Analysis of RC Framed Structure Using Base Isolation ... of RC Framed Structure Using Base Isolation Techniques by Use of Elastomeric Bearing

IJSRD - International Journal for Scientific Research & Development| Vol. 2, Issue 09, 2014 | ISSN (online): 2321-0613

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Analysis of RC Framed Structure Using Base Isolation Techniques by Use

of Elastomeric Bearing Fazil ali K

Department of Civil Engineering

KMEA Engineering CollegeAbstract— Earthquake is most extreme condition of any

building may be required to survive during its lifetime.In

this paper we are evaluating the base isolation method using

Elastomeric bearing for preventing the super structure from

damaging components of earthquake. We are comparing the

seismic Response of normal structure and base isolated

structure from the results obtained in Etab software.

Key words: Base isolation method, Rubber bearings.Static

Analysis, Model analysis, Time History Analysis

I. INTRODUCTION

Earthquake is most extreme condition of any building may

be required to survive during its lifetime. An Earthquake

would Structurally damage the building and incase the

damage is above threshold level there would be no option

but to demolish and reconstruct .While building collapse is

the primary cause of loss of life in most earth-quakes, other

contributors to earthquake loss include equipment and

contents damage, business interruption, and damage to

lifelines, such as water, power, gas, communications, and

transportation. The field of Earthquake Engineering has

existed in our country for over 35 years now. Indian

earthquake engineers have made significant contributions to

the seismic safety of several important structures in the

country. However, as the recent earthquakes have shown,

the performance of normal structures during past Indian

earthquakes has been less satisfactory. This is mainly due to

the lack of awareness amongst most practicing engineers of

the special provisions that need to be followed in earthquake

resistant design and thereafter in construction. Earthquake

motion causes horizontal and vertical ground motions.

Vertical ground motion having a much smaller magnitude is

the most usual. In general, all structures are conventionally

designed to carry the gravity loads. The vertical ground

motion due to earthquakes can be resisted by the factor of

safety provided in the design of structures. The structures

which are designed to carry only the gravity loads will not

able to resist the horizontal ground motion. The horizontal

ground motion causes the most significant effect on the

structure by shaking the foundation. Hence, it is necessary to

check the adequacy of the structures to withstand the

horizontal ground motion.

A. Base isolation

A series of devastating during the last few decades has

created considerable interest in the minds of all concerned

about the subject of earthquake resistant design. Modern

seismic design relies upon the inelastic response of

structural members and systems to dissipate the energy

imparted to a structure by an earthquake. In recent years,

base isolation technique has become an increasingly applied

structural design technique for buildings and bridges in

highly seismic areas. The objective of seismic isolation

systems is to decouple the building structure from the

damaging components of the earthquake input motion that is

to prevent the superstructure of the building from absorbing

the earthquake energy. The entire superstructure must be

supported on discrete isolators whose dynamic

characteristics are chosen to uncouple the ground motion.

Displacement and yielding are concentrated at the level of

the of the isolation devices and the superstructure behaves

very much like a rigid body. The main feature of the base

isolation technology is that it introduces flexibility in the

structure. This helps in further reducing the seismic response

of the building. Another approach for controlling seismic

damage in building and improving their seismic

performance is by installing seismic dampers in place of

structural elements, such as diagonal braces. These dampers

act like the hydraulic shock absorbers in cars much of the

sudden jerks are absorbed in the hydraulic fluids and only a

little is transmitted above the chassis of the car.Seismic

energy is transmitted though them, dampers absorb part of

it, and thus damp the motion of the building. Thus the

dampers reduce the energy available for shaking the

building. This means that the building deforms less, so the

chance of damage is reduced. The concepts of base isolation

systemand its performance during strong ground motion and

also the various applications in base isolation system and

also used various types of dampers.

B. Concept of base isolation

One of the most widely implemented and accepted seismic

protection systems is base isolation systems. Seismic base

isolation is a technique that mitigates those effects

of an earthquake by essentially isolating the structure and its

contents from potentially dangerous ground motion,

especially in the frequency range where the building is most

affected. The objective is to simultaneously reduce inter-

storey drifts and floor accelerations to limit or avoid

damage, not only to the structure but also to its contents, in a

cost-effective manner. Seismic base isolation is emerging as

on alternative approach for earth-quake protection of

structures. The basic concept in this approach is to uncouple

a structure from the ground by interposing a flexible

element/bearing between the structure and foundation. Many

buildings have been constructed on some type of rubber

bearings, and such structures have shown superior

performance in earthquakes.

C. Aim of base isolation

The aim of base isolation is to minimize the energy that is

transferred from ground motion to the structure by bu

ering it with a bearing layer at the foundation which has

relatively low stiffness. The bearing level has a longer

period than the superstructure, which reduce the force and

displacement demands on the superstructure, allowing it to

remain elastic and generally undamaged. As the result of

flexibilization, the natural period of the past fixed-base

structures undergo a jump and the new base isolation

structure has a new natural period. The flexibility of the

Analysis of RC Framed Structure Using Base Isolation Techniques by Use of Elastomeric Bearing

(IJSRD/Vol. 2/Issue 09/2014/185)


interposing layers between structure and its foundation lead

to a bigger fundamental period for structural ensemble.

