A PARAMETRIC STUDY OF SOIL STRUCTURE INTERACTION OF RAFT FOUNDATION BY USING DYNAMIC ANALYSIS

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Mohammed zubair and B R Shilpa ijesird, Vol. III, Issue I July 2016/68

A PARAMETRIC STUDY OF SOIL STRUCTURE INTERACTION OF RAFT

FOUNDATION BY USING DYNAMIC ANALYSIS

Mohammed zubair¹, B R Shilpa²

1post graduate student, ²Assistant professor

Sambhram institute of technology, M.S.Palya, jalahalli east, Bengaluru-97

Abstract— The process in which the response of the soil influences the motion of the structure and response of the structure influences the motion of the soil is known as Soil- Structure Interaction (SSI).The social economic damages caused by an earth quake depend to a great extent on the features of the strong ground motions, these motions reflect primarily from factors such as source characteristics, propagation wave path and local site conditions. In the normal design practice all will consider building frame as a fixed base but in actual case the flexible nature of soil allows the foundation for movement. Due to this the stiffness of the building frame get decreases thus increase in the natural time period of the structure and thus the overall response gets changed. Hence the present thesis work deals with the response of the building frame with raft foundation under soil flexibility due to seismic behaviour. The 3D frame is analysed by using SAP 2000 V14. The soil and the structure are considered as a single continuum model. In present study the parameters influence on the structure such as number of storeys, different seismic zones and it is evaluated as Interaction analysis (IA) and Non Interaction Analysis (NIA).The reaction obtained in this analysis are fundamental time period, base shear, joint displacement, axial force shear force and bending moment for base columns and bending moment for beam (B1).

Keywords— SSI, SAP, Continuum model, IA, NIA, Axial force, Shear force, Bending moment

I. INTRODUCTION

The procedure in which the action of soil imparts the movement of the structure and the movement of the structure affects the action of the soil is called as SSI. Impendence difference is characterized as result of speed and thickness of soil. Seismic wave ventures quicker in hard shakes in contrasted with milder shakes and silt. As the waves goes from harder to milder rocks, they turn out to be moderate and should get greater in abundance to convey the same measure of the energy, in this way shaking tends to more grounded at sides with gentler surface layers, where seismic waves move more gradually.

Resonance obtained when signal frequency matches with fundamental frequency of soil, they say that in resonance with one another, this results in to tremendous increase in ground motion amplification. Irregular basement topography when subjects to body wave incidence results in focusing and defocussing effect. This strongly depends on angle of incidence wave. The impact on a structure during an earth quake relies upon the properties of the ground soil, intensity of earth quake and structural system. When the foundation is on firm ground the foundation motion is basically taken at the soil level with the absence of structural system.

For foundation on soft soil foundation motion varies from that in the free field due to the coupling of soil and structure during earth quake this is due to the scattering of waves and energy released due to the vibration of structures. Due to these impacts the condition of displacements in the supporting soil is different from free field because of these dynamic response of a structure is differ greatly in amplitude and frequency.

II. STEPS IN SOIL-STRUCTURE INTERACTION

A structure exposed exteriorly static or dynamic loading over the ground portion of the structure, the SSI issue includes assessment of the execution of the structure which is subjected to loading considering the effect of soil foundation interaction, hence for the above situation the soil has no under lying movement for earlier loading. The effect soil foundation interaction is to provide the soil foundation system with elastic boundary conditions at the soil foundation interface. For a structure

