Loads OFFSHORE

8/11/2019 Loads OFFSHORE

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Offshore Structures Loads

ontents

Gravity Loads

Wind Loads

Wave and Current Loads Small bodies

Wave and Current Loads Large Bodies

26 May 2007 Dr. S. NallayarasuDepartment of Ocean Engineering

Indian Institute of Technology Madras-36

1


Gravity Loads

Structural Dead Loads

Facility Dead Loads

Fluid Loads

Live Loads

Environmental Loads

n oa s

Wave Loads

Current Loads

Buoyancy Loads

Ice Loads

u oa s

Seismic Loads



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Dead Loads

the platform deck, jacket, bridge and flare

structures. It includes all primary steel,

such as boat landing, padeyes, stiffeners,handrails, deck plating, small access platforms

.

The primary structural steel members will be

in the model automatically when a computerprogram is used to analyze the structure. But

shall be calculated applied to the structuralmodel at appropriate locations.



3


Facility Loads

wellhead type platform or for processt e latform su orts various

equipment and facilities. Mechanical equipment

Electrical equipment

Piping connecting each equipment

Electrical Cable trays

Instrumentation items



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Drilling Loads include reaction from Jackup

Dead loadsova e r oor oa s

Drill string weight

Depending on the type of drilling rig used, this loads will vary.For water depth less than 70m, Jackup type rig may be used.For deeper water depths, operation of Jackup type rigsbecome uneconomical and deck mounted drilling rigs will be asuitable option. Weight of such rig will be around 1500 Tonnes

with an additional support module of 1000 Tonnes.



5


Live loads

Live loads are defined as movable loads and willbe temporary in nature.

This load vary in nature from owner to owner but

is given below.

S.No. Location Load (kN/m2)

1 Stora e / la down 10

2 Walkway 5

3 Access Platform 5

4 Galley 10



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Environmenta Loa s

Wind Loads

Seismic Loads



7


Win Loa s

Wind profiles and Gusts.

U(t) (turbulence)( )V u t+

( )V meanwind

X Direction



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Quasi-static responseto wind gusting

Dynamic response towind gusting

onse

nse

Due to mean wind

Due to turbulent gusts

Resp

Due to turbulent gusts

Due to mean wind

Respo

(a) Short period structures

Time

(b) Long period Structures

Time

RESPONSE TO WIND GUSTING



9

9


.For strong wind conditions the design wind speed u (z,t) (ft/s) atheight z (ft) above sea level and corresponding to an averaging

0 0

0

( , ) ( ) 1 0 .4 1 ( ) nu

tu z t U z I z l

t

=

Where the 1 hour mean wind speed U(z) (ft/s) at level z(ft) isgiven by:

0

12 2

( ) 1 ln32.8

5.73 10 1 0.0457

zU z U C

C U

= +

= +

And the turbulence intensity Iu(z) at level z is given by:

[ ].

( ) 0.06 1 0.013132.8

u o

zI z U= +

Where Uo (ft/s) is the 1 hour mean wind speed at 32.8 ft.



10

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Dynamic response Dynamic response

STRUCTURE TYPES

(T0.5 sec

Buildin s Jacket

Most bridges(eg,concrete road

Towers

Flare booms

Some bridges (eg. Longsuspension bridges)



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11


Method Data Dynamic Calculates Calculates Accuracy

COMPARISON OF DESIGN METHODS

analysis? extremelife?

fatiguelife?

Static Static 3s gust No Yes No Poor

design analysis speed(2)

Gust Static Mean Approximate Yes No Reasonafactordesign

analysis xgust factor

windspeed

(3) ble

Windturbulenceanalysis

Spectralanalysis

Turbulent gustspectra

Yes Yes Yes Good



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WEATHER SYSTEMS

Caused by Duration Extent Wind speeds

Monsoons

Global

Weeks Large Low

circulation

pattern

1000km

dia)

max)

reva ngwinds(1)

Fullydeveloped

ays arge ow o mo era e

Thunderstorms(2)

~1hour Small(~10km

Low to moderate

pressuresystems

dia)

Tornadoes 1-10 minutes very small(~300m

Veryhigh(~100m/s

.

Local winds

Local

Varies Varies Varies (may be

high)Local heating



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13


WIND LOAD ON SUPER STRUCTURES

1. Individual members - 3 sec. gust

2. Total structure size50m - 15 sec. gust

WIND AND ASSOCIATED WAVE/CURRENT LOADS

3. Total loads -1 minute sustained wind

. - .



