Importance of Soil Structure Interaction for Framed ...

International Conference on Advances in Civil and Mechanical Engineering Systems, 23-24 Dec.2014 Government College of Engineering, Amravati in association with SVNIT, Surat, India http://acmes2014.in

Importance of Soil Structure Interaction for Framed Structure Milind V. Mohod 1, Gaurav D. Dhadse 2 1

Department of Civil Engineering, Prof. Ram Meghe Institute of Tech. and Research, Amravati, India Department of Civil Engineering, Prof. Ram Meghe Institute of Tech. and Research, Amravati, India

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ABSTRACT: Soil-structure interaction is interdisciplinary field which involves structural and geotechnical engineering. In the conventional non-interaction analysis of building frame structural designer assumed that columns are resting on unyielding support. Similarly, in foundation design, foundation settlements are calculated without considering the influence of the structural stiffness. But a realistic analysis and design procedure should include actual support flexibility, nonlinear and heterogeneous nature of the soil together with nonlinear soil-structure interaction effects. Such an analysis would result in overall stiffness of the soilfoundation-structure system, realistic to the existing conditions. Although, interaction effect is ignored to simplify the mathematical model but neglecting the interaction between soils and structures may result in a design that is either unnecessarily costly or unsafe. This work focuses on proving the importance of Soil Structure Interaction for frame structure and computational modeling of ground-structure interaction using finite element package ANSYS’11. To demonstrate the behavior of structure while considering actual nature of ground response, a simple 3bay, 3 story frames is analyzed. Portal frame is modeled as linear elastic, whereas the ground is modeled as both linear elastic and non-linear elastic-plastic behavior. The study gives insight into variation of displacement and stress intensities of portal frame while considering linear and non-linear behavior of ground and compares the variation of end moments considering non interaction analysis in STAADPRO and Interaction analysis in ANSYS’11. Keywords: Displacement, End Moments, Soil-Structure Interaction, Stress Intensities 1. Introduction When forces are applied externally to the structure, internal forces develop and both components must deform and move in a compatible manner. This is because neither the displacements of the structure nor the ground displacements are independent of each other as a result of their physical contact. Because of this mutual dependence of the structure and soil behavior, these types of problems are broadly referred to as soil-structure interaction (SSI) problems. Because of this mutual dependence, which is termed as interaction, the stress resultants in structure and, stresses and strains in soil are significantly altered during the course of loading. Therefore it becomes imperative to consider the structure-foundation and soil as components of a single system for analysis and design of the structure and its foundation. The analysis that treats structure foundation- soil as a single system is called as Soil Structure Interaction (SSI) analysis [2, 3]. 1.1 Classification of SSI based on treatment of interactions Based on the treatment of interactions of sub-domains in a system SSI analyses may be classified as 1) Monolithic or direct approach or domain, 2) Substructure approach.  Monolithic or direct approach or domain: In the direct method the soil, structure and foundation is modeled together using finite element method (FEM) and analyzed in single step.  Sub-Structure Method: The soil/foundation medium and the structure are represented as two independent mathematical models or substructures. The connection between them is provided by interaction forces of equal amplitude, acting in opposite directions of the two sub-structures [4, 5, 6, 7]. 1.2

Static soil structure interaction Linear static analysis is concerned with the linear behavior of the elastic continua under prescribed boundary conditions and statically applied loads. The analysis has been carried out on a regular multistorey frame in the present study. Different combinations of dead load and imposed load as per the relevant code provisions have been considered and the critical values of stresses and displacements are evaluated. Finite

