2C09 Design for seismic and climate changes

Lecture 10: Characterisation of seismic motion

Aurel Stratan, Politehnica University of Timisoara 14/03/2014

European Erasmus Mundus Master Course

Sustainable Constructions under Natural Hazards and Catastrophic Events

520121-1-2011-1-CZ-ERA MUNDUS-EMMC

Lecture outline 10.1 Engineering characterisation of ground motion 10.2 Factors affecting seismic motion

2

Engineering characterisation of ground motion Seismic recordings are characterised by a large

variability of their characteristics Engineering parameters:

– amplitude, – frequency content and – Duration of motion

Engineering characterisation of ground motion Analysed earthquake records:

earthquake moment

magnitude Mw

station abbreviation epicentral distance

(km)

distance to fault

(km) soil

Vrancea, 30.08.1986 7.2

Bucharest-Măgurele MAG 134 121

very soft

Vrancea, 30.08.1986 7.2 Carcaliu CAR 148 128 rock

Amplitude parameters: PGA and PGV Peak ground acceleration (PGA) maximum force

induced in very rigid structures Peak ground velocity (PGV) good correlation with

structural damage Disadvantages

– A single value is not characterising appropriately the complex shape of record

– Structural characteristics are not accounted for

Amplitude parameters: PGA and PGV Comparison between Bucharest-Măgurele and Carcaliu Acceleration Velocity

record PGA, m/s2 PGV, m/s VR86-MAG-EW 1.147 0.163 VR86-CAR-EW 0.696 0.048

Amplitude parameters: EPA and EPV Effective peak acceleration (EPA) Effective peak velocity (EPV) Scope: a parameter that is closely related to structural

response and with the damage potential of a seismic recording

There is no unique definition Lungu et al., 2003:

the maximum value of the 0.4 sec moving average of the spectral (pseudo)-acceleration.

0.4max2.5

sPSAEPA

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.00 0.50 1.00 1.50 2.00 2.50 3.00PERIOD, s

SPE

CTR

AL

AC

CEL

ER

ATIO

N (g

)

0.4 sec

Frequency content: response spectra Elastic response spectra:

– Displacement spectra (SD) – Velocity and pseudo-velocity (PSV) response spectra – Acceleration and pseudo-acceleration (PSA)

response spectra Displacement spectra (SD):

peak values of response of elastic SDOF systems with different values of the natural period of vibration and damping

Ti

i

0 1 2 3 40

0.05

0.1

0.15

0.2

TC

TD

T, s

SD,

m

Vrancea, 30.08.1986, Magurele (B), EW

Frequency content: response spectra PSV and PSA spectra

determined from SD

PSV spectra: related to the maximum strain energy induced in the system

PSA spectra: very suggestive for engineers, as it represents the equivalent static force induced in an elastic structure with a unit mass

2PSV SDT

22PSA SD

T

0 1 2 3 40

0.1

0.2

0.3

0.4

0.5

TC

TD

T, s

PSV,

m/s

Vrancea, 30.08.1986, Magurele (B), EW

0 1 2 3 40

1

2

3

4

TC

TD

T, s

PSA

, m/s

2

Vrancea, 30.08.1986, Magurele (B), EW

Frequency content: response spectra Smooth/idealised spectra used in design Control periods TB, TC, TD delimitate zones of

– Constant acceleration: TB

Frequency content: response spectra Comparison between Bucharest-Măgurele and Carcaliu

0 1 2 3 40

0.5

1

1.5

2

2.5

3

3.5

TC

TD

T, s

PSA

, m/s

2

VR86-MAG-EWVR86-CAR-EW

0 1 2 3 40

0.1

0.2

0.3

0.4

0.5

TC

TD

T, s

PSV,

m/s

VR86-MAG-EWVR86-CAR-EW

Record EPA, m/s2 EPV, m/s TC, s TD, s VR86-MAG-EW 1.069 0.164 0.97 1.58 VR86-CAR-EW 0.725 0.036 0.31 1.35

Frequency content: response spectra Seismic motion recorded on the soft soil in Măgurele

(VR86-MAG-EW) has a high frequency content in the intermediate and long period range (larger spectral accelerations and velocities in this interval).

This fact is also reflected by the TC values of the two records (0.97 and 0.31).

