Soil Structure Interaction Analysis of Multi Storey Building

IJSTE - International Journal of Science Technology & Engineering | Volume 3 | Issue 11 | May 2017 ISSN (online): 2349-784X

Soil Structure Interaction Analysis of Multi Storey Building Nimisha Ann Sunny PG Student Department of Civil Engineering Mar Athanasius College of Engineering, Kothamangalam, Kerala, India

Dr. Alice Mathai Associate Professor Department of Civil Engineering Mar Athanasius College of Engineering, Kothamangalam, Kerala, India

Abstract The analysis of building structure in contact with soil involves an interactive process of stresses and strains developed within the structure and the soil field. The response of Piled-Raft Foundation system to the structure is very challenging because there is an important interplay between the component of building structure and the soil field. Herein, soil - structure interaction of buildings founded on Piled-Raft Foundation is evaluated through Finite Element Analyses using ANSYS v17.0. The building settlement and equivalent stress is computed. The study has been conducted by modeling building with soil and without soil. It is concluded that the interaction of building foundation-soil field and super-structure has remarkable effect on the structure.

Keywords: Soil – Structure Interaction, Static Analysis, Geometric Non Linearity, Piled – Raft Foundation, ANSYS Workbench 17.0 ________________________________________________________________________________________________________ I.

INTRODUCTION

The process in which the response of the soil influences the motion of the structure and the motion of the structure influences the response of the soil is termed as soil-structure interaction (SSI) The analysis of Piled-Raft Foundation is very challenging because the load in the piled-raft structures is transferred to the soil not only by the interaction between the soil and the piles but also by the interaction between foundation structure and superstructure. In this interaction, deformations in the soils are the key factor which will affect forces and deformation in foundation and superstructure. The combined piled-raft foundation penetrates deep into the foundation soil increasing its significant depth below the ground and affects the response of structure and soil. The method of analysis of foundation and structure also affects the response of structure and soil. A simplified procedure has been followed for estimation of building settlement and equivalent stress. In this method, piled-raft foundation under vertical load considering interaction effects has been modeled and analyzed with soil ( flexible base) and without soil (fixed base ) in the finite element package of ANSYS Workbench 17.0 incorporating geometric non-linearity. The study in this paper is extended by a comparison between fixed base model and flexible base model.

II. STATIC ANALYSIS OF BUILDING IN ANSYS 17.0 Geometry of the building In the selection of the frames' span width and height of storey, it has been tried to make the width and height to be conforming to architectural principles and member section properties conforming to IS codes and structural principles. For this study, 10 storey building with piled raft foundation has been considered. To reduce the complexity in analysis, a simple structure with base dimension 12m x 12m was modeled. Cross-section properties of superstructure elements have been kept same for all floors, column – 300 x600mm, beam – 300x50mm & floor slab – 150mm and cross-section properties of foundation elements raft – 850mm, pile diameter – 750 mm & pile depth – 15m, all dimensions are as per design requirements. The soil plan area, was fixed such that, horizontal dimension should be at least five times the horizontal dimension of building and depth of soil should be at least three times depth of foundation, Therefore, soil volume modeled is 60x60x45m. The overall geometry of the two building configurations, fixed base and flexile base are shown in Fig.1.

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Soil Structure Interaction Analysis of Multi Storey Building (IJSTE/ Volume 3 / Issue 11 / 013)

Fig. 1: Fixed base and flexible base building models

Finite Element Modelling The building models are modelled as three dimensional structural solids with element types assigned automatically by FEA software, ANSYS 17.0. Mesh convergence study was done for building and used 300mm element size and for soil a coarser mesh was used. No separation type contact was used between structure and soil elements and bonded contact type between structural elements. The soil surface is taken as the contact surface; that of the structure (foundation) is taken as the target one, since its rigidity is larger than that of the soil [2].

Material models used For assessing the complete behaviour of structure and soil, proper material properties and element selection is required. Structure and soil were modelled as linear and isotropic. Properties considered for analysis are Young’s Modulus (E), Poisson’s ratio (ν) and Density (ρ). Properties of Concrete and soil are given in Table 1.