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Importance of Soil Structure Interaction for Framed Structure element models have been formulated for soil. The analysis has been repeated with and without considering the subsoil to study the soil structure interaction effects. The details of the analysis are described subsequently. SSI studies that take into account the yielding of structures and soil non –linearity are scarce, especially investigating the effects of non-linearity of SSI system on overall behaviour in terms of displacements and stresses [2, 3]. 1.3 Literature review A lot of investigations have taken place in the area of soil-structure interaction of framed structures. Various investigators have proposed different approaches for solution of interaction problems from time to time in attempt to obtain more realistic analysis. They have quantified the effect of interaction behaviour and established that there is redistribution of forces in the frame members. F. Jancy, A. Rajagopal, B. Umashankar, M. R. Madhav; Finite Element Modeling of Ground Structure Interaction Considering Non-Linear Response of the Ground (2011). This work focuses on the computational modeling of ground-structure interaction using finite element package ANSYS. To demonstrate the behavior of structure while considering actual nature of ground response [1]. Vivek Garg and M.S. Hora; Interaction Effect of Space Frame-Strap Footing-Soil System on Forces in Superstructure (2012). In the present work, the interaction analysis of a three-bay three-storey RCC space frame- footing-strap beam-soil system is carried out to investigate the interaction behavior using the finite element method. The frame, foundation and supporting soil mass are considered to be linear elastic and to act as a single compatible structural unit for more realistic analysis [6]. B. Pallavi Ravishankar and Dr. Neelima Satyam; Numerical Modelling To Study Soil Structure Interaction for Tall Asymmetrical Building (2013). In this paper asymmetrical high rise building modelled along with the homogenous sandy soil strata. The building is provided with two different type of foundation systems viz. Raft foundation and pile foundation and interaction parameters like displacements and stresses are studied at different points under consideration [8]. Dr. D. Daniel Thangaraj and Dr.K. Ilamparuthi; Interaction Analysis of MAT Foundation and Space Frame for the Non Linear Behaviour of the Soil (2012). In this paper a framed structure of 3 bay x 5 bay supported on a foundation system known as mat is considered to evaluate the influence of thickness of mat and nonlinear behavior of the soil on forces and deformation of the frame. Based on the analyses of the soil- matspace frame system, the settlement is higher for the non-linear soil condition [9]. H. J. Puttabasavegowda, Karisiddappa and K. C. Krishna; Linear Interactive and Non-Linear Interactive Analysis of Plane Frame Combined Footing Soil Systems - A Parametric Study (2012). In this paper the finite element analysis has been carried out considering the effect of soil structure interaction on two dimensional frame structure resting on combined footing subjected to pseudo static wind load. It may be concluded that the maximum settlement is observed at one edge element of the footing. This can be attributed to the corresponding footing loads being high [10]. Dr. D. Daniel Thangaraj Dr. K. Ilamparuthi; Parametric Study on the Performance of Raft Foundation with Interaction of Frame (2010). This paper examines the identification of the parameters which affect the interaction and the parameters are grouped into relative stiffness factors krs and ksb. An interaction analysis is performed for the fivebays, using ANSYS finite element code. The soil was treated as an isotropic, homogenous and elastic half space medium and the following conclusions are drawn from this analysis that the differential settlements reduced due to interaction and the performance of the raft depends on ksb values [11]. The primary objective of this work is to show the importance of SSI for framed structures and to study the SSI behaviour of structures on isolated footings which are subjected to service loads. This work has been motivated by the lack of sophisticated monolithic tools for modeling soil–structure interaction problems for static especially in regular structures. Direct method is used as a main approach for the evaluation of interactive soil–foundation–structure systems. Numerical investigations have been carried out on a three bay, three storeyed regular multistorey frames. Three types of analyses have been carried out i.e. Non-interaction analysis (NIA), Interaction analysis considering both structure and soil elastic (IAE) and Interaction analysis considering soil as elasto-plastic and frame-footing as elastic (IAEP). Each above mention analysis is divided into three sub

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Importance of Soil Structure Interaction for Framed Structure categories i.e. displacements analysis, stress intensities analysis and end moment analysis. Thus all result findings are compared.

2. Validation of Results The results obtained by the ANSYS software are validated with the results already available in the literature (F. Jancy, A. Rajagopal, B. Umashankar, and M. R. Madhav 2011, Kochi). They demonstrate the behavior of structure while considering actual nature of ground response, a simple portal frame is analyzed. Portal frame is modeled as linear elastic, whereas the ground is modeled linear elastic. The study gives insight into variation of displacement of portal frame while considering linear and non-linear behavior of ground. The geometry, material properties, loading and element discretization of the model is given below. A single-storey portal frame resting on ground as shown in Figure 1 is considered for ground-structure interaction analysis. The following cases are modeled and analyzed: a) Portal frame fixed at the base, not considering ground response and frame modeled as linear elastic (Case 1) b) Portal frame resting on ground, both frame and ground are assumed as linear elastic (Case 2) [1] Table 1: Properties Used in Modeling

