2C09 Design for seismic and climate changes

Transcription

1 2C09 Design for seismic and climate changes Lecture 10: Characterisation of seismic motion Aurel Stratan, Politehnica University of Timisoara 07/04/2017 European Erasmus Mundus Master Course Sustainable Constructions under Natural Hazards and Catastrophic Events CZ-ERA MUNDUS-EMMC

2 Lecture outline 10.1 Engineering characterisation of ground motion 10.2 Factors affecting seismic motion 2

3 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

4 Engineering characterisation of ground motion Analysed earthquake records: earthquake Vrancea, Vrancea, moment magnitude Mw 7.2 station Bucharest -Măgurele abbreviation epicentral distance (km) distance to fault (km) MAG soil very soft 7.2 Carcaliu CAR rock

5 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

6 Amplitude parameters: PGA and PGV Comparison between Bucharest-Măgurele and Carcaliu Acceleration Velocity record PGA, m/s 2 PGV, m/s VR86-MAG-EW VR86-CAR-EW

7 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. SPECTRAL ACCELERATION (g) sec max PSA EPA s PERIOD, s

8 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 SD, m i T i Vrancea, , Magurele (B), EW 0.2 T D 0.05 T C T, s

9 Frequency content: response spectra PSV and PSA spectra determined from SD PSV 2 SD T PSA PSV spectra: related to the maximum strain energy induced in the system 2 2 T SD PSV, m/s Vrancea, , Magurele (B), EW 0.5 T D T C T, s PSA spectra: very suggestive for engineers, as it represents the equivalent static force induced in an elastic structure with a unit mass PSA, m/s 2 Vrancea, , Magurele (B), EW T C T D T, s

10 Frequency content: response spectra Smooth/idealised spectra used in design Control periods T B, T C, T D delimitate zones of Constant acceleration: T B <T<T C Constant velocity: T C <T<T D Constant displacement: T>T D T C EPV EPD 2 TD 2 EPA EPV PSA PSV SD T B T C T D T T B T C T D T T B T C T D T

11 Frequency content: response spectra Comparison between Bucharest-Măgurele and Carcaliu PSA, m/s T C T D VR86-MAG-EW VR86-CAR-EW T, s PSV, m/s T C T D VR86-MAG-EW VR86-CAR-EW T, s Record EPA, m/s 2 EPV, m/s T C, s T D, s VR86-MAG-EW VR86-CAR-EW

12 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 T C values of the two records (0.97 and 0.31). PSA, m/s T C T D VR86-MAG-EW VR86-CAR-EW T, s PSV, m/s T C T D VR86-MAG-EW VR86-CAR-EW T, s

13 Frequency content: Fourier spectra

14 Time domain Frequency content: Fourier spectra Fourier transform Frequency domain N /2 i k k k 0 cos 2 / x t C ki N x(t i ) is the i-th value of the signal (taking values between 0 and N-1); N is the number of values in the signal; C k 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(t i )) and in frequency domain (C k şi k ) Power spectrum density is directly related to the Fourier amplitude spectra and may be expressed as: PSD k =C k 2

15 Frequency content: Fourier spectra Higher energy content in the period range of 1-2 sec for the Bucharest-Măgurele record Vrancea, , Magurele (B), EW Vrancea, , Carcaliu, EW s 2 -s PSD, g 1 PSD, g T, s T, s

16 Frequency content: Fourier spectra Fourier spectra and power spectrum density are best suited for characterisation of stationary random processes, Earthquake records are 1 nonstationary random processes PSD, g 2 -s SIN, Np=1 T, s acceleration, m/s SIN Np=1 Np=3 PSD, g 2 -s SIN, Np= T, s 3 4 time, s

17 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, t s " Energy an be expressed using arias intensity 2 IA a () t dt 2g 0

18 Duration parameters Comparison between Bucharest-Măgurele and Carcaliu Record t s, s I A, m/s VR86-MAG-EW VR86-CAR-EW Vrancea, , Magurele (B), EW Significant duration (5-95%) acceleratie, m/s timp, s 2 Vrancea, , Carcaliu, EW acceleratie, m/s timp, s

19 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

20 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.

21 Seismic motion: source factors Normalised response spectra in EN : Type 1: for earthquakes with surface wave magnitude M S > 5.5 Type 2: for earthquakes with surface wave magnitude M S 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) S a /a g T, s EC8 tip1 EC8 tip2

22 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. falie inversă falie normală falie transcurentă falie oblică

23 Seismic motion: source factors In the case of near-field ground motions, with the distance to the fault up to 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

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

25 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

26 Seismic motion: source factors Schematics of fault-normal (FN) and fault-parallel (FP) components in case of strike-slip earthquakes

27 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

28 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

29 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 (v S,30 ). Classification depending on v s,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.

30 Local site effects: intensity Amplification is maximum (between 1.5 and 4.0) for small intensities of acceleration at the base rock ( g) Decreases for large intensities of the earthquake (factors around 1.0 for PGA rock = 0.4 g) This effect is attributed to nonlinear response of soft soil at large intensities of the ground motion.

31 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.

32 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.

33 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 for H/Lratios= L H ridge canyon slope

34 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.

35 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 causes displacements of the foundation relative to the freefield. 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.

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