A Dynamic Behavioural Study of Structure and Foundation...

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.

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

E-mail :[email protected]

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 soil-

structure 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

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

T

8th International Conference on Electrical, Electronics and Civil Engineering (ICEECE'2016) Jan. 12-13, 2016 Dubai (UAE)

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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. 1 Arrangement of two stories at bottom

Fig. 2 Arrangement of standard story

Fig. 3 Three dimensional view for actual work problem for 15 m

pile length

Fig. 4 Three dimensional view for actual work problem

for 30 m pile length

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 0.005 s. For


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nonlinear seismic analyses, a total mass including self-weight

and floor cover „„Dead Load; DL‟‟ plus 25 % of Live Load

„„LL‟‟ (1.0DL + 0.25LL) is considered (IS1893-2000).

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.

(http://earthquake.usgs.gov)

Fig. 7 Acceleration response for clayey soil on middle

of the structure


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http://earthquake.usgs.gov/

Fig. -8 Acceleration response for medium dense sand

on bottom of the structure

Fig. 9 Acceleration response for clayey soil

on bottom of the structure

Fig. 10 Acceleration response for medium dense sand on top of the

Foundation

Fig. 11 Acceleration response for clayey soil on top of

the foundation

Fig. 12 Acceleration response for medium dense

sand on middle of the foundation

Fig. 13 Acceleration response for clayey soil

on middle of the foundation

Fig. 14 Acceleration response for medium dense sand

on bottom of the foundation

Fig. 15 Acceleration response for clayey soil on

bottom of the foundation


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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, 2002.

[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: 107-113.

[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 2002.

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 2005. E‑ mail:[email protected], [email protected]


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A Dynamic Behavioural Study of Structure and Foundation...

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