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THE C.A.S.E. PROJECT TRIAL: TECHNICAL INVESTIGATION
A. De Stefano
Politecnico di Torino, Torino, Italy
Istituzione Centenario Terremoto Marsica
13 gennaio 1915 Anno 2015
AVEZZANO 1915 –2015: CENTO ANNI DI INGEGNERIA SISMICA
Dalla tragedia alle moderne tecnologie per la protezione
sismica
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The C.A.S.E. project • 4600 provisional apartments for People
deprived of the
house by the strong earthquake of April 2009; • 183 reinforced
concrete platforms bearing twenty-five
apartment each; • 3-storey-houses, most with wooden structure; •
7380 sliding pendulum isolators, on steel or (few)
concrete columns;
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A SHORT CHRONOLOGY
• April 6, 2009 – L’Aquila earthquake, Magnitude Mw=6.3. More
than 300 people died. • May 28, 2009 –The Procuring Institution,
governmental Department for Civil Protection
(DPC), invites five Industrial Companies compete for the supply
and laying in work of seismic isolation devices. Three Companies
only accept to participate
• June 4, 2009 – One Company is excluded due to incomplete
documentation. The Selection Committee awards the contract to the
two remaining participants, parting the whole charge, for 5.7 and
3.0 M€ respectively, increased then to 7.1 and 4.2 M€ by
supplementary acts. All the isolating devices are supplied and
accepted in the time range from June 22 to December 16 2009.
• July 13, 2009 –The Technical Service of the Higher Council for
Public Works (STC having a supervision role) warns DPC that the
supplied devices have not yet an official qualification
certificate.
• DPC answers taking charge of it and promising more extended
experimental checks. • Nevertheless, months later a Technician of
th STC proposes to the L’Aquila Justice
Court a complaint file about the quality of the devices giving
start to the trial procedure.
• Coherently with the procedures of the Italian National Norms
The qualification tests are on charge of the two selected Companies
and the Director of the Works (named later DL) acquires the related
documentations (December 2009); from September to December 2009 the
EUCENTRE laboratory in Pavia certifies the verification tests
(prove di accettazione) on 396 samples extracted from the supplied
devices.
• On October 28 2010 the STC certifies officially the
qualification of the seismic isolation devices of the two
Companies.
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APPOINTMENT BY THE JUDGE
• The Judge for Preliminary Investigations committed me on July
21 2011 a technical consulting report in the context of the
“incidente probatorio”, a first step of the judicial procedure
concluded by a public debate, just to decide: to start the second
step, the criminal trial, or to stop definitely the process.
• The report was completed and submitted to the Criminal Court
on July 11, 2012 (De Stefano A., Chiaia B., 2012)
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JUDGE’S QUESTIONS
1. What kind are the seismic isolators put in place? 2. Of which
materials are they made? 3. Are they compliant or different with
respect to
the offer of the Companies to the Contracting Authority and to
the content of the contract? if different, how significantly?
4. Are they effectively working, well performing and suitable
for the purpose they were designed for?
5. Do they respect the technical norms standing at the
construction time?
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ACTIVITIES TO ANSWER THE QUESTIONS • Acquisition, study and
critical analysis of the available documentation (Chiaia, ARCOS
and myself);
• Sample extraction: a total number of 9 samples, 3 and 6
respectively for the two selected Companies (in proportion with the
number of devices installed by each Company) extracted from the
construction site;
• Physical-chemical analyses on extracted samples to identify
and classify the constituting materials (Politecnico di
Torino);
• Static and dynamic tests on the devices for the determination
of the “static” and “dynamic” friction coefficients and in general
for the verification of the good overall functioning of each
apparatus and to verify the consistency of test results with the
qualification and acceptance certificates (SRMD Laboratory, San
Diego, USA).
EXTRACTION OF DEVICES
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Packaging and sealing and carrying the devices for shipment
(L’Aquila Court area)
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Chosen type of isolators:
Single or double concave sliding surface devices (2/3 and 1/3 of
the global supply respectively)
Single surface
Double surface «slider»
Question 1 - What kind are the seismic isolators put in
place?
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Upper and lower restrained plates
Upper sliding interface (peerless steel)
Slider
Polymeric sliding material
A single concave sliding surface isolator (Example image from
the net - producer not involved in the trial)
Spherical hinge
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How to model?
Usual modelling assumes a linear equivalent damping depending on
energy dissipated per cycle
WvuR
WF )sgn(
minmax
minmax
DD
FFK
eff
22
aveff
eq
DfK
EDC
FF F
Fmax
Fmin
Dmax Dmin
2
minmaxDD
Dav
EDC = Energy Dissipated per Cycle = area enclosed in one
cycle
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A double concave sliding surface isolator (Example image from
the net - producer not involved in the trial)
Upper sliding interface (peerless steel) Slider
Polymeric sliding material
Upper restrained plate
Lower sliding interface (peerless steel)
Lower restrained plate
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Effects of isolation on the acceleration spectrum
0 0.5 1 1.5 2 2.50
0.2
0.4
0.6
0.8
1
1.2
1.4
Campano Lucano 23.11.1980 18:34 - direzione x
T (s)
Sa
(g)
smorz. 2%
smorz. 5%
smorz.10%
smorz.20%
Relative damping ratios
The seismic isolation allows a longer fundamental vibration
period and consequent much lower acceleration levels at the cost of
a larger relative displacement. The higher damping helps to reduce
the spectral ordinates.