Figure 1: Schematic Representation of Single-Story Portal Frame Resting on Ground

Property(Soil) Cohesion Friction angle Dilation angle Young‟s Modulus Poisson‟s ratio Saturated unit weight Property (frame) Grade concrete Young‟s Modulus Poisson‟s ratio

Values 0 36 degree 12 degree 10 Mpa 0.3 18 kN/m3 Values M25 25000Mpa 0.15

The finite element program ANSYS version 11.0 is used in this study. All the cases are analyzed as 2D problems for a given uniformly distributed load acting on frame and the displacements are compared. Table 2: Comparison of Total Displacement (mm) for Case 1 and Case 2 Cases Case 1 Case 2

Validated Results 4.871 mm 7.133 mm

(F. Jancy, A. Rajagopal, B. Umashankar, and M. R. Madhav 2011, Kochi)’s Results 5.782 mm 6.86 mm

Thus from Table 2, it is clear that results are matched and further study can be made.

3. Problem under Consideration In present problem a 3 bay x 3 bay three-storey RCC space frame resting on homogeneous soil mass and subjected to gravity loading is analyzed. The problem under consideration is symmetric about both axes in terms of geometry, material properties and loading. The superstructure of proposed model is depicted in Figure 3.

Figure 3: Plan Showing Columns and Footings

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Importance of Soil Structure Interaction for Framed Structure To investigate the interaction behavior, the interaction analyses are carried out for the following three cases. Case-1: The conventional non-interaction analysis (NIA) considering the columns fixed at their bases. Case-2: The interaction analysis of space frame isolated footing-soil system considering the columns supported on individual column footings and resting on soil media (Elastic Analysis). (IAE) Case-3: The interaction analysis of space frame isolated footing-soil system considering the columns supported on individual column footings and resting on soil media (Elasto-plastic Analysis). (IAEP) where, NIA - Non-Interaction Analysis, IAE - Interaction Analysis Elastic, IAEP - Interaction Analysis Elasto-Plastic The frame, foundation and supporting soil mass are considered to be elastic and to act as a single compatible structural unit for more realistic analysis. The geometric and material properties of proposed model are given in Table 3 where as loading is given in Table 4 and elevation of Frame 1, Frame 2, Frame 3, Frame 4 and plan of all beams are shown in Figure 4.

(a) (b) (c) (d) Figure 4: (a) Elevation of Frame 1 & 4 (b) Frame 2 & 3 (c) Plan Showing Ground Beams & Plinth Beams (d) Plan Showing Slab Beams for First, Second and Terrace Floor Table 3: Geometric and Material Properties of 3 Storey Frame, Footing and Soil Mass Component

Frame

Isolated Footing

Soil Mass (Medium hard clay)

Description Number of bays in X direction Number of bays in Y direction Floor to floor height Plinth height Bay width in X direction Bay width in Y direction Beam dimensions Columns C1,C13,C4,C16(A) Columns C2,C14,C3,C15,C5, C9,C8,C12 (B) Columns C6,C10,C7,C11(C) Thickness of all slabs Footing under Columns C1,C13,C4,C16(F1) Footing under Columns C2,C14,C3,C15,C5,C9,C8,C12(F2) Footing under Columns C6,C10,C7,C11(F3) Modulus of elasticity of footing for(M20) Poisson‟s ratio of concrete Density of RCC Acceleration due to gravity (g) Extent of Soil Mass Modulus of elasticity of soil Poisson‟s ratio of soil Safe Bearing Capacity (SBC) Density Cohesion Frictional angle

Data 3 3 3.0 m 1.0 m 4.0 m 4.0 m (0.23 x 0.3) m (0.23 x 0.3) m (0.38 x 0.23) m (0.23 x 0.45) m 0.15 m 1.5m x 1.5m x 0.35m 1.8m x 1.8m x 0.4m 2.2m x 2.2m x 0.5m 22360 x 106 N/m2 0.15 2500 kg/m3 9.81 m/s2 30.0m x 15.0m 100 x 106 N/m2 0.2 200 kN/m2 2000 kg/m3 50 x 109 N/m2 15 degree