D. Mechanism of base isolation

The isolation reduces the fundamental lateral frequency of

the structure from its fixed base frequency (or increases the

time period of the structure) and thus shifts the position of

structure in the spectrum from the peak-plateau region to the

lower regions. Also it brings forth additional damping due to

the increased damping introduced at the base level and thus

further reduction in the spectral acceleration is achieved.

………………(1.1)

………………(1.2)

…………(1.3) Source [8]

Tb, , are those of isolated system.

= base slab of mass -lateral stiffness.

Elements of Seismic Base Isolation System

Seismic base isolation consists of

Energy dissipation core (lead plug).

Vulcanized rubber layers.

Steel reinforcing plates.

Bottom mounting plate

Cover rubber.

Energy dissipation core reduces earthquake forces

and displacements by energy dissipation and also it provides

wind resistance. Rubber layers provide lateral flexibility to

the system. steel reinforcing plates provide vertical load

capacity and also it confines lead core. Bottom mounting

plate is integrated with the isolator and it is used to connect

the structure below and above isolator. Rubber is used to

protect the steel plate.

Fig. 1: Schematic diagram of Base Isolator

E. Response of buildings

As a result of an earthquake, ground beneath each building

begins to move, say first to left, building responds with

movement, which tends towards the right that is the

buildings undergoes displacement towards right. The

buildings displacement in the direction opposite to ground

motion is actually due to inertia. The inertial forces acting

on a buildings are the most important of all those generated

during an earthquake. But when the ground shakes, isolated

buildings do not move. No inertia force is transferred to the

buildings due to shaking of ground. The inertia force acting

on buildings have been reduced. Acceleration is decreased

because base isolation system lengthens a buildings period

of vibration. Structure with longer periods of vibration tends

to reduce acceleration, while those with shorter periods tend

to increase or amplify acceleration. The inertial forces which

the buildings undergoes are proportional to the buildings

acceleration during ground motion

Fig. 2: Comparison of fixed base with isolated base

F. Application of base isolation system

Base isolation systems are found useful for short period

structures, sayless than 0.7s including soil-structure

interaction. Base isolation provides an alternative to the

conventional,fixed-base design of structures and may be

cost-effective for some new buildings in locations where

very strong ground shaking. Base isolation is an alternative

for buildings that must remain functional after a major

earthquake(e.g.hospital,emergency, communications centre,

computer processing centre, etc.) Several new buildings

have been isolated using rubber or elastomeric bearings

.several commercial brands of base isolators are now

available in the market especially in foreign countries. Care

should be taken to identify the most suitable type of device

for a particular building. The first base-isolated building in

the United States is the foothill communities law and

justices centre at Los Angeles completed in 1985, the

building is four stories high with a full basement and sub-

basement for the isolation system, which consists of 98

isolators of multilayered natural rubber bearings reinforced

with steel plates.

Fig. 3: Diagram showing Rubber bearing in building

II. MODELLING AND ANALYSIS

A Institutional building with 10 storey was selected. The

number of bay is assumed as 3 in X direction with a spacing

of 5m. The number of bay is assumed as 3 in y direction




with a spacing of 4m. The bottom storey is of 3.5 height and

all other typical floors are of 3m height. The plan of the

building is as shown in the below figure.

Fig. 4: Plan of the building

The institutional building constructed using base

isolation techniques at Calicut .At ground floor, slabs are

not provided and the floor will directly rest on ground. The

beams rest centrally on columns to avoid local eccentricity.

For all structural elements, M20 grade concrete will be used.

The building rest on fixed base and base isolation. Centre-

line dimensions are followed for analysis and design.

Preliminary sizes of structural components are assumed by

experience. Rubber bearings base isolation is used The

modeling was done using Etab Software. The two building

was modeled with same properties of the members and the

support condition was fixed. The first model is the building

with fixed base condition and the second model with base

isolated condition.