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Mohammed zubair and B R Shilpa ijesird, Vol. III, Issue I July 2016/69 subjected to seismic conditions, dynamic loadings

are forced on the structure. These loadings, which start with movements of the solid medium, are transmitted to the structure through its footing;

consequently, the general SSI issue for this situation includes, although the foundation impedance, foundation stiffness interaction, and foundation limit issues represented over, the assessment of seismic dynamic forces and impacts of free field ground movement impelled soil deformities on the soil foundation frameworks. Keeping in mind the end goal to assess the seismic dynamic forces on the footings and the impacts of the free-field ground distortions on compliances and limits of the soil–

foundation frameworks, it is important to decide the varieties of free-field movement inside the ground areas which associate with the foundations. This issue of deciding the free-field ground movement varieties will be referred in this as the "free-field site response problem". The issue of assessing the seismic dynamic forces on the foundations is identical to deciding the "effective or spread foundation input movements" actuated by the free- field soil Motions. This issue will be referred to here as the "foundation spread problem" Thus, the general SSI issue for a structure subjected to externally applied static and/or dynamic loadings can be isolated into the assessment of Foundation toughness or impedance, foundation–structure interactions and Foundation capacity.

III. LITERATURE REVIEW

M.N. Viladkar (Revised 10 April 1990) conducted SSI in plane frames using coupled Finite-Elements. In this case study, a beam bending element, which accounts for transverse shear deformation and axial-flexural interaction has been used for frame members and a combined footing which is treated on a part of the frame. A hyperbolic stress-strain model is used for the treatment of soil non-linearity. The percentage variation of bending moment values between the non-interactive and interactive analysis has been found to be in the range of -50 to +90 %, however the percentage variation because of non-linearity of soil mass over and above those owing to linear interactive analysis has been found to be in the

range of 1 – 20%. Contact pressure values obtained on the basis of non-linear analysis are much higher than those from linear analysis.

Vivek Garg (Nov 2012) conducted Interaction effect of space frame-strap footing -soil system on forces in superstructure. In this present work, RCC space frame footing-strap beam-soil system carried out to investigate in interaction behaviour of 3 storeys and 3 bay. Frame foundation soil is considered as linear elastic. Evaluation of shear force, bending moment and axial force in columns, floors, plinth and strap beams and comparison is made between non-interaction and interaction analysis. Analysis is carried out by ANSYS software using two node beam bending element with six degree of freedoms and joints assumed as perfectly rigid. The roof slab is discretised with four node plate bending element SHELL 181 the footing is discretised with SHELL 281 with six degree of freedoms. 200x100x90 m semi-infinite soil model is considered. Soil is homogeneous, isotropic, half space, SOLID 92 @ interface characteristics between the raft and soil is represented by TARGET 170 and CONTA 174. The comparison of axial force due to NIA and LIA reveals that interaction effect causes redistribution of the forces in column members. The inner columns are relieved of the forces and corresponding increase is found in the corner columns due to interaction. No significant interaction effect Is found in axial force of side column. LIA (Isolated footing) provides variation of -14 to 31 % in axial force compared to NIA. LIA with strap footing provides variation of - 9% in inner corners to 18 % in corner columns in axial forces compared to NIA. A significant increase in bending moment of outer columns at the column footing junction is found in LIA-ISO as well as reversal in the sign takes place because of rotation of eccentrically loaded isolated footing.

LIA-STR provides variation of -90 to 375% in bending moment compared to NIA. -90% at corner column and 375% at side column. LIA-ISO provides variation of 65% is found in the inner end of plinth beam, +67% is found in the outer end of plinth beam. Variation of -39% to 42% found in the shear force due to LIA-STR compared to NIA. The maximum decrease of nearly 39% at inner end of

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Mohammed zubair and B R Shilpa ijesird, Vol. III, Issue I July 2016/70 plinth beam, 41% at outer end of plinth beam. LIA-

STR provides variation of -16 to 75% in shear force compare to LIA-ISO. LIA-ISO provides variation of -74% (inner end of floor beam of top storey) whereas the maximum increase of 219% is found in the outer end of plinth beam in bending moment.

The variation of -47% for inner plinth beam and 125% for outer plinth beam found in bending moment due LIA-STR. LIA_STR provides variation of -88 to 124% in bending moments compare to LIA-ISO.