Boundary conditions and loading Soil is modelled as rectangular solid around the foundation due to scope limit. Since soil is extended in infinity in actual case, we modelled it as finite sized solid and applied boundary conditions. Bottom layer is considered as hard strata so it restrains the movement in any direction, hence a fixed boundary condition is provided at the bottom soil solid element. That is, if soil lateral side is perpendicular to x axis, displacement in x direction is restrained and displacement in y axis & z axis is free to move. Thus, soil is restrained in the corresponding direction and allows free movement in other directions.

Table – 1 Material Properties of Concrete and Soil Properties Structure Soil Material Concrete Soft Soil Young’s Modulus, E (Pa) 2.9 x 1010 2.5 x 107 Poisson’s ratio, ν 0.15 0.25 Density, ρ (kg/m3) 2500 1900

Fig. 2: Meshed models

Dead load and Live load are given as per IS 875 (Part I) and (Part II) 1987, respectively. Dead load includes, self-weight of all elements and wall load, considering wall thickness of 200mm thick. All loads are applied as pressure.

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Soil Structure Interaction Analysis of Multi Storey Building (IJSTE/ Volume 3 / Issue 11 / 013)

III. RESULTS OF STATIC ANALYSIS AND DISCUSSIONS Static analysis gives performance of structure under static loads, in form of total deformation, stress in members etc. Here we have considered the total deformation (settlement) of particular column in both models and equivalent stress in the building models Total deformation of building

Fig. 3(a): Total deformation of building with fixed base

Fig. 3(b): Total deformation of building with flexible base

Fig. 3(c): Total deformation of building with flexible base (sectional view)

Total deformation occurred to the column under consideration is greater in flexible base model (0.795m) compared to fixed base model (0.0086m). Figure above shows how soil settlement occurs in actual case.

Equivalent stress in building models

Fig. 4(a): Equivalent Stress in building with flexible base

Fig. 4(b): Equivalent Stress in building with fixed base

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Soil Structure Interaction Analysis of Multi Storey Building (IJSTE/ Volume 3 / Issue 11 / 013)

Above figure shows the equivalent stress in building considering both cases. From which we can conclude that there is a difference in stress distribution when we consider soil along with the building while analyzing. Average stress developed in model with flexible base is more than the other model.

IV. CONCLUSIONS The analysis of combined piled-raft foundation of multi-storey building is very challenging because of complexities involved in the interaction between the components of building structure and soil field. The analysis of the tall building structure with complex foundation system in uniform soil field should include the interaction of structure-foundation-soil. In this study, the finite element soil structure interaction analysis of building structure having piled-raft foundation with soil model and without soil model is conducted. The complete interaction among the soil field depth, settlement of building and stress developed in building has been evaluated. From the study, it is seen that total deformation (vertical) of building is more in flexible base model than in fixed base model, which means that in actual case settlement occurs and it depends on type of soil beneath. In the model where soil is considered stress distribution pattern has varied. Average stress developed in model with soil is greater than other model .This study indicates that building should be modeled along with the soil in which it is resting considering all properties of soil for the analysis and design purpose.

ACKNOWLEDGMENT I am extremely thankful to my beloved project guide Dr.Alice Mathai for her valuable guidance, encouragement and immense support throughout my humble endeavour. It is beyond words to express my sincere gratitude to all my friends and teachers for the constant inspiration in completing this work.

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[8] Shehata E. Abdel Raheem • Mohamed M. Ahmed, Tarek M. A. Alazrak (2015),Evaluation of soil–foundation–structure interaction effects on seismic response demands of multi-story MRF buildings on raft foundations, International Journal Advanced Structural Engneering volume7:11–30 [9] Mohd Ahmed, Mahmoud H. Mohamed, Javed Mallick, Mohd Abul Hasan (2014), 3D-Analysis of Soil-Foundation-Structure Interaction in Layered Soil, Open Journal of Civil Engineering, 4, 373-385 [10] Rahul Sawant and Dr. M. N. Bajad (2016), Effect of Soil-Structure Interaction on High Rise RC Building, IOSR Journal of Mechanical and Civil Engineering, Volume 13, Issue 1 Ver. IV,PP 85-91.

[11] Anil K Chopra, Dynamics of Structures, Pearson (3rd Edition), 2016.

[12] J A Hemsley, Design Applications of Raft Foundation, Thomas Telford, 2000.

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