INFLUENCE OF BNWF SOIL MODELLING ON DYNAMIC BEHAVIOUR OF PILE FOUNDATION FOR RC FRAME WITH STRUCTURAL WALL

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1 ICOVP, 3 th International Conference on Vibration Problems 29 th November 2 nd December, 27, Indian Institute of Technology Guwahati, INDIA INFLUENCE OF BNWF SOIL MODELLING ON DYNAMIC BEHAVIOUR OF PILE FOUNDATION FOR RC FRAME WITH STRUCTURAL WALL A. SINHA PR, N. SHARMA, K. DASGUPTA CR, A. DEY Department of Civil Engineering, Indian Institute of Technology Guwahati, 7839, Guwahati, Assam, India anantasinha6@gmail.com, nsharmanishant@gmail.com, arindamdeyiitg6@gmail.com, kd@iitg.ac.in Abstract. Multistoried Reinforced Concrete (RC) wall-frame buildings are nowadays frequently constructed in severe seismic zone due to their large lateral stiffness and lateral strength. Conventionally, the effect of soilstructure interaction (SSI) on the dynamic behavior of a wall-frame structure is not considered in its seismic design. As a result, the considered system is stiffer as compared to the actual condition in which the pile-soil system imparts more flexibility and can induce nonlinear behaviour of piles through large deformations. Therefore, it becomes imperative to study the nonlinear behaviour of a wall-frame-pile-soil system. In this study, two different soil domains are created for influence of soil-structure interaction on the response of pile foundation, namely (i) the first model incorporating soil-pile interaction effect and (ii) the second model without considering the interaction effect. The response from two different soil domains are applied on a twodimensional RC wall-frame. The soil-pile system for the RC building frame is modelled as Beam on a Nonlinear Winkler Foundation (BNWF). It is observed that after the application of ground motion, significant bending moment is generated near the bottom of the pile and nonlinearity extends beyond 5% of the pile length. The observations suggest the necessity for further assessment of the state of the practice for simulating seismic soil-pile interaction. Keywords. Soil-Structure Interaction, BNWF approach, Dynamic P-y curves, continuum model.. Introduction Reinforced Concrete (RC) frame building with structural walls or shear walls constitutes a large fraction of the highrise building stock constructed in severe seismic zones. Due to the large lateral strength and stiffness of the shear wall, these buildings show better seismic behaviour as compared to the conventional RC frame buildings. Although a number of studies have been carried out in the past on the behaviour of the superstructure of wallframed buildings by Yasushi et al. [], Inoue et al. [2] and Zang [3], the seismic behaviour of such building with pile foundation has not been studied extensively. Soil-structure interaction plays a great role on the overall response of a highrise building during any earthquake event. It imparts flexibility to a building which reduces the overall stiffness of the multistoried building. Also, during strong earthquake shaking, nonlinear behaviour may occur in piles also. So the study of the nonlinear behaviour of pile becomes imperative. There are two approaches of seismic analysis to represent the soil-pile soil interaction effect... Beam on a Nonlinear Winkler Foundation (BNWF) BNWF method is a very simple method of representing the soil-pile interaction effect and requires less computational effort. Having some advantages of predicting the soil stiffness