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Importance of Soil Structure Interaction for Framed Structure Table 4: Factored UDL Intensities on Beams Structural Component U.D.L. Intensities (N/m) Ground Beams 25530 Plinth beams 9315 First and Second floor Outer slab beams 34630 Inner slab beams 41315 Terrace floor Outer slab beams 22900 Inner slab beams 38900 Loads are calculated by usual way i.e. for slab Live load = 3kN/m2, Floor finish = 1.25kN/m2, and after this factored it and distributed on beams as per IS 456-2000. For beams, the loads of walls are calculated and factored it [12]. As far as dead load is considered, Ansys‟11 calculated dead weight of members from mass density and acceleration due to gravity hence these properties are required while assigning. Table 4 shows factored UDL intensities on all beams. 3.1 Finite element modeling The interaction analysis of the problem is carried out using ANSYS software (Version 11). The frame structure, footing and soil mass is discretized with 8 noded plane stress element (PLANE 82) for case 1 and case 2, with two degree of freedom per node (Ux and Uy). It is assumed that the joints between various members are perfectly rigid. The vertical displacements in soil mass are restrained at the bottom boundary whereas horizontal displacements are restrained at vertical boundaries. The soil mass is idealized as isotropic, homogeneous, halfspace model. For Case 3, elastic-plastic behavior of the ground is modeled using Drucker-Prager criterion in ANSYS‟11[13, 14]. The interface characteristics between the isolated footing and soil are represented by TARGE169 and CONTA172 elements. The element size for frame and footings are taken as 0.1m x 0.1m. The soil mass is discretized with 1m x 1m finer meshes in close vicinity of footing where stresses are of higher order. As Plane 82 is plane stress element thus it used to find displacement as well as stress intensity but it is not used for finding out rotation and hence moments in frame can‟t be found out. So for finding out moments in the frame, frame is modeled with 2D Elastic BEAM 3 element having two nodes and three degree of freedom per node (Ux,Uy and θz) [15].

4. Interaction Analysis 4.1 Displacement analysis As we know that when the supports are fixed we consider deflection of beams but i.e. basically the part of structure displace from original place. In case of SSI analysis we cannot judge the fixity of support thus support may be fixed, pinned, or something else. Thus in SSI structure deflects as well as settle and we call collectively them as displacements in members. Thus displacements are compared for NIA, IAE and IAEP respectively and presented in bar chart as show in Figure 5(a) and (b).

Figure 5(a): Comparison of Displacement In Beams of Ground and First Floor

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Importance of Soil Structure Interaction for Framed Structure

Figure 5(b): Comparison of Displacement in Beams of Second and Terrace Floor From above bar charts it is clear that displacement goes on increasing from NIA to IAE and IAE to IAEP. Thus all the analysis cases are important. If we compare the allowable settlement with the given results then as per IS 1904-1986 allowable settlement for isolated footing is limited to 50 mm and results shows maximum displacement of 39.72mm [16]. Here the maximum displacement value is less than allowable hence question may rise that is SSI really important?????? Therefore 10 storey building is considered having column and beam sizes as per loading and footing is raft slab as area of isolated footing is more than 50% total area hence raft footing is adopted [17]. Properties of soil mass are same as above example. Thus displacements are checked with allowable as shown in Figure 6 and 7. Thus from Figure 7 displacement of frame is 118 mm which is more than allowable (75 mm for raft footing on hard clays). Hence SSI is important for frame structure.

Figure 6: Meshing of 10 Storey Frame

Figure 7: Displacement of Inner Frame for IAEP

4.2 Stress analysis As far as stress intensities due to bending and settlement are concern, stresses are increasing rapidly from NIA to IAE but slight change for IAEP. This is because soil offer resistance in IAE than IAEP. Also in IAE displacement is due to deflection and settlement both but in IAEP displacement is due to settlement only as soil is plastic. Hence stresses are nearly same for IAE and IAEP. Following bar charts in Figure 8, 9 and 10 shows the exact behavior of stresses in beams, columns and footings respectively.

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Importance of Soil Structure Interaction for Framed Structure

Figure 8: Comparison of Stresses in Beam

Figure 9: Comparison of Stresses in Columns

Figure 10: Comparison of Stresses in Footings “Therefore displacements in IAEP and stresses in IAE are important from design consideration”.

