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TitleSeismic Analysis for offshore structures
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Total Pages7
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Page 1

SEISMIC ANALYSIS
SEISMIC ANALYSISSEISMIC ANALYSIS

To generate forces on the structure due to base motion arising
from earthquake.

Structural
response

H i t l

Vertical
motion

Horizontal
motion

14 June 2009
Dr. S. Nallayarasu

Department of Ocean Engineering
Indian Institute of Technology

Madras-36

1

SEISMIC ANALYSIS

API RP 2A IS 1893

STRENGTH LEVEL EQ
(200 YEARS)

DESIGN BASIS EQ
(DBE)

DUCTILITY LEVEL EQ
(2000 YEARS)

MAXIMUM
CONSIDERED EQ

(MCE)(MCE)

14 June 2009
Dr. S. Nallayarasu

Department of Ocean Engineering
Indian Institute of Technology

Madras-36

2

Page 2

SEISMIC ANALYSIS
GEOMETRY

AND Seismic Analysis Steps AND
SEASTATE

LINEARISE
FOUNDATION

SOIL MODEL
PSI INPUT

FILE

Seismic Analysis Steps

FOUNDATION
SUPERELEMENT

DYNAMIC ANALYSIS
MODE SHAPES DYNINPMODE SHAPES

& FREQUENCIES

DYNAMIC RESPONSE
RESPONSE

SPECTRA METHOD

DYNINP

SEISMIC
SPECTRA

API / IS 1893SPECTRA METHOD API / IS 1893

LOAD COMBINATION
MODEL: CQC

DIRECTIONAL: SRSSDIRECTIONAL: SRSS

NUMBER CHECK
PRST 1.0
PRSC

14 June 2009
Dr. S. Nallayarasu

Department of Ocean Engineering
Indian Institute of Technology

Madras-36

3

PRSC

SEISMIC ANALYSIS

Peak Ground Acceleration (PGA)– Indication of Strength
of earthquake

PGA = 0.3 X Spectral average between period 0.1 to 0.3
sec.

Structure

PGA

14 June 2009
Dr. S. Nallayarasu

Department of Ocean Engineering
Indian Institute of Technology

Madras-36

4

Page 3

SEISMIC ANALYSIS
CALCULATED PGA FOR ZONE I TO IVCALCULATED PGA FOR ZONE I TO IV
AS PER IS: 1893-2000

ZONE SEISMIC ZONE ZONE SEISMIC
INTENSITY

ZONE
FACTOR

II Low 0 10II Low 0.10
III Moderate 0.16

IV Severe 0 24IV Severe 0.24

V Very 0.36

14 June 2009
Dr. S. Nallayarasu

Department of Ocean Engineering
Indian Institute of Technology

Madras-36

5

SEISMIC ANALYSIS

E th k D i C it iEarthquake Design Criteria

Specific design criteria depends on the country, location and
type of soil stratum etc.yp

IS 1893

Design Basis Earthquake (DBE)Design Basis Earthquake. (DBE)

Maximum Considered Earthquake (MCE)

API RP 2AAPI RP 2A

Strength Level Earthquake (SLE)

Ductility Level Earthquake (DLE)Ductility Level Earthquake (DLE)

14 June 2009
Dr. S. Nallayarasu

Department of Ocean Engineering
Indian Institute of Technology

Madras-36

6

Page 4

SEISMIC ANALYSIS

E th k D i C it iEarthquake Design Criteria

OPERATIONAL EARTHWUAKE (200 YEARS RETURN PERIOD)

• No damage to structure • No damage to structure

• conventional structural design

• Allowable stresses can be increased

EXTREME EARTHQUAKE (2000 YEARS RETURN PERIOD)

• structure to be stable

• Redundant framing

• Ductile failure expected• Ductile failure expected

14 June 2009
Dr. S. Nallayarasu

Department of Ocean Engineering
Indian Institute of Technology

Madras-36

7

SEISMIC ANALYSIS

Th k d l ti f ti l it d d The peak ground acceleration of a particular site depends on
the following parameters.

• Soil stratum

• Distance from epicenter

It is specified in terms of ‘G’ values as a relative significanceIt is specified in terms of G values as a relative significance.

IS 1893 defines PGA as

Sa Sa
0.1 0.30.4

2

Sa Sa
G Gs sPGA

14 June 2009
Dr. S. Nallayarasu

Department of Ocean Engineering
Indian Institute of Technology

Madras-36

8

Page 5

SEISMIC ANALYSIS
Response Spectra—Spectra Normalized to 1.0 Gravity (API RP 2A)p p p y ( )

14 June 2009
Dr. S. Nallayarasu

Department of Ocean Engineering
Indian Institute of Technology

Madras-36

9

SEISMIC ANALYSIS

Response spectra for rock and soil sites for 5% damping (IS 1893)

14 June 2009
Dr. S. Nallayarasu

Department of Ocean Engineering
Indian Institute of Technology

Madras-36

10

Page 6

SEISMIC ANALYSIS

M th d f A l iMethods of Analysis

• Seismic co-efficient method

• Time series method

• Response spectra method

R id tiResponse consideration

• Horizontal acceleration both orthogonal directions

• Vertical acceleratione t ca acce e at o

• Number of modes to be combined by complete quadratic
combination (CQC)

Directional combination to be combined using SRSS • Directional combination to be combined using SRSS.
(Square Root of Sum of Sequences)

e.g. 2 2 2
X Y ZER ER ER ER

14 June 2009
Dr. S. Nallayarasu

Department of Ocean Engineering
Indian Institute of Technology

Madras-36

11

SEISMIC ANALYSIS

S i i C ffi i t th dSeismic Co-efficient method

Design horizontal co-efficient

2
h

ZISa
A

Rg

Where

Z =zone factor

I =importance factor
Sa

=average response acceleration co-efficient

R =response reduction factor

Sa
g

14 June 2009
Dr. S. Nallayarasu

Department of Ocean Engineering
Indian Institute of Technology

Madras-36

12

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