2 2 Sinha, Sharma, Dasgupta, Dey under dynamic as well as static loading, it also has several disadvantages. Shear transfer between the layers of soil is ignored and the subgrade reaction at a particular depth is only governed by the lateral displacement of the pile at that depth only. The spatial effect of lateral displacement of the pile on the overall soil-pile system is ignored..2. Continuum Modelling The modelling of soil as a continuum facilitates coupled analysis of soil-pile-superstructure system. Although the method requires extensive computations, this method has been used by several researchers to obtain a more realistic insight into the behaviour of the soil domain. However, it has still not been widely accepted as a practical tool. In this paper, the BNWF approach is used to reflect the soil stiffness under dynamic loading. Two different soil domains are considered for the response of pile foundation, namely (i) the first model incorporating soil-pile interaction effect and (ii) the second model without considering the interaction effect (i.e., the free field response). Ground response analysis is performed to estimate the dynamic soil response at different depths of soils and the same is given as input to the pile at those corresponding depths. The structural nonlinearity is modelled by assigning the user-defined lumped plastic hinges at appropriate locations. The extent of nonlinearity and the maximum depth of occurrence is compared with the possible depth proposed by past researchers. 2. Modelling and Inputs In this study, an eight-storied RC wall-frame building with symmetric floor plan (Figure 3.) has been considered with the location in Guwahati city of seismic zone V of Indian code IS:893 (Part I) [4]. The shear wall is modelled as an equivalent frame element having the same cross-sectional properties as the original wall. The building is modelled using the computer program SAP2 [8] and designed as per the Indian Standards IS: 456 [5], IS:893 (Part I) [4] and IS: 29 (Part I) [6]. The grades of concrete and steel reinforcement are considered as M3 and Fe5 respectively. Bay Width m Storey Height m Beam Section mm mm Column Section mm mm Table.. Building details Shear wall Thickness mm Under Column mm Pile Diameter Under Shear wall mm Pile Length Under Column m Under Shear wall m The soil medium is considered as dry loose sand. The API [7] guidelines are followed to calculate the soil stiffness which is based on the BNWF approach. The soil is represented by the nonlinear springs located at an interval of.5 m throughout the pile length (API [7]). The angle of internal friction, angle of wall friction and the unit weight of the soils are considered as 3, 22.5 and 6 kn/m 2 respectively. For the springs, the axial force-displacement relationship is represented by P-y springs, where P is the axial force and y is the axial deformation of the spring. Pu is the ultimate lateral load bearing capacity. The pile skin friction is represented by the t-z curves where t is the skin friction force and z is the vertical

3 Mobilized End Bearing Capacity,Q (kn/m 2 ) P u value (kn/m) Skin Friction,T (kn) Influence of BNWF approach of Soil Spring Model on the Dynamic Behaviour of Pile Foundation 3 displacement of the pile. The end bearing behaviour of the pile is represented by Q-z curve, where Q is the tip resistance of pile. Typical P-y, t-z and the Q-z curves are shown in Figures, and (c) respectively Depth 4m Depth 2m Lateral Deflection (y), m depth m -3 depth 4m z, (m) z/d Dia.5 m Dia.6 m (c) Figure.. P-y curves of.5m dia pile, t-z curves of.5m dia pile, (c) Q-z curves for.5m and.6m diameter piles To observe the occurrence of plasticity in frame elements during dynamic loading, user defined lumped plastic hinges are assigned in computer program SAP2. The plastic hinge lengths are calculated for the superstructure by the different empirical equations proposed by Park and Paulay [9] and by Paulay and Priestley []. The hinge properties of piles are calculated by the Goel [] and Saeedy [2]. The typical moment-curvature relation obtained for frame sections is shown in Figure.2.

4 Moment (kn) Moment/Yield moment Moment/Yield moment Moment/Yield moment Moment/Yield moment 4 Sinha, Sharma, Dasgupta, Dey Curvature (/m) Curvature (/m) Curvature (/m).5 Curvature (/m) (c) (d) Curvature (/m) Figure.2. Moment-curvature curves for Pile of diameter.5m Pile of diameter.6m (c) Shear Wall (d) Column (e) Beam sections (CSI, 2) (e) 2.. Application of Ground Motion In the present study, Nonlinear Time History Analysis (NLTHA) is performed on the framepile-soil model by applying displacement time-histories obtained at various depths of pile