14

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Wind Pressure

The wind pressure can be calculated as

2V

gfw

=

22 6.0 N/mV=



15


e o a orce on e p a orm can ecalculated as

sxwx =sywy



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Wind load on oblique directions can becalculated using following relationship.

)sin()cos( yx

FFF +=

The projected areas can be calculated asA1 = Axcos() and A2 = Aysin ()

)( 21 w AAfF +=

)cos())sin()cos((

yxwx

yxw

AAfF +=

=

)sin())sin()cos(( yxwy AAfF +=



17


Where Fx and Fy are the components of Fein x and y directions respectively. Ratiobetween Fx and Fx can be expressed as

)cos()sin()cos(( yxwx

AAfF +=

xwx

Fcoss ncos xy

x

x

F+=



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Similarly, ratio between Fy and Fy can be

expressed as

yw

yxw

y

y

AfF=

)cos()sin()/()(sin2

yx

yAA

F+=

y



19


WAVE FORCES ON SMALLBODIES



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Methodology

In applying design waves load on to the offshore

structures, there are two ways of applying it

Spectral MethodIn design wave method, a discrete set of design waves

to generate loads on the structure. These loads willbe used to compute the response of the structure.

In the spectral method, a energy spectrum of the sea-state for the location will be taken and a transfer

function for the res onse will be enerated. Thesetransfer function will be used to compute thestresses in the structural members



21


The forces exerted by waves are most dominant inovernin the acket structures desi n es eciall the

foundation piles. The wave loads exerted on thejacket is applied laterally on all members and itgenerates overturning moment on the structure.

Period of wind generated waves in the open sea can bein the order of 2 to 20 seconds. These waves arecalled gravity waves and contain most part of waveenergy.

Maximum wave shall be used for the design of offshorestructures. The relationship between the significant

s (Hmax) is

Hmax= 1.86 Hse a ove equa on correspon o a compu a on ase

on 1000 waves in a record.



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The desi n wave hei ht for various re ions istabulated below

Region 1 year 100 year Maximum

Bay of Bengal 8 18

Gulf of Mexico 12 24

es gn waves nvarious regions

Arabian Sea 8 18

Gulf of Thailand 6 12

Persian Gulf 5 12

North sea 14 22

API RP2A requires both 1 year and 100 year recurrence waveshall be used for the design of jacket and piles. Appropriate

design. A one-third increase in permissible stress is allowed for100 year storm conditions.



23




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The wind driven current variation with depth can

be expressed as:

1

7

=h

yVV oTT

Where VT is the tidal current at any height fromsea e , oT s e a curren a e sur ace, yis the distance measure in m from seabed and h



25


The current variation with depth can be expressed as:

y=

ho

W from sea bed, VoW is the wind driven current at thesurface is the distance measure in m fromseabed and h is the water depth



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Flow is assumed to be not disturbed by the

Force calculation is empirical calibrated by

Suitable Coefficients need to be usedde endin on the sha e of the bod

Validity range shall be checked before useand generally suitable for most jacket typestructures where D/L


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Wave Loa on a Mem er

applied to all members in the offshore structure. It

cylinder like pile of a structure. But in reality, themembers of the offshore structure may behorizontal or inclined in space and can not usedwithout modification



29


TEP Establish Wave Height, Period and Current

Distribution along the depth Establish Wave Theory applicable for H,T,d

s ma on o a er par c e nema csincluding wave current interaction

Establish Marine Growth Establish Wave Kinematics factor Conductor Shielding (if applicable) Current Blockage factor

Morison Equation used to estimate the forces



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or shortens it depending on the direction ofcurrent. This is called Do ler shift.

The apparent wave period need to becalculated to use in the load calculation

Drag term is nonlinear and hence the waterarticle velocities due to wave and current

needs to be added vectorialy before using it

in Morison equation.



