3. From the analysis it has been observed that time-period increases in building having SSI.MH soil has largest time-period and 15% more than regular building. For CL soil, time-period increases to narrow extent. Due to the flexibility of the base time-period increases which leads to larger roof displacements in buildings.
B. Storey Shear
Comparison of storey shear forces is shown in figure 4. It has been found that on considering flexible base, storey shear increases at lower storeys and decreases in upper storeys. From the above result it can be concluded that as storey shear decreases in upper storey leads to economical design of structural elements. From the design prospect ground storey building should be designed for higher seismic loads.
C. Roof Displacement
Results for displacement in roof is shown in figure 5. Due to the flexibility incorporated in term of physical modelling of medium and soft soil, roof displacement increases by 15% due to interaction of soil and structure.
D. Storey Drift
Storey drift limitation in buildings is 0.004 times the storey height. CL and MH soil with building having fix base is represented in 6. It is found that storey drift increases on incorporating flexibility. Effect is more notable at bottom stories, while at upper storey drift low stiff soil shows more storey drift.
Traditionally, seismic analysis of building is carried out by considering structures as fixed in ground. Since, seismic response of structure is arduous because soil shows nonlinear behaviour. In this study three design approaches for an irregular building with different base condition have been studied. Comparison of seismic analysis parameters for linear static have been studied in this study considering SSI effect for medium stiff CL Soil and low stiff MH Soil.
1) For G+9 irregular building, it is observed that on incorporating soil structure interaction, bottom stories attract more storey shear as compared to fix base structure.
2) On considering SSI effect bottom stories should redesign with higher seismic forces.
3) For upper stories on considering SSI effect, storey shear decreases as compared to fix base structure which leads to economical design of upper storey.
4) SSI effect building shows increase in time period, roof displacement and storey drift and among the three cases, building with soft soil is more susceptible to earthquake effects.
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