ambient vibration analysis for the characterization of soil and

Transcription

SAHC2014 – 9th International Conference on
Structural Analysis of Historical Constructions
F. Peña & M. Chávez (eds.)
Mexico City, Mexico, 14–17 October 2014
AMBIENT VIBRATION ANALYSIS FOR THE CHARACTERIZATION
OF SOIL AND COVERINGS OF VILLA DEI MISTERI IN POMPEII
I. Bergamasco1, G. Bongiovanni2, B. Carpani3, P. Clemente2, A. Paciello2, S. Serafini2
1
Soprintendenza Speciale Beni Archeologici Pompei Ercolano Stabia,
Via Villa dei Misteri 2, 80045 Pompei, Italy
[email protected]
2
ENEA, Casaccia Research Centre
Via Anguillarese 301, 00123 Rome, Italy
[email protected], [email protected], [email protected], [email protected]
3
ENEA, Brasimone Research Centre
Loc. Brasimone, 40032 Camugnano, Italy
[email protected]
Keywords: experimental dynamic analysis, operational modal analysis, cultural heritage.
Abstract. Villa dei Misteri, one of the most famous domus in the ancient city of Pompeii, was
protected by building reinforced concrete frames roofs; in recent years timber and steel structures have been preferred. The evaluation of their health status requires a detailed study by
means of a multidisciplinary approach, which should include geometrical surveys, damage
assessment based on both in situ and laboratory diagnostic tests, UAV (Unmanned Aerial Vehicles) remote sensing to inspect area and coverings not easy to reach in safe, etc. In this paper the preliminary results of ambient vibration measurement, as basis for seismic safety
assessment are presented. The evaluation of the structural capacity, to be compared to the
seismic hazard at the site, is not easy due to the presence of quite different elements, such as
the original masonry walls, the concrete frame roofs as well as wooden and steel structures.
15 seismometers connected to a recorder were used in two different layouts. The first one was
used to analyse the dynamic characteristic of the soil with the underground structures and the
effects of the trench, which surrounds the Villa at three sides In the second layout sensors
were deployed on the structural elements of the coverings to analyse their behaviour and the
interaction with the original masonry walls.
.
I. Bergamasco, G. Bongiovanni, B. Carpani, P. Clemente, A. Paciello, S. Serafini
1
INTRODUCTION
Villa dei Misteri is one of the most famous domus in the ancient Roman city of Pompeii, a
UNESCO World Heritage Site and one of the most popular tourist attractions in the world. It
is located just outside the archeological area of Pompeii and is so called for its frescoes depicting mystery rites (Figure 1). The excavations of 1909-10, led early on by Giulio De Petra,
the archaeologist director then in office, and the systematic research of the years 1929-1930,
carried out under the direction of Amedeo Maiuri, revealed a great quadrangular plant rich in
about 90 rooms, oriented from East to West and built on land slope with daring jumps level
won through earthworks terraces and porches along the South and East sides.
The Villa is in a particular topographical situation, being partially surrounded by a moat of
about 3.5 m height, on the West, North and South sides. The outer part of the moat, on North,
East and West sides, is an escarpment with a height up to 8 m. The moat reveals the presence
of a cryptoporticus below the floor of the Villa, which suggests that the body of the building
is extended downward, and the whole monument is partially within the grounds surrounding it.
Figures 2 shows a porch and the escarpments on the East side and the moat and cryptoporticus
on the West side. In the decades after the Second World War the structures of the Villa were
protected by reinforced concrete roofs (Figure 3). In recent years timber and steel structures
have been preferred for the same purpose. At the present the analysis of the health status of
the structures is necessary [1]. With this aim a multidisciplinary approach has been used,
which includes historical analysis, geometrical and structural surveys, damage assessment
based on both in situ and laboratory diagnostic tests, UAV (Unmanned Aerial Vehicles) remote sensing to inspect area and coverings not easy to reach in safe, and vibration analysis to
characterize the dynamic behaviour of the soil and some coverings structures [2].
According to the Italian seismic classification, the seismic hazard at the site of Pompeii,
expressed in terms of expected PGA on rigid soil (Vs > 800 m/s), ranges from 0.133 g, with
10 % exceedance probability in 50 years, up to 0.219 g, with 2 % exceedance probability in
50 years. Aside from the question of the hazard level to be used for this type of structures, it is
apparent that the evaluation of the structural capacity, to be compared to the seismic hazard at
the site, is not easy due to the presence of quite different elements, such as the original masonry walls, the concrete roofs as well as timber and steel structures.
In this paper the preliminary results of ambient vibration measurements, as basis for seismic safety assessment are shown. Two layouts were considered. In the first one sensors were
deployed in the three orthogonal directions at five locations on the ground. The second layout,
was devoted to obtain information on specific structural elements, such as reinforced concrete
beams and roofs, wooden beams, and their relation with the original masonry walls.
