A Dynamic Behavioural Study of Structure and Foundation for 25 Storey Structure with Variable Sub-Soils by Time History FEM Model

Transcription

1 A Dynamic Behavioural Study of Structure and Foundation for 25 Storey Structure with Variable Sub-Soils by Time History FEM Model Shruti. Shukla, Dr.Atul Desai, and Dr.Chandresh Solanki Abstract A piled raft foundation is a combination of a shallow foundation and a deep foundation with the best characteristics of each of its components. The piled raft foundation is a composite construction consisting of three bearing elements, piles, raft, and subsoil. In this foundation, the piles usually are not required to ensure the overall stability of the foundation but to reduce the magnitude of settlements, differential settlements and the resulting tilting of the building and guarantee the satisfactory performance of the foundation system. In this paper author has analyzed piled rafts are analyzed as a plate on elastic foundation with the representation of the foundation media using the Winkler idealization. The elastic constant of the Winkler springs is derived using the sub-grade modulus. Perusal of literature reveals that very few investigations were done on the effect of variable sub soil on the behavior of structures supported on pile raft foundations. So in this research, an iterative dynamic analysis was performed using SAP: 2000 program to carry out three dimensional time history analysis of non-linear soil-foundation-building models under a great earthquake ground motions. The interaction between the soil and structure is represented by Winkler spring model. The obtained results confirmed that the dynamic characteristics of soil structure system should be recommended for conservative nonlinear seismic response of the high building since it mitigates of earthquake hazards. Keywords Soil structure interaction, time history analysis, modulus of sub grade reaction, spring constant, acceleration response, dynamic loading. T I. INTRODUCTION HE study of Dynamics begins with an introduction of the concepts of force and mass, then goes on to introduce the basic laws of Dynamics, Newton's Three Laws. All real physical structures behave dynamically when subjected to loads or displacements. The additional inertia forces, from Newton s second law, are equal to the mass times the acceleration. If the loads or displacements are applied very slowly, the inertia forces can be neglected and a static load Shruti Shukla Assistant professor, Applied Mechanics Department, Sardar Vallbh Bhai National Institute of Technology, Surat, Gujarat, India -sdv@amd.svnit.ac.in Dr.Atul.Desai & Dr (Prof.) Chandresh Solanki Professors, Applied Mechanics Department, Sardar Vallbh Bhai National Institute of Technology, Surat, Gujarat, India. analysis can be justified. Hence, dynamic analysis is a simple extension of static analysis. In addition, all real structures potentially have an infinite number of displacements. Therefore, the most critical phase of a structural analysis is to create a computer model with a finite number of mass less members and a finite number of node (joint) displacements that will simulate the behaviour of the real structure. The mass of a structural system, which can be accurately estimated, is lumped at the nodes. Also, for linear elastic structures, the stiffness properties of the members can be approximated with a high degree of confidence with the aid of experimental data. However, the dynamic loading, energy dissipation properties and boundary (foundation) conditions for many structures are difficult to estimate. This is always true for the cases of seismic input or wind loads. To reduce the errors that may be caused by the approximations summarized in the previous paragraph, it is necessary to conduct many different dynamic analyses using different computer models, loading and boundary conditions. This ground acceleration is descritized by numerical values at discrete time intervals. Integration of this time acceleration history gives velocity history, integration of which in turn gives displacement history. The post earthquake study of the structures reveals that the interaction of soil and foundation is playing a major role in the damage/response of structure. Perusal of literature reveals that very few investigations were done on the effect of variable sub soil on the behaviour of structures supported on pile raft foundations. So in this paper, an attempt has been made to find out the prominent investigations on soilstructure interaction analysis of structures supported on piled raft foundations with variable subsoil. To address this problem, a Finite Element Method is used to model soil-structure interaction analysis of piled raft foundation using Wrinkle approach. A parametric study piled raft foundation is carried out to understand the effects pile length, pile diameter, number of piles of the pile group and effect of different earthquake on the response. As the dynamic response of the structure and the pile to large extent is inelastic, the primary focus is on the understanding of the behavior of superstructure by modeling the nonlinearities of soil, modeling the interface of soil and pile. For this purpose 28