Saugata Dasgupta (1999) determined the Effect of soil-structure interaction on building frames on isolated footings. In this study, the effect of these parameters on soil structure interaction is studied through the variation of axial force in column &

change in differential settlement supported on isolated footing. Effect of SSI is marginal for single story, single-bay frame irrespective of its span &

hence, irrespective of ratio stiffness of beams &

columns. The study clearly shows that building frames with isolated footing may have large increase in column axial loads and settlements with increase in the number of stories and number of bays. A maximum increase of 104% is observed for a four-story four-bay frame. The structure which has larger effect of such interaction exhibits a reduction in differential settlement calculated in reference state.

Sekhar Chandra dutta (5 April 2002) conducted A critical review on idealization & modelling for interaction among soil-foundation-structure system.

This paper is review on different models of SSI, it draws conclusion that SSI should be considered in both static and dynamic loading. Winkler hypothesis, despite its obvious limitations yields reasonable performance & it is easy to exercise.

Non-linear modelling desired for clayey soil because of stress settlement relationship. To get more realistic modelling & soil should be taken visco-elastic model with time dependent process.

Lumped mass analysis is gives comparable good realistic results. This paper helps to arrive at a suitable method of analysis by properly weighing the strength and limitation of the same against the particular characteristics and need the problem at hand.

IV. METHODOLOGY

The model of 5 storeys, 10 storeys and 15 storeys were modelled in the SAP 2000 software and there were subjected to the seismic loading conditions of Zone 3, Zone 4 and Zone 5.The live load of 4 KN/m² (including Floor Finishes) in all cases. A raft 800mm thick is taken in analysis which kept constant in all cases. Soil-Structure system modelled as continuum model which means the soil and structure considered as one system.

For this modelling soil is modelled as solid.

Three layers of soil considered in study with shear modulus of 30000 KN/m², 20000 KN/m² and 10000 KN/m²for soil layers from top to bottom variation respectively. Thickness of each layer is 10m.

Soil system is considered as multi-linear elastic link element of different load- deflection curve in vertical uplift, bearing and axial loading. Gap element is used between soil structure interfaces to facilitate uplift of foundation from soil.

Sl n o

Multi-linear elastic link element properties

Vertical uplift Bearing Axial

Disp (mm)

Force (KN)

Disp (mm)

Force (KN)

Disp (mm)

Force (KN)

1 -60 -35.82 -0.0001 0 -3 -12.36

2 -30 -35.82 0 0 0 0

3 0 0 40 355.86 3 12.36

4 0.001 0 80 355.86 6 12.36

Table 1: Link properties

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Fig 1: Assigning link properties in SAP 2000

Fig 2: 15 storey fixed model

Fig 3: 15 storey SSI model

V. RESULTS

Dynamic analysis of structure-soil framework (Continuum model), by the strategy for 'Reaction Spectrum' exhibited in IS1893:2002, is done utilizing SAP2000* Ver.14 software. The parameters are considered by differing soil property (Shear modulus) between the Raft-foundation and the soil underneath. Significant piece of this study is confined for the examination of estimations of set parameters, between the Non-Interaction case (Fixed base condition) and the Interaction case. The accompanying 5 parameters of the 2bay structures of 5, 10 and 15 stories with Raft foundation and by varying seismic zones (3, 4 & 5) are examined, viz,

1. Fundamental Time Period 2. Base Shear

3. Maximum Lateral Displacement 4. Axial force in the base columns 5. Bending moment in beam (B1)

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Graph 1: Fundamental time period Table 2: Fundamental time period

Table 3: Maximum lateral displacement and Table 4: Base reactions

Sl No

No of

storeys Zone

With interaction

(sec)

Without interaction

(sec)

Change In %

1 5

zone 3 17.9867 14.1695 26.939 zone4 17.9867 14.1695 26.939 zone 5 17.9867 14.1695 26.939

2 10

zone 3 34.7176 29.5614 17.442 zone4 34.7176 29.5614 17.442 zone 5 34.7176 29.5614 17.442

3 15

zone 3 53.107 47.2046 12.504 zone4 53.107 47.2046 12.504 zone 5 53.107 47.2046 12.504

No of

storeys Zone

With interaction

(KN)