Figure 1: Frescoes depicting mystery rites
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Ambient Vibration Analysis for the Characterization of Soil and Coverings of Villa dei Misteri in Pompeii
Figure 2: Porch and escarpment on the East side (left) and the moat on the North and West side (right)
Figure 3: General view of the coverings (left) and typical timber roof (right)
2
INSTRUMENTATION LAYOUTS AND TESTS
The measurement campaign was carried out on October 28th and 29th, 2013. The following
equipment was used:
 15 velocimetric sensors Kinemetrics SS-1, short-period (1 Hz nominal), connected
by wires to a centralized system of acquisition;
 1 Kinemetrics Granite recorder, 24-channel, 24 bit A/D converter, 200 Hz sampling
rate, GPS timing.
Considering the topographical features previously discussed, it was decided to make measurements in two configurations: the first (Layout1) was dedicated to the soil and particularly
to the effects of the moat, the second (Layout2) was relative to the structure, or, more correctly, to some portions of the structure. The two configurations are described in Figure 4.
3
DATA ANALYSIS
3.1 Layout 1
The RMSs for each channel were calculated with a 0.08 sec moving window. The maximum values of RMS are in figure 5. It is worth noting that the maximum value in the transversal direction are always higher than the corresponding ones in the longitudinal direction,
e.g., RMS of ch13 is higher than that of ch15.
Spectral ratios as the square root of the PSDs between the horizontal and the vertical components (HVSR) for each measurement point were calculated, in order to point put any stratigraphic amplifications according to the Nakamura’s method [3], even though the stratigraphic
3
conditions are largely far from the conditions required by method. Figure 6 shows the ratios
for the sensors at the top of the moat (ch10, ch11 and ch12, and ch13, ch14 and ch15). There
is significant amplification of the ratios around 9.5 Hz for both the sites, higher for the second
one where 8.5 Hz is also present.
Figure 4: Sensor layout in the two configurations. Numbers with * refer to Layout 2.
In order to point out any amplification effects between the moat and/or cryptoporticus level
and the upper level, the calculation of spectral ratios between the homologous components
was carried out. Figure 7 shows the relevant graphs. No amplification was detected for the
vertical channels. For horizontal channels there are no individual peaks, but amplifications are
apparent with factors up to 4 in the range of frequencies between 9.5 and 15 Hz for the channels ch10 and ch12 compared to the homologous ch01 and ch03, respectively. The peak at 9.5
Hz in y direction is present only for ch10. Channels ch13 and ch15 compared to the homologous ch07 and ch09, and ch04 and ch06, show a range of amplification at frequencies up to 18
Hz. The amplification of the channels ch10 and ch12, compared to ch04 and ch06, respectively, takes place in the range between 9 and 15 Hz with factors up to 8.
4
RMS max (mm/sec)
0.12
0.08
0.04
ch15
ch14
ch13
ch12
ch11
ch10
ch09
ch08
ch07
ch06
ch05
ch04
ch03
ch02
ch01
0.00
Sensor
Figure 5: Maximum values of RMS for each sensor
4
4
Pompei - Villa dei Misteri - Layout1
ch10/ch11
ch15/ch14
3
Ratio
Ratio
ch13/ch14
ch12/ch11
3
2
1
2
1
0
0
0
5
10
f (Hz)
15
20
0
5
10
f (Hz)
15
20
Figure 6: HVSR at the top of the moat
12
10
ch13/ch04
ch10/ch04
12
ch11/ch05
8
ch14/ch05
ch15/ch06
ch12/ch06
Ratio
Ratio
16
6
8
4
4
2
0
0
0
6
5
10
f (Hz)
15
20
0
8
10
f (Hz)
15
20
ch10/ch01
ch13/ch07
ch11/ch02
4
5
6
ch14/ch08
ch12/ch03
Ratio
Ratio
ch15/ch09
4
2
2
0
0
0
5
10
f (Hz)
15
20
0
5
10
f (Hz)
15
Figure 7: Spectral ratio between homologous components at the top and the bottom of the moat
5
20
PSD (mm/s)^2/Hz)
Analyses carried out so far, while showing amplification of the complex soil-structure surrounded by the moat at some frequencies, do not allow to extract any significant conclusion.
So the cross-spectral analysis between selected pairs of signals was performed. The common
element is the value generally very low of coherence and a non-significant phase angle (≠ 0°,
≠ ± 180°). Only for the couple ch10-ch13 (Figure 8), the coherence values were acceptable in
the range between 9 and 15 Hz but no the phase angles.
The analysis highlights the following aspects:
• the front portion of the Villa, surrounded by the moat, is characterised by the presence
of the cryptoporticus;
• an amplifying behaviour, in the range between 9 and 20 Hz, is present along the two
main direction of the whole soil/structure;
• amplifying behaviour can be attributed to the soil-structures system, as non-linear
summation of the motion of the present masonry walls, visible and not, and the motion
of the portions of soil that include the masonry walls.