2 Finite Element Program SAP: 2000 is used. Its detailed analysis and results are shown in the preceding sections. II.STATEMENT OF THE ACTUAL PROBLEM The building to be constructed in the engineering field is a frame shear-wall structure with total 25 stories and 2-story basement, the total height of aerial part is 90m, and the size of building plane is 43.2 m 20.7 m. Plane layout of the building is shown in Fig.1 and the basic information is listed in Table. The primary dimension of beam section is 250 mm 600 mm; and column dimension is 600 mm 600 mm; the thickness of shear wall is 300 mm. Pile-raft foundation is taken to support the super structure. Fig. 3 Three dimensional view for actual work problem for 15 m pile length Fig. 1 Arrangement of two stories at bottom Fig. 4 Three dimensional view for actual work problem for 30 m pile length Fig. 2 Arrangement of standard story Nonlinear Time History Analysis Nonlinear time history analysis is by far the most comprehensive method for seismic analysis. The earthquake record in the form of acceleration time history is input at the response of the structure is computed at each second for the entire duration of an earthquake. Furthermore, nonlinearities that commonly occur during an earthquake can be included in the time history analysis. Furthermore, this method is equivalent to getting 100 % mass participation using response spectrum analysis. Full mass participation is necessary to generate correct earthquake forces. All types of nonlinearities is accounted for in this analysis Furthermore, input earthquake is never known with certainty. Hence, three to five different histories should be used the base of the structure Nonlinear time history analysis was performed. A constant damping ratio of 0.05 has been taken for RC buildings. The inelastic time history analysis is preformed using the direct integration technique considering a time step of s. For 29

3 nonlinear seismic analyses, a total mass including self-weight and floor cover Dead Load; DL plus 25 % of Live Load LL (1.0DL LL) is considered (IS ). For analysis purpose, 6 different nodes of central frame of the structure were selected and they are shown in figure:-5. Result of medium duration earthquake (El-Centro) is presented in this paper. Fig. 5 Different heights and its nodes to check the pseudo spectral acceleration To check the behaviour of above building with piled raft foundation in soft soil, three different types of soils are considered. They are classified as under:- Purely cohesive soils (c-soils):- by cohesion is meant the shearing resistance inherent in soil which does not require any normal pressure or other outside influence for its development. It is the property which holds the particle to gather in a soil mass mainly due to interparticle molecular attraction and bonds. A soil in which interparticle attraction and adsorbed water work together to produce a mass that holds together and deforms plastically at varying moisture contents is called a cohesive soil. Cohesionless soils (ϕ - soils):- t a soil which does not exhibit cohesion is termed cohesion less or non cohesive soils. Cohesionless soils possess no shearing resistance except as developed by normal pressure between their particles. Soils composed of bulky particles are cohesionless regardless of the fineness of particles. These soils are also known as granular soils. These soils are the soils which do not have cohesion and they derive the strength from the intergranular friction. They are also referred as cohesionless soils i.e. sands and gravels. Cohesive-cohesion less soils (c-ϕ soils):- These are the composite soils having both cohesion and friction. Composite soils are mixture of cohesive and cohesionless soil so they are referred as c- ϕ soils. I.e clayey sand, sandy clay, silty sand etc To check the behaviour of above building with piled raft foundation in soft soil, three different types of soils are considered. They are classified as under:- Purely cohesive soils (c-soils):-cohesion is meant the shearing resistance inherent in soil which does not require any normal pressure or other outside influence for its development. It is the property which holds the particle to gather in a soil mass mainly due to interparticle molecular attraction and bonds. A soil in which interparticle attraction and adsorbed water work together to produce a mass that holds together and deforms plastically at varying moisture contents is called a cohesive soil. Cohesionless soils (ϕ - soils):- It a soil which does not exhibit cohesion is termed cohesion less or non cohesive soils. Cohesionless soils possess no shearing resistance except as developed by normal pressure between their particles. Soils composed of bulky particles are cohesionless regardless of the fineness of particles. These soils are also known as granular soils. These soils are the soils which do not have cohesion and they derive the strength from the intergranular friction. They are also referred as cohesionless soils i.e. sands and gravels. Cohesive-cohesion less soils (c-ϕ soils):- These are the composite soils having both cohesion and friction. Composite soils are mixture of cohesive and cohesionless soil so they are referred as c- ϕ soils. I.e clayey sand, sandy clay, silty sand etc Fig: - 6 Accelogram for El Centro earthquake, duration 40 sec. ( Fig. 7 Acceleration response for clayey soil on middle of the structure 30

