site-specific seismic studies for optimal structural design - a case study (2012)

Prof. Dr. Llambro DUNI

Polytechnic University of Tirana Institute of Geosciences, Energy, Water and Environment

Departament of Seismology

Dr. Faruk KABA InfraTransProject Ltd., Albania

Prof. Dr.Neki KUKA Polytechnic University of Tirana

Institute of Geosciences, Energy, Water and Environment Departament of Seismology

Site-specific seismic studies for optimal structural design: A case study

1. Introduction Seismicity and seismic hazard of Albania

2. Code requirements for bridge design Seismic action into the KTP-N.2-89 code Seismic action requirements according to the EC8 standard

3. Application to the bridge design. A case study Assessment of the horizontal and vertical UHRS Deaggregation of seismic hazard at the Viaduct site construction Development of ground motion time histories


Introduction: Seismicity and seismic hazard of Albania

Distribution of the earthquake epicenters in Albanian and surrounding area (510 B.C.31/12/2010, MW4.0),


Intense microseismic activity (1.0 < M 3.0) Many small earthquakes (3.0 < M 5.0) Seldom by moderate size earthquakes (5.0 < M 7) Very seldom by strong earthquakes (M > 7.0)

From the evidences we possess today, it results that Since the period of IIIII century B.C. up to now, Albania was striken by: 55 strong earthquakes with intensity Io VIII degree

15 of them have had intensity of Io IX degree From these 55 earthquakes of a period of more than

2000 years, 36 belongs to the 19-th century

Seismic zonation map of Albania

(Sulstarova et al., 1980)

Probabilistic Seismic hazard map of Albania

(Duni & Kuka 2010)


The seismic action in the KTP-N.2-89 design code is expressed by an elastic ground acceleration response spectrum:

Sa(T) = kE (T) g kE is the so-called seismic coefficient, (T) is the dynamic coefficient , g is the acceleration gravity Both kE and (T) are dependent on local soil conditions Introducing the coefficients kr (building importance coefficient) and (ductility and

damping structures coefficient), the design acceleration values are obtained. Values of various parameters defining the spectral shape of (T) curves

Code requirements for bridge design:Seismic action into the KTP-N.2-89 code

00.5

11.5

22.5

3

0 1 2 3T(sec)

Dyn

amic

coe

ffici

ent

Category ICategory IICategory III

Soil category TC(sek) TD(sek) ( 0TTC ) (TCTTD) (TDT)

I 0.30 1.08 2.3 0.7/T 0.65 II 0.40 1.23 2.0 0.8/T 0.65 III 0.65 1.69 1.7 1.1/T 0.65

The structure design is performed following two methods: (i) the response spectrum method; (ii) acceleration time-history method Acceleration time history selection is recommended to be based on site-specific

seismic studies. The amplitude of the chosen accelerograms should not be lower than the value kEg.

For lifeline systems (railways, roads, bridges, etc.) some specifications are introduced: The value of the vertical component of acceleration should be specfied in the case of

bridge design Specific engineering-seismological studies for the definition of kE and parameters for

the tunnels with large length, etc. are recommended

Code requirements for bridge design:Seismic action into the KTP-N.2-89 code

The seismic hazard is described in terms of reference peak ground acceleration on type A ground, agR, with reference return period (RP) 475 years of the seismic action for the no-collapse requirement.

The effect of soil conditions on the seismic action is accounted for through seven ground types A, B, C, D, E, S1 and S2

The earthquake motion at a given point on the surface is represented by: (i) an elastic ground acceleration response spectrum, called elastic response spectrum o Two types of spectra are recommended: Type 1 and Type 2. If the earthquakes, that

contribute most to the site seismic hazard, have a surface-wave magnitude Ms not greater than 5.5, it is recommended that the Type 2 spectrum is adopted.

o Recommendations are given in EC8 for the five ground types A, B, C, D and E and values of the parameters S, TB, TC and TD, as well.

Code requirements for bridge design: Seismic action requirements according to the EC8 standard

The earthquake motion at a given point on the surface is represented by: (ii) time-history of the earthquake motion.

For this kind of presentation, artificial, recorded or simulated accelerograms of the earthquake motion can be used.

The general rules for their use are as follows: 1. Artificial accelerograms shall be generated so as to match the elastic response

spectra for 5% viscous damping ( = 5%). 2. Duration of the accelerograms shall be consistent with the magnitude and the other

relevant features of the seismic event underlying establishment of ag. 3. When site-specific data are not available, the minimum duration Ts of the stationary

part of the accelerograms should be equal to 10 sec.