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Importance of Soil Structure Interaction for Framed Structure 4.3 End moments analysis In this section end moments in frame from STAAD-PRO (fixed supported frame) and ANSYS (IAE) are compared. Table 5 shows the values of redistributed end moments in some of critical beams and columns of frame-footing-soil system due to STAAD-PRO (NIA) and ANSYS (IAE). The comparison of end moments due to NIA and IAE reveals that the interaction effect causes redistribution of the moments in beam as well as column members. The insignificant interaction effect is found in ends moment of inner beams. IAE provides variation of -100 to 26% in end moments compared to NIA. The maximum decrease of nearly 42% is found in the outer end of terrace floor beam (member SB3) whereas the maximum increase of nearly 26% is found in the inner terrace beam (member SB4). The reversal in the sign of end moments is found in column (C). Table 5: Comparison of End Moments Mz (kN-m) in Beams End Moments From Sr. No. Member (Beams) STAAD-PRO(NIA)(kNm) 1 GB4 30.105 -38.48 First floor 2 SB1 33.263 -53.932 3 SB3 53.932 -33.263 4 SB4 51.499 -58.498 5 SB6 58.498 -51.499 6 SB10 42.466 -62.787 7 SB12 62.787 -42.466 Second floor 8 SB1 36.545 -52.533 9 SB4 54.961 -57.208 10 SB6 57.208 -54.961 Third floor 11 SB1 19.155 -36.863 12 SB3 36.863 -19.155 13 SB4 43.598 -56.36 14 SB6 56.36 -43.598 15 SB10 32.325 -61.683 16 SB12 61.683 -32.325 Columns 17 C6,C10,C7,C11(C) 2.869 6.866

End Moments From ANSYS (IAE) (kNm) 33.62 -34.36 26.95 58.53 30.75 68.03 42.11 64.31

-58.53 -26.96 -68.03 -30.76 -64.31 -42.11

31.51 35.98 65.66

-56.44 -65.66 -35.98

7.98 45.55 14.58 71.47 29.97 66.79

-45.55 -7.98 -71.47 -14.58 -66.79 -29.97

-7.23

-7.6

5. Conclusions 1.

2.

Comparison have been done over NIA, IAE and IAEP for displacements in frame and it has been observe that,  In NIA displacement is purely a deflection of beams and in IAE displacement is due to the settlement as well as deflection. Whereas in case of IAEP displacement is majorly due to settlement and very small due to deflection this proves in stress analysis.  Also as per IS 1904-1978 allowable settlement for isolated footing is limited to 50 mm and results shows maximum displacement of 39.72mm. So for this, the 10 storey building model is analyse for raft footing and maximum displacement of 118mm was found out which is more than allowable settlement of 75mm.  It is found that displacement increases as no. of storyes increases.  Thus it is proved that for high rise RCC frame structure the SSI analysis must be done. Comparison have been done over NIA, IAE and IAEP for stress intensities in frame and footing and it has been observe that,  Stresses increases majorly from NIA to IAE but slight change from IAE to IAEP.

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Importance of Soil Structure Interaction for Framed Structure  3. 4.

5.

6. 7. 8. 9.

Due to soil stiffness in IAE stress are induced in frame and in IAEP stiffness decreases and soil offer only settlement to frame hence nearly same stress are present. Therefore displacements in IAEP and stresses in IAE are important from design consideration. This will also save computer memory Comparison of ends moments (NIA in STAAD-PRO and IAE in ANSYS‟11) shows that,  The comparison of end moments due to NIA and IAE reveals that the interaction effect causes redistribution of the moments in beam as well as column members.  IAE provides variation of -100 to 26% in end moments compared to NIA.  The maximum decrease of nearly 42% is found in the outer end of terrace floor beam (member SB3).  The maximum increase of nearly 26% is found in the inner terrace beam (member SB4).  The reversal in the sign of end moments is found in column (C). The study also gives some important note that for stress analysis PLANE 82 element is used and for end moment analysis BEAM 3 element is used.. Thus this shows that for different analysis different elements are used. For plastic analysis Drucker-Prager criterion in ANSYS‟11is used. For which the cohesion value must > 0 thus cohesion less soil mass is not analyses in ANSYS11. From whole study it is indirectly observed that modulus of elasticity for soil is most important parameter. Settlement of footing can be calculated thus type of footing is decided The present study is useful for designing the structure for SSI effect.