4 Fig. -8 Acceleration response for medium dense sand on bottom of the structure Fig. 12 Acceleration response for medium dense sand on middle of the foundation Fig. 9 Acceleration response for clayey soil on bottom of the structure Fig. 13 Acceleration response for clayey soil on middle of the foundation Fig. 10 Acceleration response for medium dense sand on top of the Foundation Fig. 14 Acceleration response for medium dense sand on bottom of the foundation Fig. 11 Acceleration response for clayey soil on top of the foundation Fig. 15 Acceleration response for clayey soil on bottom of the foundation 31

5 From all Fig.:- 7 Fig. : - 15 graphs it was observed that, overall piled raft foundation with Medium dense sand soil like dense sand is a very good combination for the reasonable behaviour of the structure in earthquake. II. OBSERVATIONS AND CONCLUSIONS For l = 15 m pile length, ϕ soils gave higher same acceleration as c-ϕ soil at top of the pile and as the depth of pile increases, ϕ soils acceleration reduces where as in case of pile length l = 30 m, ϕ soils gave least accelerations in all time histories for all types of subsoil. This behaviour was because of increased density of soil as the depth increases and it reflects in the acceleration behaviour at top of the pile. Maximum acceleration and displacement values are in decreasing manner from top to bottom of superstructure. The reason for this to happen is that long duration earthquake with high PGA have more energy flux and it takes large time for the structure to dissipate energy. The energy gets dissipated after getting transferred up to full length of structure hence the top portion has maximum acceleration. The difference of response in both cases is also noteworthy. REFERENCES [1] N.Dharmarajan, K.Ilamparuthi Piled raft analysis and design methodology proceedings of indian geotechnical conference, December 15-17, 2011, Kochi (Paper No. N-138.) [2] Mossallamy Yaser Ei, (2002) Innovative application of piled raft foundation in stiff and soft subsoil, Deep Foundation, [3] Mehta D, Gandhi N (2008) Time response study of tall chimneys, under the effect of soil structure Interaction and Long period earthquake Impulse. The 14th World Conference on Earthquake Engineering, October 12-17, 2008, Beijing, China. [4] Maharaj D.K. (2004) Non Linear finite element analysis of piled raft foundations Geotechnical Engineering, Issue 157, GE3, PP: [5] Polous H. G., Grahame B., (2008) Foundation design for The Burj Dubai the world s tallest building 6th International Conference on Case Histories In Geotechnical Engineering,Arlington,August -, Paper 147. [6] Polous H.G. (2002), Simplified design procedure for piled raft foundations, Deep Foundations Shruti J. Shukla Assistant Prof. Applied Mechanics Department S V National Institute of Technology, Surat. B. E. (CIVIL) in 2002 from Gujarat University, Gujarat, India. M.TECH. (CIVIL) specialization in Soil Mechanics & Foundation Engineering in 2006 from S.V.N.I.T, Surat, Gujarat, India. Mrs. Shruti j. shukla field of specialization is Geotechnical Engineering and Soil improvement techniques; research is going on in the field of piled raft foundation. The author became life member of Indian geotechnical society (LM- 2376) in E mail:sdv@amd.svnit.ac.in, vaidya_shruti2001@yahoo.com 32