Without interaction

(KN)

Change in %

5

zone 3 26.983 17.01 36.96 zone4 40.474 10.631 73.734 zone 5 60.711 38.273 36.95 10

zone 3 46.498 34.02 26.83 zone4 69.747 51.03 26.83 zone 5 104.62 76.545 26.83 15

zone 3 65.743 51.03 22.37 zone4 98.614 76.545 22.37 zone 5 147.921 114.818 22.37

No of storey

Zone

With interaction (m)

Without interaction

(m) Along

X

Along Y

Along X

Along Y

5

zone 3 0.3913 -0.0348 0.1863 -0.0833 zone4 0.5848 -0.0338 0.0786 -0.0851 zone 5 0.8750 -0.0322 0.5452 -0.0773 10

zone 3 1.0521 -0.0002 0.5650 -0.1786 zone4 1.5783 -0.0002 0.9473 -0.1728 zone 5 2.3677 -0.0001 1.5207 -0.164 15

zone 3 1.7347 -0.0002 0.9937 -0.231 zone4 2.6023 -0.0002 1.6194 -0.223 zone 5 3.9036 -0.0002 2.5578 -0.209

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Graph 2: Maximum lateral displacement

Table 5: Axial force for 5 storeys Graph 3: Base reactions

Table 6: Axial force for 10 storeys

Axial forces for 10 storey building

Non-Interactive case (KN) Interactive case (KN) Col

no

Zone 3

Zone 4

Zone 5

Zone 3

Zone 4

Zone 5

C1 -1566 -1566 -1566 -1682 -1682 -1682 C2 -1917 -1917 -1917 -2767 -2767 -2767 C3 -1405 -1405 -1405 -1682 -1682 -1682 C4 -1941 -1941 -1941 -2842 -2842 -2842 C5 -2447 -2447 -2447 -28.35 -28.35 -28.3 C6 -1742 -1742 -1742 -2842 -2842 -2842 C7 -1403 -1430 -1403 -1682 -1682 -1682 C8 -1714 -1714 -1714 -2767 -2767 -2767 C9 -1252 -1252 -1252 -1682 -1682 -1682 Axial forces for 5 storey building

Non-Interactive case(KN) Interactive case(KN) Col

no

Zone 3

Zone 4

Zone 5

Zone 3

Zone 4

Zone 5

C1 -712.1 -712.1 -712. -735.5 -735.5 -735 C2 -994.2 -994.2 -994 -1486 -1486 -1486 C3 -625.9 -625.9 -625 -735.5 -735.5 -735 C4 -1006 -1006 -1006 -1529 -1529 -1529 C5 -1423 -1423 -1423 -28.35 -28.35 -28.3 C6 -886.8 -886.8 -886 -1529 -1529 -1529 C7 -624.9 -624.9 -624 -707.7 -707.7 -707.

C8 -872.6 -872.6 -872 -1460. -1460. -1460 C9 -547.5 -547.5 -547 -707.7 -707.7 -707

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Graph 4: Axial force for 5 storeys Graph 5: Axial force for 10 storeys Table 7: Bending moment along X & Y axis

Axial forces for 15 storey building

Non-Interactive case (KN) Interactive case (KN) Col

no

Zone 3

Zone 4

Zone 5

Zone 3

Zone 4

Zone 5

C1 -2476 -2476 -2476 -2701 -2701 -2701 C2 2811 -2811 -2811 -3983 -3983 -3983 C3 -2246 -2246 -2246 -2701 -2701 -2701 C4 -2840 -2840 -2840 -4072 -4072 -4072 C5 -3330 -3330 -3330 -28.3 -28.35 -28.3 C6 -2571 -2571 -2571 -4072 -4072 -4072 C7 -2244 -2244 -2244 -2701 -2761 -2761 C8 -2538 -2538 -2538 -3983 -3983 -3983 C9 -2024 -2024 -2024 -2701 -2701 -2701

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Table 8: Axial force for 15 storeys

Graph 6: Bending moment along X axis

Graph 7: Bending moment along Y axis

Graph 8: Axial force for 15 storeys

VI. CONCLUSION

 Fundamental time period of 5, 10 &

15 storey structures considering NIA is less than that of IA.