5 Influence of BNWF approach of Soil Spring Model on the Dynamic Behaviour of Pile Foundation 5 through ground response analysis in the entire soil domain. The displacement time histories are applied at the ends of the soil springs for carrying out NLTHA. The current study aims to check the extent of nonlinearity in piles under the applied motions. As nonlinear behaviour gets mobilized more under strong ground motion (with high PGA), one of the recorded motions during the 995 Kobe earthquake is chosen for the study. In the present model, the 2m long pile is divided into 4 segments with the length of each segment as.5m segment. At different depths of the piles, different ground response is obtained due to variations in shear wave velocity. All the nodes of the piles, with soil interface, are connected with the p- y spring, q-z spring and t-z springs. One end of a spring is connected to a pile node and the other end is restrained against all the possible translations and rotations. For obtaining the ground response at different depths of the soil, two different models are created in OpenSEES [3]. In the first model (Case I), the soil springs in the SAP2 model and the soil domain in the OpenSEES model are assigned the same soil parameters. In the OpenSEES model (Case I), a continuum soil medium is modelled having pile embedded inside the medium. In this case at each pile node, a recorder has been used to record the ground response from the OpenSEES model. The domain of soil medium is taken as 8m (length) 4m (depth). The width of the domain is considered as 2D, where D is the overall width of the structure and the depth is taken as twice the length of the pile, which is 4m (Sharma et al., 25) [4]. The Kobe earthquake acceleration record has been applied as an input at the base of the OpenSEES model. After analysis, the different relative ground displacement time histories at different levels are obtained. A representative diagram of Case I is shown in Figure.3. In the second model (Case II), the continuum soil medium is modelled in OpenSEES without considering the presence of the piles (Figure.3) The ground response obtained in Case II is known as the free field ground response. The obtained displacement time histories records at every depth of the pile from the two different domains are applied at the restrained spring ends in the SAP2 model. Finally the two models, one considering the soil-pile-soil interaction and another is without considering the interaction effect into the input time history data, are analyzed in SAP2. Figure.3. Representative diagram of the SSI model in OpenSEES (Mazzoni et al., 26) for Case I and Case II 2.2. Ground Response Analysis Results From the ground response analysis, the ground motions are extracted at different depths of soil. In the present study, the ground response are obtained for two different conditions. In one model, the domain is incorporated with pile-soil-pile interaction and in another model response are obtained as a free field domain analysis. The difference in responses at the various depths are shown in Figure.4. The ground response are applied to the building frame

6 Displacement (m) 6 Sinha, Sharma, Dasgupta, Dey at the spring ends at the various levels as input ground motion. The application of earthquake motion in the link elements is shown in Figure Case I Case II Time (sec) Figure.4. Comparison of obtained ground response of two models of P3 pile at.5 m depth Figure.5. Application of earthquake motion at the ends of horizontal soil springs 3. Results and Discussions The main objective of this study is to observe the pattern of hinge formation in the pile, the bending moment profile of pile and the deformed shape of the entire structure during possible earthquake shaking. After the analysis of Case I and Case II in computer program SAP2, the differences in bending moment profile and the hinge formation are also compared in this section. 3.. Results of Case I In the first Case (Case I), where the soil domain incorporates the pile-soil-pile interaction effect, the ground response are applied at different elevation and the model is analyzed. After the analysis completed, it is observed that plastic hinges are formed over almost half the length of the piles under the shear wall. The beginning of hinge formation in piles is observed at the time instant of.38 sec at junction of the pile and the pile cap. After that hinges start

7 Influence of BNWF approach of Soil Spring Model on the Dynamic Behaviour of Pile Foundation 7 forming throughout the pile length. The soil attains permanent deformation beyond some particular time instant which further leads to almost constant bending moment for the rest of the duration of the applied ground motion. The hinges are formed till the depth of 7m at the pile under shear wall and 8.5m at the end pile of the building model which is more than the active depth mentioned for pile design. The hinge formation and the bending moment profiles are shown in Figure.6 and Figure.7 at various steps of analysis. The spring force relative displacement curves of multi-linear plastic springs are also obtained for the framepile-soil model at the various levels. One sample spring force relative displacement curve is shown in Figure.8. Figure.6. First hinge formation (.38 sec) and hinge formation in the last step of NLTHA (26 sec) for Case II Figure.7. Bending moment profiles at time instants of 2.2 sec and 26 sec for Case I

8 Spring Force (kn) 8 Sinha, Sharma, Dasgupta, Dey Seismic Response -4 Backbone Relative Spring Compression (m) Figure.8. Variation of spring force with relative spring deformation for pile P4 at.5m depth in Case I 3.2. Results of Case II For Case II, while modelling the soil domain, the pile-soil-pile interaction effect has not been considered to extract the ground response at the various soil depths. The output results, obtained from ground response analysis are applied to the building frame. The generated ground motions from Case I and Case II are compared in Section 2.2. Significant changes are noticed in the ground response values at the various soil depths. After the application of ground motion to the frame-pile-soil model in SAP2, the changes in plastic hinge formation are also noticed (Figure.9). Figure.9. First hinge formation (7.48 sec) and hinge formation in the last step of NLTHA (26 sec) for Case II