31


2

)tanh(2

dL

L

=

22

L

)2

tanh( dL

g



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Following three equations needs to be solved to obtain the Ta

IVLL

+=app

22 L=

)2

tanh( dL

gapp

+=0

)/)(4cosh()(

)/4sinh(

)/4(

d

cIdzLdzzU

Ld

LV

Uc(z) is the current profile elevation z



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on near rag erm n or son equa on

aCD

VVDCFWMwDT

+=

1 2

VwVcV +=

Vc = Current Velocity

Vw = Wave Water Particle Velocity

Example

Lets assume Vc=2m/sec, Vw=3m/sec

If we calculate the drag forces separately, add, we will get 2*2 +3*3 = 13

we a e ve oc es rs an compu e e oa s, we ge

(2+3)*(2+3) = 25It under predicts the forces as much as by 50%



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Source : API RP 2A



35


a er ave nema csAir wave theor is considered in the calculation ofwave kinematics. Consider a progressive wave with

water surface elevation depicted by cosine curve,

cos( )H kx t =

and the corresponding velocity potential is given by:

)sin()(cosh

tkxzhkH

+

=



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Source : API RP 2A



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Source : API RP 2A



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Marine growth around submerged structuralmembers increases the wave/current loads

as the diameter is increased It varies from 50mm to 150mm thickness

along the depth from seabed

At also adds to additional weight

This is to be modelled such that the above istaken in to account

Density of marine growth is around 1300kg/m3



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41


urren oc age ac or

Source : API RP 2A



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Source : API RP 2A



43


AND INERTIA COEFFICIENTS



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d an m

These are empirical Coefficients to be used in

Morison equation and they have been corelated withexperimental data

These coefficients vary due to shape of the structure,surface roughness, flow velocity and direction of flow

API RP 2A has enough information for circularcylinders

DNV recommendation can be used for non-circularshapes



45


d an m

For Smooth cylinders Cd = 0.65, Cm=1.6

For rough cylinders Cd = 1.05, cm=1.2

The values shall be used only if UT/D > 30

For other region of flow, charts available literatureshall be used



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eu egan- arpenter um er

K m 2

=

Where K is Keulegan-Carpenter Number, Um is the maximumvelocity including current and T2 is the duration of half wave



47


eyno s um er

R m=

Where R is Reynolds Number, Um is the maximum velocityincluding current and D is the diameter of the member



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-



51


water particle can be calculated using the following

cosh hkH +

)(sinh

)cos(sinh2

zhkH

txkhx

Vh

+

=

=

)(cosh

s nsinh2

2 zhkHV

xkhz

h

v

+

=

=

cos)(sinh

sinh2

2 tkxzhkHV

a

kht

v

h

+

=

=

sinh2 khtv

Where k is the wave number defined by 2/T, is the wave

c rcu ar requency e ne y , s t e wave engt , an

x is the distance of the point in consideration from origin.



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Consider a case of a surface piercing cylinder such asile of a structure or a le of a acket the combined

drag and inertia force (total force) varies with time

and will be maximum only at one occasion. In order,

maximum force occurs shall be found first.

Let us express the total force on the pile bysu s u ng e ve oc y an acce era on componen sand integrating between the limits (from surface toseabed, i.e. 0 to h)

h

k

kh

khT

HDCF DT

24

)2sinh(

sinh

coscos

2

122

22

+=

kT

HD

sin2

4C-

2

22

M



53


Storm Wave



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T e tota orce wi e maximum w en,

0=

Substituting the values of velocity and accelerationcomponents in to the drag and inertia forceequation and differentiating with respect to andrearranging the terms, we get

= khCD M sinh2

cos

21

max

+D s n



55


Consider a case of horizontal cylinder such as brace of a

acket, the combined drag and inertia force (total force) varies

with time and will be maximum only at one occasion. In order

find the maximum force, phase angle at which the maximum

.

Let us express the total force on the pile by substituting the

velocity and acceleration,

+hzkH )(cosh1 222

=

khDT

sinh4

2

222

kh-CM sinhsin24



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T e tota orce wi e maximum w en,

0=

Substituting the values of velocity and accelerationcomponents in to the drag and inertia forceequation and differentiating with respect to andrearranging the terms, we get

= sinh

sin1

max

khCD M

+cos zD



57


The resultant force on a arbitrarily oriented circular

vector analysis combined with Morison equation

tn

FFF

+=

written as

The force in normal direction an be expressed as:

nnn

inertiaanddratheareandwhere

1

nn

D

FF

FFF +=

ly.respectiveforces



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1=

nnn

D

n

D VVDCF

1 2=

nn

M

n

D aIDCF

where

n =

cylinderthetonormalflowfortcoefficienInertiaC

n

M

D

=

=

seawaterofDensity=

axiscylinderthetonormalparticlefluidofonAcceleratia

n

n

=

=



59


The equation for tangential force can bewritten as

tt

1=

tt

D

2=

n

ttDD

=

D



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ese orces can e summe an expressein terms of cylinder local axis as below:

= tt

t

Dx VVDCF

1

nn 11= yMynDy a

42

+= z

n

Mzn

n

Dz aIDCVVDCF

211



61


Maximum Global Loads

Maximum global loads on a platform

principles

Maximum Base Shear Method

Maximum Overturning Moment Method



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Maximum base shear or maximum total force on a

structure has to be determined for the global analysis of.

force on each member is different and all the locations willnot be attaining the maximum forces. To find themaximum total force a structure, following steps need to

.

Position the wave crest at the origin of the structure as shownin fi ure.

Divide one wave cycle into number of segments either interms of or in terms of length

Compute the wave forces on all members at that instant of

Sum up the forces in horizontal direction for all the members Repeat the calculation in step 4 for all the points for one wave

cycle The maximum of all the total forces computed in step 5 is the

maximum base shear or total force.



63


Maximum Overturning Moment

Maximum overturning moment on a

procedure for the maximum bas shear

members shall be multiplied by the leverarm from mud-line. This shall be summedup an t e proce ure s a e repeate orall the steps in the wave.



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Hydrodynamic Factors

Wave Kinematics factor

This varies between 0.8 to 0.95. This is to be applied

since the calculated wave loading is based twodimensional wave theory while the actual loading isfrom three dimensional wave climate.

Conductor Shielding factor

shielding effect to the conductors behind and thisdepends on the spacing and number of conductors

This current blockage factor is used to account for the

reduction in the current due to the presence of theru ur r r .



65


Water Depth d= 60m

Wave hei ht H= 12m

Wave Period Tapp= 10 Sec

Calculate H/gTa2 =0.012

Calculate d/gTapp2 =0.06

Refer to API RP 2A and the Stokes Wave Theoryis applicable



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H/gTapp2 =0.012

d/gTapp2 =0.06



67


ave urrenDirection

assumed to be acting insame direction

be set to maximize thetotal loads and pile loads

for 4 or 8 legged jacketsand 12 for tripods

-directional depending onthe design requirement



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400

300

350

m)

Shear

Moment

150

200

250

(kN

(or)kN-

0

50

100

(or)Mome

nt

-150

-100

-50

Shear

-200

0 90 180 270 360

Phase angle (deg)

Maximum shear and moment of a single pile along phase angle



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500

600

)

Shear

Moment

300

400

kN(or)kN-

0

100

200

r)Moment(

-200

-100

Shear(o

-400

-300

0 90 180 270 360

Phase angle (deg)

Maximum shear and moment of two pile group along phase angle



71


900

700

-m)

Moment

300

500

t(kN

(or)kN

100

(or)Momen

-300

-

Shear

-500

0 90 180 270 360

Phase angle (deg)

Maximum shear and moment of two p ile group structure along phase angle



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200

150

-m)

ear

Moment

100

(kN

(or)kN

0

50

(or)Momen

-50Shear

-100

0 90 180 270 360

Phase an le de

Maximum shear and moment of a member in two pile group

structure along phase angle26 May 2007 Dr. S. Nallayarasu

Department of Ocean EngineeringIndian Institute of Technology Madras-36

73


Wave slamming predominant in horizontal members

Needs to be taken in to account together with globalloads

Slamming Force Co-efficient is to assumed and therecommended value is 5.5

sswSSVVDCF

2=



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Wave slamming predominant in Vertical members

The wave breaking force coefficient Csis to assumed

as 5.98 for breaking wave and 2.74 for broken wave

to taken as 0.48 for breaking wave and 0.70 forbroken wave

CVb

=

A VVCFbbwbb

=

2092.1C=



75


FLOW PAST HORIZONTAL CYCLINDER



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77


Assymetric Vortices



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79


BOUNDARY LAYER



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LIFT FORCES

sswLLUUDCF =

= dL

.



81


FLOW PAST HORIZONTAL PLATES



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WAVE FORCES ON LARGEBODIES



83


Assumption of No disturbance is not valid

.