4. The suite of artificial accelerograms should observe the following rules: 4.1 a minimum of three accelerograms should be used; 4.2 the mean of the zero period spectral response acceleration values (calculated

from the individual time histories) should not be smaller than the value of agS for the site in question;

4.3 in the range of periods between 0.2T1 and 2T1, where T1 is the fundamental period of the structure in the direction where the accelerogram will be applied; no value of the mean 5% damping elastic spectrum, calculated from all time histories, should be less than 90% of the corresponding value of the 5% damping elastic response spectrum.


Bridge design has some specific requirements A site-dependent, horizontal and vertical elastic response spectrum should be specified

for the design. The horizontal component depends on the ground type and should be applied at the

foundation of the supports of the bridge. Near source effects, describing the directivity phenomenon of the earthquakes when

the site is located within 10 km horizontally of a known active seismogenic fault that may produce an event of moment magnitude higher than 6.5, should be assessed.

At least three pairs of horizontal ground motion time-history components shall be used.

The ensemble spectrum shall be scaled so that it is not lower than 1.3 times the 5% damped elastic response spectrum of the design seismic action, in the period range between 0.2T1 and 1.5 T1, where T1 is the natural period of the fundamental mode of the structure in the case of a ductile bridge, or the effective period (Teff.) of the isolation system in the case of a bridge with seismic isolation.


Near source effects, describing the directivity phenomenon of the earthquakes when the site is located within 10 km horizontally of a known active seismogenic fault that may produce an event of moment magnitude higher than 6.5, should be assessed


-40

-20

0

20

40

0 0.5 1 1.5 2 2.5 3

Koha (sek)

Nxi

timi (

cm/s

/s)

TIR-04-1 E-W komp (TIR3)

-70

-35

0

35

70

0 0.5 1 1.5 2 2.5 3

Koha (sek)

Nxi

timi (

cm/s

/s)

TIR-04-1 E-W komp (TIR2)

0.01

0.1

1

10

0.01 0.1 1Perioda (sek)

PSRV

(cm

/sec

)

TIR-04-1(TIR3)

Earthquake Code

Station Code

Acceleration(cm/s/s) Velocity (cm/s) Displacement (cm)

Z E-W N-S Z E-W N-S Z E-W N-S

TIR-04-1 TIR2 27.88 -46.87 -3.99 -0.358 0.639 -0.081 -0.009 -0.011 -0.003

TIR-04-1 TIR3 4.05 35.47 7.33 -0.101 1.121 -0.248 -0.004 -0.043 -0.010

0.01

0.1

1

10

0.01 0.1 1Perioda (sek)

PSRV

(cm

/sec

)

TIR-04-1 (TIR2)

0.01

0.26

0.51

0.76

0.1 0.35 0.6 0.85

Period (sec)

PSRV

(cm

/sec

)

Map of active faults of Albania (Aliaj et al, 2000) Blue: Middle Pleistocene-Holocene; Green: Pliocene-Lower Pleistocene; Red:Pre-Pliocene, active also during Pliocene-Quaternary

-0,6

-0,3

0

0,3

0,6

0 2 4 6 8 10 12

Acce

lera

tion

(g)

Time (sec)

E - W comp

-0,2

-0,1

0

0,1

0,2

0 2 4 6 8 10 12

Acce

lera

tion

(g)

Time (sec)

N - S comp


48 m high and 160 m long viaduct at Bulqiza-Ura e Vashes road section.

Application to the bridge design: A case study Assessment of the horizontal and vertical UHRS

Probabilistic evaluation of seismic hazard for rock conditions expressed in the form of: (i) horizontal peak ground acceleration (PGA), (ii) vertical PGA, (iii) 5% damped, uniform hazard response spectra (UHRS)

Period

(sec)

Spectral acceleration, g

RP=95 years

RP=145 years

RP=475 years

RP=975 years

RP=2475 years

PGA 0.179 0.202 0.270 0.316 0.383

0.10 0.259 0.299 0.432 0.527 0.677

0.20 0.344 0.395 0.560 0.682 0.860

0.30 0.304 0.348 0.499 0.613 0.781

0.50 0.196 0.227 0.334 0.415 0.543

1.00 0.082 0.096 0.146 0.185 0.248

2.00 0.044 0.052 0.079 0.100 0.133 0 0.2 0.4 0.6 0.8 1

0.1 0.3 0.5 0.7 0.9

Nxitimi spektral, g

1E-005

0.0001

0.001

0.01

0.1

1

Frek

uenc

a vj

etor

e e

tejk

alim

it

LegjendaPGASA 0.1sSA 0.2sSA 0.3sSA 0.5sSA 1.0sSA 2.0s

PP 975 vjet

PP 475 vjet

PP 95 vjet

PP 2475 vjet

PP 5000 vjet

PP 145 vjet

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 1 2 3 4 5

Perioda (sek)