9 Spring Force (kn) Influence of BNWF approach of Soil Spring Model on the Dynamic Behaviour of Pile Foundation 9 Figure.. Bending moment profiles at the time instants of 8.56 sec and 26 sec for Case II In this model, plastic hinges are formed to a greater depth along with generation of significant bending moment (Figure.). At the bottom part of the piles, unlike the previous model, no such bending moment is observed. Many hinges at piles reach the collapse stage in this model. Below the shear wall, hinges are formed till the depth of 7m from the ground level which is almost 35% of the pile length and under the column pile cap, hinges are formed up to a depth of 6m which is 6% of the pile length. Unlike the previous model, the permanent deformation of soil is observed near the top surface of soil medium. The spring response at various depth are extracted and a sample curve is shown in Figure Seismic Response -4 Backbone Relative Spring Deformation (m) Figure.. Variation of spring force with relative spring deformation for pile P4 at.5m depth for Case II

10 Sinha, Sharma, Dasgupta, Dey Conclusions Based on the present study, the following salient conclusions are drawn: From NLTHA, it is concluded that the concept of the active length of pile may not be valid for dynamic behaviour of the piles. Consideration of nonlinear pile-soil interaction shows the possibility of nonlinear behaviour at the bottom of the pile. Thus, code-prescribed reinforcement detailing needs to consider the mentioned nonlinear behaviour. The ground response results of a soil domain in presence of embedded piles differ significantly from the free field analysis results References [] Yasushi S., Toshimi K., Yoshiaki N., Analyses of Reinforced Concrete Wall-Frame Structural Systems Considering Shear Softening of Shear Wall, 3th World Conference on Earthquake Engineering Vancouver, B.C., Canada, 24 [2] Inoue N., Yang K., Shibata A., Dynamic Non-Linear Analysis of Reinforced Concrete Shear Wall by Finite Element Method with Explicit Analytical Procedure, Earthquake Engineering & Structural Dynamics, John Wiley & Sons, Ltd., 26, , 997 [3] Zhang L., Nonlinear Analysis of Laterally Loaded Rigid Piles in Cohesionless Soil, Journal of Computers and Geotechnics, 36, , 29 [4] Bureau of Indian Standard (BIS 22), Criteria for Earthquake Resistant Design of Structures Part : General Provision and Buildings, IS: 893 (Part ), Bureau of Indian Standards, New Delhi, 22. [5] Bureau of Indian Standard (BIS 2), Plain and Reinforced Concrete-Code of Practice, IS 456, New Delhi, India, 27. [6] Bureau of Indian Standard (BIS 2), Design and Construction of Pile Foundation-Code of Practice (Part-): Concrete Piles, IS 29, New Delhi, India, 2. [7] American Petroleum Institute (API), Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms-Working Stress Design, Washington, D.C., 2. [8] SAP2 CSI., Computers and Structures Inc. Berkeley, CA, USA 23. [9] Park R., Paulay T., Reinforced Concrete Structures, John Wiley & Sons, 975. [] Pauley T., Priestley M. J. N., Seismic Design of Reinforced Concrete and Masonry Structures, J. J. Wiley & Sons, INC USA, 992. [] Goel, Rakesh K., Evaluation of In-Ground Plastic-Hinge Length and Depth for Piles in Marine Oil Terminals, Journal of Earthquake Spectra, 3, , 25. [2] Saeedy NE., Plastic Hinge Length and Depth for Piles in Marine Oil Terminals Including Nonlinear Soil Properties, Proceedings of 5th World Conference on Earthquake Engineering, 25, 22. [3] Mazzoni S., McKenna F., Scott M. H., Fenves G. L., The Open System for Earthquake Engineering Simulation (OpenSEES) User Command-Language Manual, 26. [4] Sharma N., Dasgupta K., Dey A., Finite Element Modeling Intricacies for SSI Studies, Sixth International Conference on Recent Advances in Geotechnical Earthquake Engineering and Soil Dynamics, 26.