 Fundamental time period of the system does not vary with seismic zones.

 Base shear values increments with ascent in number of stories further more increments with higher dynamic seismic zones.

 Maximum lateral displacement value increases with ascent in number of storeys along both X & Y direction of raft foundation and also increases with higher active seismic zones.

 Maximum lateral displacement value is observed more in IA models than compared to NIA models.

 Axial forces in the base storey columns are more in IA case than compared to NIA case.

 Axial forces increases with increase in number of storeys and also

5 zone4 -2032.9 -934.4 -21.37 -17.97 zone 5 -2032.4 -934.9 -27.2 -17.94

10

zone 3 -639.1 -649.6 -28.43 -20.26 zone4 -834.6 -844.4 -32.11 -20.28 zone 5 -19.65 -20.52 -37.64 -20.27

15

zone 3 -879.9 -1022.3 -32.56 -20.74 zone4 -880.01 -1022.3 -38.11 -20.76 zone 5 -879.9 -1022.3 -46.44 -20.76

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Mohammed zubair and B R Shilpa ijesird, Vol. III, Issue I July 2016/76 increases with higher active seismic

zones.

 From the study it is found that bending moment in interaction case is much higher than non-interaction case.

REFERENCES

[1] Agarwal. P and Shrikhande. M (2006) ―Earthquake resistant Design of structures‖ Prentice-Hall of India Private Limited, New Delhi, India.

[2] SOIL-STRUCTURE INTERACTION IN PLANE FRAMES USING COUPLED FINITE-ELEMENTS, M.N. Viladkar, P.N. Godbole and J.

Noorzaei Civil engineering Department, University of Roorkee, Roorkee-247 667 (U.P),India (Revised 10 April 1990)

[3] INTERACTION EFFECT OF SPACE FRAME-STARP FOOTING – SOIL SYSTEM ON FORCES IN SUPERSTRUCTURE, Vivek Garg¹ and M.S Hora2, ARPNJournal of Engineering and applied sciences Vol 7, No 11, Nov 2012

[4] Effect of soil-structure interaction on building frames on isolated footings, Saugata Dasgupta^*, Sekhar Chandra Dutta^** and Gautam

Bhattacharya^***, Journal of Structural Engineering VOL 26 NO 2 July 1999

[5] INTERACTION ANALYSIS OF SOIL-RAFT-FRAME – A PARAMETRIC STUDY, D. Daniel Thangaraj, Assistant professor, Department of Civil Engineering, Anna University College of Engineering Tindivanam, India, K. Ilamparithi, Professor and head, Division of Soil Mechanics and Foundation Engineering, Anna University Chennai, India, IGC 2009, Guntur, INDIA

[6] A CRITICAL REVIEW ON IDEALIZATION & MODELLING FOR INTERACTION AMONG SOIL-FOUNDATION-STRUCTURE SYSTEM, Sekhar Chandra dutta, rana roy (5 April 2002)

[7] RESPONSE OF LOW RISE BUILDING UNDER GROUND EXCITATION INCORPORATING SSI, Sekhar Chandra dutta , koushik bhattacharya & rana roy (2 July 2004)

[8] ATC-40 (Applied technology Council) ―Seismic evaluation and retrofit of concrete buildings‖ Volume 1.

[9] IS: 456-2000, ―Code of practice for plain and reinforced concrete‖.

Bureau of Indian standards, New Delhi, India.

[10] IS: 1893(part 1): 2002, ―Criteria for Earthquake Resistant Design of structures‖ part 1-General provisions and buildings, fifth revision, Bureau of Indian Standards, New Delhi, India

[11] Veletsos .A.S., Jethro W.meek, (1974). ―Dynamic behavior of building-foundation systems‖ Earthquake engineering and structural dynamics, vol.3, 121-138.