Dynamic control of historical buildings through interferometric radar technique.

Transcription

1 . An useful approach for Structural Health Monitoring on earthquake damaged structures. Sergio Vincenzo Calcina, Luca Piroddi and Gaetano Ranieri Università di Cagliari Dipartimento di Ingegneria Civile, Ambientale e Architettura DICAAR 32 Convegno Nazionale GNGTS Trieste, novembre 2013

2 Theoretical Background of the radar The IBIS-S sensor detects the position and the displacement of target points placed at different distances from the equipment using two well-known radar s: 1. the Stepped-Frequency Continuous Waves (SF-CW), adopted to resolve the scenario in the range direction, i.e. to detect the position in range of different targets placed along the radar line of sight LOS; 2. the Interferometric, implemented to compute the displacement of each target comparing the phase information of the back-scattered electromagnetic waves collected at different times.

3 SFCW is based on the transmission of sweep consisting of a series N monochromatic electromagnetic pulses of T tone duration (Continuous Waves) at different frequencies (Stepped Frequency). Δf= frequency step f 0 f 1 =f 0 + Δf f 2 = f 1 + Δf = f 0 + 2Δf f n =f 0 + n Δf Stepped-Frequency Continuous Waves (SF-CW)

4 The transmitted SFCW signal: f(t) Sweep duration T tone Next sweep f 0 f 0 +Df f 0 +2Df f 0 +(N-1)Df t I 0 +jq 0 I 1 +jq 1 I 2 +jq 2 I N-1 +jq N-1 At the end of a sweep IBIS-S system has acquired a vector of complex numbers that corresponds to a frequency sampling of the observed scenario.

5 F(f) n-th frequency Virtual B f [Hz] The SFCW allows IBIS to obtain the same performance of a synthetic pulse of duration 1 B to which corresponds a range resolution of D R c 2 c 2B

6 The Stepped Frequency - Continuous Wave allows the resolution of the scenario along range direction independently from the distance. IBIS Range Resolution up to 0.75 m One-dimensional radar profile Range direction: straight direction from IBIS to the target; Range resolution: capability to distinguish two targets in the range direction; Range bin: range resolution area.

7 Time-Frequency Duality The interferometric analysis provides data on object displacement by comparing phase information, collected in different time periods, of reflected waves from the object. The IBIS sensor is able to evaluate the displacement for each range-bin resolved by mean of SFCW.

8 The phase ambiguity bounds the maximum displacement measurable between two consecutive acquisition to ± λ/ mm?

9 IBIS-S radar sensor: capabilities Maximum distance: 1Km (static surveys), 500 m (dynamic sampling frequency of 40Hz) Maximum sampling frequency: 200 Hz One Dimensional radar imaging Range resolution: 0.75 m independent from the range Measurement of the displacement of the entire scenario at the same time Displacement accuracy: up to 0.01mm Ku band system Band of 200 MHz, between 17.1GHz and 17.3 GHz

10 IBIS-S radar sensor: capabilities The Ku band IBIS-S system is provided with two identical IBIS-ANT3 antennas operating in vertical polarisation and characterised by a maximum gain of 19dBi.

11 β=los angle The displacement is measured in LOS direction (Line Of Sight) target-sensor. To calculate the true displacements: -acquisition geometry; -identification of natural reflectors.

12 Church of San Giacomo e Filippo ( ) - San Giacomo Roncole This structure was damaged by the earthquakes of 20 th and 29 th May The damage consists in a collapse of the top and in many cracks at the base.

13 Morphology of the southern side. The dashed lines are fractures directly observed by visual inspection and along which the maps show a clear color discontinuity. By Pesci et al. (2012). Study of the base where there is a crack extended to the whole horizontal section. The deformation maps show slippage of the body of the bell tower fractured area and to identify clearly the contours.

14 Stations of radar sensor

15 The IBIS system was configured with the following acquisition parameters: Antenna type 3 (Gain max = 19 db); Survey duration: min; Max distance R max : m; Distance resolution ΔR: 0.75 m; Sampling frequency: Hz.

16 Scenario -Location St01 Scenario - Location St02 Scenario - Location St03

17 Interometric Data Processing: 1) Radar pixel selection: selection of the echoes characterized by better signal to noise ratio, attributable to reflections from structural and decorative details of the building, such as eaves, arches and balustrades and other natural backscatterers. The identification of the different back-scatterers points on the structure was performed taking account of the range bin in which is included the peak selected (i.e. the distance between the point and the sensor) and the position and inclination of the radar antenna respect to the base of the tower.

18 Step 1: interpretetation of the echos Dynamic control of historical buildings Reflection point on the structure Radar configuration St2 Horizontal distance 23 m Central LOS angle 30

19 Step 2: Frequency Domain Decomposition method (Brincker et al. 2001) The FDD shows a frequency response recognizable to about 0.96 Hz, obtained by integrating over 15 minutes of acquisition Top Base Frequency analysis of the natural oscillations induced by the wind and the other sources of random vibrations

20 Comparison between the time-series at the several floors of the building f ( t) 0.15sin t

21 Vibration time history and frequency analysis of the tower (55 th range bin) f=0.96 Hz Time Domain Fourier Domain (f 0 =0.96 Hz) f=0.96 Hz

22 IBIS-S configuration St01 52 nd range bin shows further frequency peaks, probably related to the asymmetrical distribution of the loads within the structure or in the top of the building.

23 IBIS-S configuiration St03 Effect produced by the vibrations of the cable 22 nd range bin

24 First experimental mode shape (eigen-frequency 0.96 Hz) Displacement projected in the horizontal direction

25 Italian Code (NTC-08): Several empirical relationships available to estimate the Spanish Code (NSCE-02): fundamental frequency of masonry structures provide results in the range between 1.4 and 1.5 Hz. The significant difference (31-36%) between the Linear relationship proposed by Faccio et al. (2010): Rainieri et al. (2012): experimental value and the estimated values could be imputed to the damage caused by the earthquake. It is generally recognised that the period of vibration grows while increasing the mass of the vibrating system and while reducing the stiffness.

26 Finite Element Model: comparison between the period of vibration of the tower before the earthquake (not damaged condition) and obtained experimentally on the structure damaged. Finite Element Model Subdomain Setting Young modulus 2800 MPa Density 1800 Kg/m 3 Poisson ratio 0.2

27

28 Mode 1: 1.22 Hz simple bending (X direction)

29 Mode 2: 1.23 Hz simple bending (Y direction)

30 Mode 3: 5.64 Hz bending (XY direction)

31 Mode 4: 5.64 Hz bending (YX direction)

32 Mode 5: 6.22 Hz first torsional

33 Mode 5: 6.22 Hz first torsional

34 Mode 6: 9.89 Hz

35 Mode 7: Hz

36 Mode 8: Hz - bending

37 Mode 9: Hz - bending

38 Mode 10: Hz - torsional

39 Conclusions o o o This approach has proven to be effective for vibration monitoring of damaged structures. By means of the Ground Based Radar Interferometry we have assessed main vibration properties of the building after a few minutes of recording, highlighting levels of the building characterized by anomalous vibration amplitudes and probably related to earthquake damages. In general, this method may be used to measure displacements ranging from a few microns up to several millimetres for large structures, ranging from thin and tall structures to other types of buildings (towers, skyscrapers and bridges). The possibility of working remotely makes this approach suitable for the dynamic control of buildings that have reported structural damage after an earthquake, especially for civil structures of strategic interest during the emergency and for cultural heritage buildings.