Seismic Analysis for offshore structures

SEISMIC ANALYSIS SEISMIC ANALYSIS To generate forces on the structure due to base motion arising from earthquake. Structural response

Horizontal H i t l motion Vertical motion

14 June 2009

Dr. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36

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SEISMIC ANALYSIS

14 June 2009

API RP 2A

IS 1893

STRENGTH LEVEL EQ (200 YEARS)

DESIGN BASIS EQ (DBE)

DUCTILITY LEVEL EQ (2000 YEARS)

MAXIMUM CONSIDERED EQ (MCE)

Dr. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36

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SEISMIC ANALYSIS Seismic Analysis Steps SOIL MODEL PSI INPUT FILE

GEOMETRY AND SEASTATE

LINEARISE FOUNDATION SUPERELEMENT DYNAMIC ANALYSIS MODE SHAPES & FREQUENCIES DYNAMIC RESPONSE RESPONSE SPECTRA METHOD

DYNINP

SEISMIC SPECTRA API / IS 1893

LOAD COMBINATION MODEL: CQC DIRECTIONAL: SRSS

NUMBER CHECK PRST 1.0 PRSC 14 June 2009

Dr. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36

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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

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

14 June 2009

ZONE

SEISMIC INTENSITY

ZONE FACTOR

II

Low

0 10 0.10

III

Moderate

0.16

IV

Severe

0 24 0.24

V

Very

0.36

Dr. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36

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SEISMIC ANALYSIS E th Earthquake k D Design i C Criteria it i Specific design criteria depends on the country, location and type yp of soil stratum etc. IS 1893 Design Basis Earthquake. Earthquake (DBE) Maximum Considered Earthquake (MCE) API RP 2A Strength Level Earthquake (SLE) Ductility Level Earthquake (DLE)

14 June 2009

Dr. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36

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SEISMIC ANALYSIS E th Earthquake k D Design i C Criteria it i OPERATIONAL EARTHWUAKE (200 YEARS RETURN PERIOD) • 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

Dr. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36

14 June 2009

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SEISMIC ANALYSIS The peak Th k ground d acceleration l ti off a particular ti l site it depends d d on the following parameters. • Soil stratum • Distance from epicenter It is specified in terms of ‘G’ G values as a relative significance significance. IS 1893 defines PGA as Sa ⎡ ( Sa G )0.1s + ( G )0.3 s ⎤ PGA = 0.4 ⎢ ⎥ 2 ⎢⎣ ⎥⎦

14 June 2009

Dr. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36

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

14 June 2009

Dr. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36

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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

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SEISMIC ANALYSIS M th d off Analysis Methods A l i • Seismic co-efficient method • Time series method • Response spectra method R Response consideration id ti • Horizontal acceleration both orthogonal directions • Vertical e t ca acceleration acce e at o • Number of modes to be combined by complete quadratic combination (CQC) • Directional combination to be combined using SRSS. SRSS (Square Root of Sum of Sequences) e.g. ER =

( ERX ) + ( ERY ) + ( ERZ ) 2

2

2

Dr. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36

14 June 2009

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SEISMIC ANALYSIS S i Seismic i C Co-efficient ffi i t method th d Design horizontal co-efficient

Ah =

ZISa 2 Rg

Where Z =zone factor I =importance factor Sa =average response acceleration co-efficient g R =response reduction factor

14 June 2009

Dr. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36

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SEISMIC ANALYSIS Design of Design Forces Total base shear can be calculated as VB=A Ah.W Where Ah=design horizontal seismic co-efficient W=total weight of structure Distribution of Design Forces Design seismic force at each floor is to be calculated by Qi = VB

Wi hi n

∑W h j =1

j

2 j

n=number of storage or points of mass located along the height. Wi=seismic i i weight i ht off floor, fl i 14 June 2009

Dr. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36

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