0 1 2 3 40

0.5

1

1.5

2

2.5

3

3.5

TC

TD

T, s

PSA

, m/s

2

VR86-MAG-EWVR86-CAR-EW

0 1 2 3 40

0.1

0.2

0.3

0.4

0.5

TC

TD

T, s

PSV,

m/s

VR86-MAG-EWVR86-CAR-EW

Frequency content: Fourier spectra Fourier transform Time domain Frequency domain

– x(ti) is the i-th value of the signal

(taking values between 0 and N-1); – N is the number of values in the signal; – Ck represents the amplitude of cosine functions, while k – their

phase angle.

The Fourier transform provides a two-way connection between the signal in time domain (x(ti)) and in frequency domain (Ck şi k)

Power spectrum density is directly related to the Fourier amplitude spectra and may be expressed as: PSDk=Ck2

/2

0cos 2 /

N

i k kk

x t C ki N

Frequency content: Fourier spectra Higher energy content in the period range of 1-2 sec for

the Bucharest-Măgurele record

0 1 2 3 40

0.5

1

1.5

2

T, s

PSD

, g 2

-s

Vrancea, 30.08.1986, Magurele (B), EW

0 1 2 3 40

0.1

0.2

0.3

0.4

T, s

PSD

, g 2

-s

Vrancea, 30.08.1986, Carcaliu, EW

Frequency content: Fourier spectra Fourier spectra and power

spectrum density are best suited for characterisation of stationary random processes,

Earthquake records are nonstationary random processes

0 1 2 3 40

1

2

3

4

T, s

PSD,

g 2

-s

SIN, Np=1

0 1 2 3 40

10

20

30

T, s

PSD,

g 2

-s

SIN, Np=3

0 1 2 3 4-4

-2

0

2

4

time, s

acce

lera

tion,

m/s

2

SIN

Np=1Np=3

Duration parameters Spectra provide no information

on the duration of seismic action Ground motion duration

increases with earthquake magnitude

Definitions of duration: – interval between first and last

exceedance of a threshold value of (usually 0.05g)

– interval between a built-up of energy of (5-95% or 5-75%) – "significant duration, ts"

Energy an be expressed using arias intensity

2

0

( )2A

I a t dtg

Duration parameters Comparison between

Bucharest-Măgurele and Carcaliu

Significant duration (5-95%)

Record ts, s IA, m/s VR86-MAG-EW 16.0 0.183 VR86-CAR-EW 29.6 0.095

0 10 20 30 40 50-2

-1

0

1

2

-1.15

timp, sac

cele

ratie

, m/s

2

Vrancea, 30.08.1986, Magurele (B), EW

0 10 20 30 40 50-2

-1

0

1

2

-0.70

timp, s

acce

lera

tie, m

/s2

Vrancea, 30.08.1986, Carcaliu, EW

Factors affecting seismic motion The main factors that influence seismic motions can be

grouped in four categories: (1) source factors, (2) path effects, (3) site effects, (4) soil-structure interaction

1

4

2

3

Seismic motion: source factors There are three generally recognized tectonic regimes:

– active regions (inter-plate earthquakes) – the interior of tectonic plates (intra-plate earthquakes) – subduction zones

Inter-plate earthquakes: – large magnitude events, characterised by – large peak ground accelerations, – long durations and – intensities that can affect large areas (hundreds of km). – more energy in the low-frequency range.

Intra-plate earthquakes – lower magnitude, – lower frequency of occurrence, – smaller duration and – smaller affected area.

Seismic motion: source factors Normalised response

spectra in EN 1998-1: – Type 1: for earthquakes with

surface – wave magnitude MS > 5.5

– Type 2: for earthquakes with surface – wave magnitude MS ≤ 5.5

Type 1 earthquakes (large magnitude – long distance events) have a larger frequency content in the long period range than type 2 (local events of small and moderate magnitude)

0 1 2 3 40

1

2

3

4

T, s

S a/a

g

EC8 tip1EC8 tip2

Seismic motion: source factors Seismicity of a source is characterised by

– length (or area) of rupture surface, – probability of occurrence of earthquakes of a given magnitude, – slip rate

Fault types: – Strike-slip fault: are vertical (or nearly vertical) fractures where the

blocks have mostly moved horizontally. – Normal fault: fractures where the blocks have mostly shifted

vertically, while the rock mass above an inclined fault moves down.

– Reverse fault: fractures where the blocks have mostly shifted vertically, while the rock above the fault moves up.

– Oblique fault: the most general case, a combination of vertical and horizontal movement.

Seismic motion: source factors In the case of near-field

ground motions, with the distance to the fault up to 20-60 km, the azimuth of the site with respect to the hypocenter may affect considerably the characteristics of the seismic motion.