8.E-06
6.E-06
ch10
ch13
4.E-06
2.E-06
0.E+00
9
11
13
15
f (Hz)
6.E-06
1.0
COHER
4.E-06
0.5
2.E-06
0.E+00
Coherence
CSD (mm/s)^2/Hz)
CROSS
0.0
9
11
13
15
f (Hz)
180
CROSS
PHASE
4.E-06
90
0
2.E-06
Phase (°)
CSD (mm/s)^2/Hz)
6.E-06
-90
0.E+00
-180
9
11
13
15
f (Hz)
Figure 8: PSDs and CSD of ch10 and ch13
3.2 Layout 2
The power spectral densities of all the records (Figure 9) show the following features:
• there is a wide variety of amplitude and frequency content;
• even for close sensors, such as the horizontal ones ch02 and ch08 and the vertical ones
ch03 and ch07, amplitude and frequency content are significantly different;
6
ch01
1.E-06
5.E-08
0.E+00
15
2.E-03
1.E-03
5.E-04
10
f (Hz)
15
20
ch10
1.E-04
0.E+00
20
ch03
5
2.E-04
0
PSD (mm/S)^2/Hz)
10
f (Hz)
PSD (mm/S)^2/Hz)
2.E-06
5
0
20
5
10
f (Hz)
15
6.E-05
20
ch11
4.E-05
2.E-05
0.E+00
0.E+00
15
3.E-05
ch04
2.E-05
1.E-05
0.E+00
5
10
f (Hz)
15
2.E-06
0
20
PSD (mm/S)^2/Hz)
10
f (Hz)
2.E-06
1.E-06
5.E-07
0.E+00
10
f (Hz)
15
20
ch12
1.E-05
5.E-06
0.E+00
20
ch05
5
2.E-05
0
PSD (mm/S)^2/Hz)
5
5
10
f (Hz)
15
2.E-05
20
ch16
1.E-05
5.E-06
0.E+00
0
5
10
f (Hz)
15
4.E-07
20
ch06
3.E-07
2.E-07
1.E-07
0.E+00
0
PSD (mm/S)^2/Hz)
PSD (mm/S)^2/Hz)
PSD (mm/S)^2/Hz)
PSD (mm/S)^2/Hz)
15
4.E-06
0
PSD (mm/S)^2/Hz)
10
f (Hz)
ch02
0
PSD (mm/S)^2/Hz)
5
6.E-06
0
5
10
f (Hz)
15
4.E-05
20
ch17
2.E-05
0.E+00
0
PSD (mm/S)^2/Hz)
ch09
0.E+00
0.E+00
0
5
10
f (Hz)
15
3.E-04
20
ch07
2.E-04
1.E-04
0
5
10
f (Hz)
15
4.E-04
20
ch18
2.E-04
0.E+00
0.E+00
0
PSD (mm/S)^2/Hz)
1.E-07
PSD (mm/S)^2/Hz)
2.E-06
PSD (mm/S)^2/Hz)
PSD (mm/S)^2/Hz)
5
10
f (Hz)
15
2.E-05
0
20
5
ch08
1.E-05
0.E+00
0
5
10
f (Hz)
15
20
Figure 9: PSDs of records in Layout 2
7
10
f (Hz)
15
20
ch18 shows a single resonance frequency, but there is no close sensor to compare
with;
• sensors at the lowest level (ch04 ch05 and ch06) show one peak at 1.4 Hz with very
low amplitude, which is more relevant for ch04. The same sensors were present in
Layout 1, but this frequency was not present.
Cross spectral analysis of ch02 and ch08 (Figure 10), shows that there are several common
frequencies, in phase and with high values of the coherence function, that cannot be attributed
to any particular behaviour, but can be considered as part of propagation through different
structures. Cross spectral analysis of ch03 and ch07 (Figure 10) shows that their frequency
content is very different, probably due to the different stiffness of their locations. In fact, ch07
was on a beam while ch03 was at about mid span between two beams.