Nxi

timi s

pekt

ral (

g)

Spektri elastik horizontal i reagimit i Tipit te Isipas EC8 per truallin e tipit A; Ag=0.270 g;Shuarja 5%Spektri elastik horizontal i reagimit me rrezikuniform (RP=475 vjet)

Identification of the earthquake scenarios (distance-magnitude pairs) through the procedure known as seismic hazard deaggregation.

This analysis was performed for two periods of vibrations: SA=1.0 sec and SA=2.0 sec, vibration periods of the viaduct: T1=1.815 sec on the transversal direction and T2=1.428 sec on the longitudinal one.

Application to the bridge design: A case study Deaggregation of seismic hazard at the Viaduct site

SA

Modal event Mean event

Magnitude Distance Epsilon Magnitude Distance Epsilon

SA 1.0 s 6.8 4.6 1.03 6.62 5.2 1.41

SA 2.0 s 5.8 5.8 0.93 6.26 21.5 1.15 Probabilistic Seismic Hazard Deaggregation

Rruga e Arberit - Viadukti (9+850)(Lat=41.4648N, Lon=20.1373E)SA period 1.0sec. Acceleration 0.146 gMean Return Period: 475 yearsMean (R,M,e 0) = 5.2 km, 6.62, 1.41 Modal (R,M,e 0) = 4.6 km, 6.80, 1.03 from peak R,M binMean (R,M,e*) = 4.6 km, 6.80, eps interval: 1 to 2 sigma %c=9.1Binning: DeltaR=10km, deltaM=0.2, deltae=1.0

c)

Probabilistic Seismic Hazard DeaggregationRruga e Arberit - Viadukti (9+850)(Lat=41.4648N, Lon=20.1373E)SA period 2.0 sec. Acceleration 0.079 gMean Return Period: 475 yearsMean (R,M,e 0) = 21.5 km, 6.26, 1.15 Modal (R,M,e 0) = 5.8 km, 5.80, 0.93 from peak R,M binMean (R,M,e*) = 6.1 km, 5.80, eps interval: 1 to 2 sigma %c=3.3Binning: DeltaR=10km, deltaM=0.2, deltae=1.0

d)

Application to the bridge design: A case study Development of ground motion time histories

Generally, for important projects, input not covered by Code guidelines is required.

The structural engineer may perform time domain analysis that requires acceleration time histories instead of the spectral acceleration input.

When soil-structure interaction is accounted for, typically a profile of ground accelerations and displacements versus depth is required.

The same holds for evaluation of slope stability risk and calculation of dynamic earth pressures.

In cases of liquefiable soils, analyses would be performed to study the effects of this phenomenon on a proposed structure.

In all this examples, a site-specific study would be necessary to provide the required input.

Site-specific seismic studies for optimal structural design: A case study CONCLUSIONS

A site-specific seismic hazard analysis using the probabilistic approach is

performed for a viaduct site construction at the Bulqiza-Ura e Vashes road section. Besides calculation the uniform hazard response spectra for different safety levels,

the earthquake scenarios that have high likelihood of occurrence at this site are also identified.

Then, the earthquake ground motion time histories are developed using the stochastic point-source method.

The obtained results can be used to derive structural design parameters for each usage and performance of the structure.

Site-specific seismic studies for optimal structural design: A case study CONCLUSIONS

Thank You !


Slide Number 1Slide Number 2Slide Number 3Introduction: Seismicity and seismic hazard of AlbaniaIntroduction: Seismicity and seismic hazard of AlbaniaIntroduction: Seismicity and seismic hazard of AlbaniaIntroduction: Seismicity and seismic hazard of AlbaniaSlide Number 8Slide Number 9Slide Number 10Slide Number 11Slide Number 12Slide Number 13Slide Number 14Slide Number 15Slide Number 16Slide Number 17Slide Number 18Slide Number 19Slide Number 20Slide Number 21

site-specific seismic studies for optimal structural design - a case study (2012)

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