The properties of the model are as follows:

Support condition - Fixed Support

Location of the building – Calicut ,Seismic Zone

factor (Z) - 0.16

Type of soil – Medium

Type of Building – Institutional

Type of Frame – SMRF

Damping - 0.05

Cross section of Column - 300mmx500mm

Cross section of Beam - 300mmx600mm

Thickness of Slab - 150mm

Width of masonry Infill wall - 230mm

Height of ground Floor - 3500mm

Height of other Floor - 3000mm

Number of storey - ten (G+9)

Spacing in x direction - 5m

Spacing in y direction - 4m

A. Rubber bearing properties:

The seismic isolators are link component 0.5 m length

placed between the fixed base and the coloumns. The

parameters selected to define the seismicisolators in Etab

program are as follows

Link property:

Link type : Rubber

Linear effective stiffness:

U1 = 2500000 ( kN)m

U2 =1600( kN)m

U3 =1600( kN)m

Nonlinear stiffness :

U1 = 5000 (kN)m

U2 =160( kN)m

U3 =160( kN)m

Post yield stiffness Ratio:

U2 = 0.1

U3 = 0.1

source[25]

B. Modelling Details:

The three dimensional view of the two models are as shown

in the below figures

Fig. 5: building with fixed base condition

Fig. 6: building with base isolated condition




C. Static Analysis:

The static analysis was done and the storey displacement

storey drift and storey shear were obtained and tabulated and

plotted graphs obtained from software are as shown below.

1) Storey Displacement:



Storey no Story displacement in x direction in mm

fixed base base isolated

10 12.4 13.78

9 11.8 13.6

8 11 13.4

7 9.9 13.1

6 8.7 12.21

5 7.3 11.8

4 5.8 10.6

3 4.3 10.3

2 2.7 9.1

1 1.2 8.21

Base 0 6.15

Table 5.1: Story displacement in X-direction for both Fixed

and base isolated

From the above table and graph we can see that the storey

displacement is decreased for the base isolated compared to

fixed base. In base isolated structures the story displacement

is decreasing for upper stories

2) Storey Drift:



Storey no Storey drift in x direction

Fixed base Base isolated

10 0.000191 0.000179

9 0.000277 0.000217

8 0.000359 0.000297

7 0.000424 0.000368

6 0.000470 0.00391

5 0.000504 0.000498

4 0.000529 0.000515

3 0.000562 0.000519

2 0.000696 0.000503

1 0.001769 0.000347




Base 0 0

Table 5.2: Story drift in X-direction for both Fixed and base

isolated


drift is decreased for the base isolated compared to fixed

base. In base isolated structures the base story cause a high

drift and on going to upper stories the storey drift is reduced.

3) Storey Shear:



Storey no Storey shear in x direction

Fixed base Base isolated

10 -69.192 -67.19

9 -130.949 -124.95

8 -179.96 -171.92

7 -217.65 -210.716

6 -245.335 -235.67

5 -265.28 -257.284

4 -278.34 -271.04

3 -285.42 -276.42

2 -288.87 -280.86

1 -289.78 -265.16

Base 0 0

Table 5.3: Story shear in X-direction for both Fixed and

base isolated


shear is decreased for the base isolated compared to fixed

base. The storey shear for the bottom story is reduced in

base isolated compared to fixed base.

D. Model Analysis

The elastic properties and mass of building causes to

develop a vibratory motion when they are subjected to

dynamic action. The dynamic properties of the structure

such as natural period, Damping, and mode shape play a

crucial role in determining response of building. The

building with higher natural frequency and short natural

period will suffer higher accelerations but smaller

displacement. The period and mode shape of the building

for both the cases are compared below.

Fig. 13: Fixed base Mode 1

Period = 1.377

Fig. 14: Base isolated Mode 1

Period = 2.274





Period = 0.456


Period = 0.597


Period = 0.188


Period = 0.205

From the mode shape obtained for the both models. we can

clearly say that the the period is increasing for base isolated

structures compared to fixed base. Here the mode 1 show a

65% increase in period for base isolated and that of mode 10

it shows 20 % increase in period

E. Dynamic Analysis -Time History Analysis

The time history analysis method technique represents the

most sophisticated method of dynamic analysis for

buildings. In this method the mathematical model of the

building is subjected to acceleration from earthquake

records that represents the expected earthquake at the base

of the structure. It consist of step by step direct integration

over a time interval. The Earthquake motions are applied

directly to the base of model of the structure. Instantaneous

stresses throughout the structure are calculated at small

intervals of time for the duration of earthquake. The time

history analysis were done and the time history analysis

results are as follows

1) Time-Acceleration plot

Fig 19: Fixed base




Fig 20: Base isolated

2) Time-Velocity plot

Fig 21: Fixed base


3) Time-displacement plot

Fig 23: Fixed base


From the time History Analysis the above graphs indicate

that the Acceleration ,Velocity, Displacement are reduced

for a structure with base isolation compared to Fixed base

III. CONCLUSIONS

From the comparison of fixed base and base isolation

methods the following conclusions are made:

Base isolation is very promising technology to

protect different structures from seismic excitation

It is observed that when increasing the number of

stories maxium storey displacement is become

considerable

Story drift and storey shear are also reduced in base

isolated building making the super structure

flexible

From time history Analysis the Acceleration,

velocity, displacement are low for base isolated

structures. It make the structure rigid and stiffer

It can be concluded that the performance of base

isolated structure is efficient in seismic prone areas

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Analysis of RC Framed Structure Using Base Isolation ... of RC Framed Structure Using Base Isolation Techniques by Use of Elastomeric Bearing

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