•
1.2E-03
ch02
8.0E-06
ch08
ch03
8.0E-04
ch07
4.0E-04
2.0E-04
0.0E+00
15
6.0E-06
0
20
1.0
CROSS
2.0E-06
0.0E+00
Coherence
0.5
0.0
0
5
10
f (Hz)
15
-90
0.0E+00
CSD (mm/s)^2/Hz)
Phase (°)
2.0E-06
-180
0
5
10
f (Hz)
15
8.0E-05
0.5
4.0E-05
0.0
1.6E-04
90
0
1.0
COHER
0
180
PHASE
20
0.0E+00
CROSS
4.0E-06
15
CROSS
1.2E-04
20
6.0E-06
10
f (Hz)
1.6E-04
COHER
4.0E-06
5
5
10
f (Hz)
15
20
180
CROSS
PHASE
1.2E-04
90
8.0E-05
0
4.0E-05
-90
0.0E+00
20
Coherence
10
f (Hz)
Phase (°)
5
CSD (mm/s)^2/Hz)
0
CSD (mm/s)^2/Hz)
1.0E-03
6.0E-04
4.0E-06
0.0E+00
CSD (mm/s)^2/Hz)
Pompei - Villa dei Misteri -Layout2
PSD (mm/s)^2/Hz)
PSD (mm/s)^2/Hz)
1.2E-05
-180
0
5
10
f (Hz)
15
20
Figure 10: PSDs and CSDs of couples ch02-ch08 and ch03-ch07
An interesting aspect of the analysis is the presence of low frequencies at the lowest monitored point (ch04, ch05 and ch06) inside the cryptoporticus. This frequency could play an important role in case of an earthquake. In figure 11 the time-histories in terms of RMS with
moving window of 0.5 sec are compared. The presence of temporally localized loads that last
up to several tens of seconds can be attributed to vehicular traffic and those of longer duration
to trains from the nearby railway.
To assess whether the frequency content at 1.4 Hz depends on the these loads, timefrequency analysis has been carried out on ch04, both for Layout1 for Layout 2, in order to
detect if these frequencies are present on all the recording (Figures 12 and 13). It is apparent
8
that there is not a direct correlation between the traffic and the low frequency content, as well
as it is apparent the effect of train transit on the frequency range previously analyzed.
RMS (mm/s)
RMS (mm/s)
ch04
ch03
ch02
ch01
0
400
800
t (s)
1200
1600
ch05
0.04
0.03
ch04
0.02
0.01
0
RMS (mm/s)
0.05
0.04
0.03
0.02
0.01
0
ch05
RMS (mm/s)
0.05
0.04
0.03
0.02
0.01
0
Pompei Villa dei Misteri
Layout2 RMS 0.5 s
0.01
0.008
0.006
0.004
0.002
0
RMS (mm/s)
RMS (mm/s)
RMS (mm/s)
0.05
0.04
0.03
0.02
0.01
0
RMS (mm/s)
0.025
0.02
0.015
0.01
0.005
0
RMS (mm/s)
RMS (mm/s)
Pompei Villa dei Misteri
Layout1 RMS 0.5 s
0.02
0.016
0.012
0.008
0.004
0
0.4
0.3
ch03
0.2
0.1
0
0.03
0.02
ch02
0.01
0
0.12
0.08
ch01
0.04
0
0
400
800
t (s)
1200
1600
Figure 11: RMSs, moving window, 0.5 sec
Figure 12: Time-frequency analysis of ch04, Layout 1
4
CONCLUSIONS
The dynamic behavior of the Villa of the Mysteries is characterized by:
• the presence of soils that include the underground structure and by the moat that surrounds partially it; these circumstances determine amplification with factors ranging
from 2 to 8 in the frequency range 9-15 Hz, between the lowest monitored point and
the top of the moat both for the horizontal and the vertical component;
9
the variety of roof structures, concrete frames wood and steel, added to protect the
original masonry and the frescoes; these structural elements, both for vertical and horizontal motion, have significantly different behavior in terms of frequency content and
amplitude; frequency content extends from 5 up to 15 Hz, then includes the frequencies observed for the soils.
Dynamic excitation arise mainly from the close railway. Frequency content shows the
presence of significant peak at about 1.4 Hz, higher in direction of the railway, that could play
a crucial role in case of earthquake.
•
Figure 13: Time-frequency analysis of ch04, Layout 2
ACKNOWLEDGMENTS
The activities here described were carried out in the framework of an agreement between
the Soprintendenza Speciale Beni Archeologici Pompei Ercolano Stabia and ENEA.
REFERENCES
[1] Bergamasco I., Carpani B., Clemente P., Papaccio V. (2012). "Seismic preservation of
archeological sites: the case of Pompeii". 8th Int. Conf. on Struct. Analysis of Historical
Constr., SAHC (Wroclaw, 15-17 Oct), Paper No. 195.
[2] Carpani B., Marghella G., Marzo A., Candigliota E., Immordino F., Bergamasco I.
(2014). A Methodology for the Safety Assessment of Protective Roofs Covering Archaeological Sites: the Case of the “Villa dei Misteri” at Pompei. 9th Int. Conf. on
Struct. Analysis of Historical Constr., SAHC (Mexico City, 14-17 Oct).
[3] Nakamura Y. (1989). A method for dynamic characteristics estimation of subsurface using microtremor on the ground surface. QR of RTRI, Vol. 30, No. 1.
10

ambient vibration analysis for the characterization of soil and

Transcription

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