The effect of forward directivity is produced when the rupture propagates towards a site and the slip takes place also towards the site

Seismic motion: source factors Ground motion in a site

affected by forward directivity effect has the form of a long duration pulse. This effect is characteristic of the fault-normal component of the ground motion.

Rupture propagates away from the site: backward directivity, characterised by longer duration and lower amplitudes of the seismic motion.

Seismic motion: source factors Velocity of rupture

is close to the shear wave velocity

Forward directivity: an accumulation of energy is observed at the rupture front.

Backward directivity: when the rupture propagates away from the site, seismic waves arrive distributed in time.

Seismic motion: source factors Schematics of fault-normal (FN) and fault-parallel (FP)

components in case of strike-slip earthquakes

Travel path effects Motion recorded in a site will depend on

– focal depth, – source-site distance, – geologic structure between them

Motion recorded in a site is affected by multiple

reflections, refractions, diffractions and interferences, etc.

As the distance to the seismic source increases, – earthquake intensity decreases, – while the duration increases – Importance of vertical component decrease

Local site effects Seismic motion recoded at the surface will be sometimes

substantially different from the one recorded at the base rock.

Schematically, the effect of soil layers beneath the structure may be represented by a dynamic oscillator, which modifies the motion at the base rock depending on its linear and non-linear characteristics.

Investigation methods: – Comparison of two recordings: at the base rock and at the soil

surface – Comparison of

horizontal and vertical components (spectral H/V ratios)

– Analytical procedures

Local site effects: soil classification Surface geology: generally separate materials according

to geologic age (e.g., Holocene-Pleistocene-Tertiary-Mesozoic)

Average shear wave velocity in the upper 30 m (vS,30). Classification depending on vs,30 was adopted by most recent codes.

Geotechnical data, including stiffness, thickness and type of material.

Depth to basement rock (defined as having a shear wave velocity of 2.5 km/s). This parameter is used ti supplement the schemes above, which provide data only fir topmost layers.

Local site effects: intensity Amplification is maximum

(between 1.5 and 4.0) for small intensities of acceleration at the base rock (0.05 - 0.1 g)

Decreases for large intensities of the earthquake (factors around 1.0 for PGArock = 0.4 g)

This effect is attributed to nonlinear response of soft soil at large intensities of the ground motion.

Local site effects: frequency content Stiff soils: amplification of spectral ordinates in the

short-period range Weak soils: amplification of spectral ordinates in the

long-period range Maximum

amplification of response for periods of vibration close to the predominant period of soil layers.

Local site effects: basin effects Soils with horizontal layers

– the incident wave can resonate in the soil layer, but – part of the energy is refracted, limiting the effects of

amplification of seismic waves Basins:

– the seismic wave enters the basin through its edge, – larger than critical incident angles may develop, leading to the

eave being be "trapped" inside the basin. – effects of multiple reflections are amplification of amplitude of

motion and increase in duration.

Local site effects: surface topography Amplification of seismic

motion may be observed as well for irregular topographies, such as crest, canyon, and slope

In case of crests, analytical studies found base/ridge amplifications of 1.2-2.0 for H/Lratios=0.3-0.5

L

H

ridge canyon slope

Soil-structure interaction (SSI) Structural response to free-field motion is influenced by

SSI. – SSI modifies the dynamic characteristics of the structure, and – characteristics of ground motion at the foundation level.

For structures situated on deformable soils, seismic motion at the foundation level is generally different from the one in the "free-field", having an important rotational component, beside the translational one.

The rotational component, and SSI in general, have important effects on rigid structures located on flexible soils.

Another effect of SSI is the dissipation of energy from the foundation to the soil, through radiation of waves and nonlinear response of the soil.

Soil-structure interaction (SSI)

Inertial Interaction: Inertia developed in the structure due to its own vibrations gives rise to base shear and moment, which in turn cause displacements of the foundation relative to the free-field. – Increase in period of vibration of the structure due to

flexibility of the soil – modification (usually increase) of soil damping due to

energy dissipation through radiation of waves and nonlinear response of the soil

Kinematic Interaction: The presence of stiff foundation elements on or in soil cause foundation motions to deviate from free-field motions as a result of ground motion incoherence, wave inclination, or foundation embedment. – reduction of translational component of the ground motion, – increase of the torsional and rotational components, and – filtering of high frequencies of the seismic action.

aurel.stratan@upt.ro

http://steel.fsv.cvut.cz/suscos