Part of Wave reflected once the wave

around

This henomenon is called diffraction

These forces also can be measuredexperimentally

Many research papers exist for differenttypes and shapes of structures



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s r u on o oun ary con onsfor the linear diffraction problem



85


potential can be expressed as follows.

22 ss

=+ in022 yx

The velocity potential due to an incidentmonochromatic wave traveling in the

follows:

)(,

expcosh

kx

kd

=26 May 2007 Dr. S. Nallayarasu


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Boundar conditions

Free surface boundary condition The kinematic boundar condition on = can be

written as

0=

+

+

xxt

which after linearisation by neglecting the nonlinear terms

in equation

0yat0 ==t

where the atmospheric pressure and surface tension aretaken as zero can be written as

at0.2

1==++

yg

t26 May 2007 Dr. S. Nallayarasu


87


The second term in the above equation isnonlinear and is neglected in the presentformulation leading to

s

0yat0 ==+ gt

The kinematic and dynamic boundaryconditions can also be s ecified b combinin

22 sss yator

22 =+

=

+

gyy

g

t



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Body boundary condition The boundary condition on the body boundary should

the body boundary should be equal and opposite to

the incident wave velocity as follows:

oqnn

=

=

Bed boundary condition

sea bed can be specified as a no flow condition asfollows:

0=n



89


Radiation boundary condition

The boundary condition on the left and rightboundaries of the computational domain should

scattered waves. This can be specified using theSommerfeld radiation condition

s=+

x

x



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Diffraction forces can be calculated manually

or s mp e s apes

For Complex shapes Numerical Methodss a e use suc as

Finite Difference Method

n e e emen me o

Source and Sink Method

Many commercial codes including SACS,



91


Solution of Boundary Value Problem for

e oc y o en a

Calculation of Pressures

Integration of Pressures to obtain Forces



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Based on Empirical and Measured values



95




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SEISMIC ANALYSIS

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

Structuralresponse

Horizontalmotion

Verticalmotion



97


STRENGTH LEVEL EQ DESIGN BASIS EQ

DUCTILITY LEVEL E MAXIMUM(2000 YEARS) CONSIDERED EQ

(MCE)



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AND

SEASTATESOIL MODELPSI INPUT

FILE

Seismic Analysis Steps

LINEARISEFOUNDATION

SUPERELEMENT

DYNAMIC ANALYSISMODE SHAPES

& FREQUENCIESDYNINP

DYNAMIC RESPONSERESPONSE

SPECTRA METHOD

SEISMICSPECTRA

API / IS 1893

LOAD COMBINATIONMODEL: CQC

DIRECTIONAL: SRSS

NUMBER CHECKPRST 1.0PRSC



99


Peak Ground Acceleration (PGA) Indicationof Strength of earthquake

PGA = 0.3 X Spectral average between period. . .

Structure

PGA



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AS PER IS: 1893-2000

ZONE SEISMICINTENSITY

ZONEFACTOR

II Low 0.10

IV Severe 0.24

V Very 0.36



101


Earthquake Design Criteria

Specific design criteria depends on the country, location andtype of soil stratum etc.

IS 1893

Design Basis Earthquake. (DBE)

Maximum Considered Earthquake (MCE)

API RP 2A

Strength Level Earthquake (SLE)

Ductility Level Earthquake (DLE)



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Earthquake Design Criteria

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



103


The peak ground acceleration of a particular site depends onthe following parameters.

Distance from epicenter

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

IS 1893 defines PGA as

( ) ( )0.1 0.30.4

Sa SaG Gs sPGA

+=

2



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.



105


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



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Methods of Analysis

Seismic co-efficient method

Time series method

Response spectra method

Response consideration

Vertical acceleration

combination (CQC)

Directional combination to be combined using SRSS.quare oo o um o equences

e.g. ( ) ( ) ( )2 2 2

X Y ZER ER ER ER= + +



107


Seismic Co-efficient method

Design horizontal co-efficient

ZISa=

2Rg

Where

Z =zone factor

I =importance factor

=average response acceleration co-efficientSa

R =response reduction factor

g



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Design of Design Forces

Total base shear can be calculated as

VB=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

2

i ii B n

j j

W hQ V

W h

=

1j=n=number of storage or points of mass located along the height.

Wi=seismic weight of floor, i26 May 2007 Dr. S. Nallayarasu


109

Documents

Loads OFFSHORE