References [1] [2]

[3] [4] [5] [6] [7] [8] [9]

[10]

[11] [12] [13] [14] [15] [16]

F. Jancy, A. Rajagopal, B. Umashankar, M. R. Madhav, „Finite Element Modeling of Ground - Structure Interaction Considering NonLinear Response of the Ground‟, Proceedings of Indian Geotechnical Conference December 15-17, 2011, Kochi pp. - 281 H.M. Rajashekhar Swamy, „Non-linear Dynamic analysis of Soil Structure Interaction of Three Dimensional Structure for Varied Soil conditions’, Doctor of Philosophy In Civil Engineering Thesis, Department of Civil Engineering, Manipal Institute of Technology Manipal April-2012 Deepa Balakiiishnan S., ‘Numerical Investigations On Soil Structure Interaction of Multistory Frames’, Doctor of Philosophy Thesis, Division of Civil Engineering School of Engineering Cochin University of Science & Technology Cochin- 682022 January 2008 R. R. Chaudhari, Dr K. N. Kadam, „Effect Of Piled Raft Design On High-Rise Building Considering Soil Structure Interaction‟, International Journal Of Scientific & Technology Research, ISSN 2277-8616, Volume 2, Issue 6, June 2013 H.S. Chore, R.K. Ingle and V.A. Sawant, „Building frame - pile foundation - soil interaction analysis: a parametric study‟, Interaction and Multiscale Mechanics, Vol. 3, No. 1 (2010) pp. 55-79 Vivek Garg and M.S. Hora , „Interaction Effect of Space Frame-Strap Footing-Soil System on Forces in Superstructure‟, ARPN Journal of Engineering and Applied Sciences, ISSN 1819-6608, VOL. 7, NO. 11, November 2012 Siddharth G. Shah, Solanki C.H., Desai J.A., „Soil structure interaction analysis methods - State of art-Review‟, International Journal of Civil and Structural Engineering, ISSN 0976 – 4399, Volume 2, No 1, 2011 Pallavi Ravishankar, Dr D Neelima Satyam, „Numerical Modeling to Study Soil Structure Interaction For Tall Asymmetrical Building‟, International Conference on Earthquake Geotechnical Engineering Istanbul, Turkey, Report No: IIIT/TR/2013/-1. Dr.D. Daniel Thangaraj and Dr.K. Ilamparuthi, „Interaction Analysis of MAT Foundation and Space Frame for the Non Linear Behaviour of the Soil‟, Bonfring International Journal of Industrial Engineering and Management Science, Vol. 2, No. 4, December 2012 H. J. Puttabasavegowda ,Karisiddappa and K. C. Krishna, „Linear Interactive and Non-Linear Interactive Analysis of Plane Frame Combined Footing Soil Systems - A Parametric Study‟, International Journal of Earth Sciences and Engineering, ISSN 0974-5904, Volume 05, No. 02, April 2012, pp. 341-346 Dr. D. Daniel Thangaraj, Dr. K.Ilamparuthi, ‘Parametric Study on the Performance of Raft Foundation with Interaction of Frame‟, EJGE, Vol. 15 [2010] IS 456 – 2000, ‘Indian standard Criteria for Plain and Reinforce Structure’, (ICS 91.100.30 New Delhi) S. S. Bhavikatti, „Finite Element Analysis’, (2nd edition, New Age International Publishers, New Delhi, 2012) C. S. Krishnamoorty, „Finite Element Analysis’, (2nd edition, Mc Graw Hill, New Delhi, 2010) Madenci E, Guven I, „The Finite Element Method and Applications in Engineering’, ISBN: 978-0-387-28289-3, www.springer.com, 2006. IS 1904 – 1986, ‘Indian standard Code of Practice for Design and Construction of Foundations in Soils: General Requirements’, (UDC 624‟15‟04: 006‟76 New Delhi)

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Importance of Soil Structure Interaction for Framed Structure [17] R. M. Jenifer Priyanka, N. Anand , Dr. S. Justin, „Studies on Soil Structure Interaction of Multi Storied Buildings with Rigid and Flexible Foundation‟, International Journal of Emerging Technology and Advanced Engineering, ISSN 2250-2459, Volume 2, Issue 12, December 2012

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