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TESTS ON MATERIALS AT POLITECNICO DI TORINO: Polymeric sliding
layer
Laboratory for Plastic and Polymeric Materials, Alessandria
• Thermo-gravimetric analysis (TGA) to identify the polymeric
matrix;
• Scanning Electronic Microscopy (SEM) to the morphologic
characterization of the specimen and to evaluate the size, shape
and quantity of inorganic embedded particles;
• Differential Scanning Calorimetry(DSC) to identify the thermal
behaviour of the material
• Fourier Transform based Infrared Spectroscopy (FT-IR) on
attenuated total reflectance (ATR) to identify the chemical nature
of polymers
Question 2 - Of which materials are they made? Question 3 - Are
they compliant or different with respect to the offer of the
Companies to the Contracting Authority and to the content of the
contract? if different, how significantly? Question 5 - Do they
respect the technical norms standing at the construction time?
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• Measure of the thickness of the layer (not less than 2.5 mm as
imposed by contract and technical norms)
• Chemical analysis for identification of steel alloy weight
percentage composition using Optical Emission Spectrometry
• (Requested: X2CrNiMo17-12-2 EN 10088/AISI 316L. Some other
alloy compositions can suffer pitting and fragility phenomena due
to lack of stability of austenitic structures. A limited amount of
carbonium and the addition of a useful quantity of stabilizing
elements like Mo, Ti, Nb can guarantee a batter durability and
resistance to fatigue).
TESTS ON MATERIALS AT POLITECNICO DI TORINO: Peerless steel
sliding interface layer
Laboratory for Metallurgy, Turin
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MECHANICAL TESTS ON SPECIMEN: SRMD Laboratory, UC San Diego,
CA
• UCSD-SRMD Facility, San Diego (USA)
Question 4 - Are they effectively working, well performing and
suitable for the purpose they were designed for? Question 5 - Do
they respect the technical norms standing at the construction
time?
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SRMD: PERFORMANCE NUMBERS
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PLAN AND VERTICAL SECTION OF THE TESTING MACHINE
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The Lab for Materials and Structures of Politecnico di
Torino verifies all the certification documents
related to the calibration and validation of actuators and
sensors of the SRMD Facility,
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Calibration of the vertical force actuator
Calibration of the vertical load actuators as certified and
checked on site by the staff of Politecnico di Torino using
calibrated load cells:
Error of the vertical load related to the corresponding force
level (%)
Force [kN] Relative accuracy error (%)
500 ±2
1000 ±1
3000 ±1
Let’s now compare the above shown calibration with the label
error percentage:
0.5% of 53,400 kN means 267 kN (far from 2% of 500 kN). So,
then?
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Some considerations about the vertical force actuator resolution
and the apparent contraddiction of the previous page
• The calibration table above shows that the potential
resolution of the actuator is significantly higher than the label
value.
• The vertical force is imposed by 4 independently controlled
jacks each one with the same label resolution relative error.
• The main 4 jacks are not the only-ones; other vertical
actuators control the pitch effect and have a regularization
effect.
• Due to the pitch control the four main jacks are forced to
operate in parallel. We can suppose with conservative mind that
inside each minimum graduation of force allowed by the metric
resolution the real effective force values have uniform
distribution and equal probability.
• Forcing the jacks to work in parallel the global distribution
changes to normal, the probability in no more equal and the
standard deviation significantly decreases, leading to a better
sensitivity and resolution around the nominal force value.
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Qualification tests for the seismic isolators
Italian Technical Norms – NTC 2008
• Slow tests- quasi-static con alternate linear imposed
displacement growth (evaluation of the «static» friction)
• «Dynamic tests» – Harmonic, with period near to the design
period
European Technical Norms – EN 15129
• Not so far from the Italian-ones, but including the possible
choice of bi-directional clover leaf, relevant to a better
evaluation of the friction coefficiant and tha equivalent viscous
damping.
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Carrelli di aeroplani
Horizontal «clover leaf» time-history
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Test program for SRMD
Two different sets of tests were defined: The first-one (SET 1
or SERIES 1) is mostly following the qualification procedures of
NTC 2008. The second-one (SET 2 or SERIES 2) follows too the rules
of NTC 2008, but it starts with a simulated earthquake and a clover
leaf displacement history (EN 15129). The simulated earthquake is
imposed as displacement history on the base of a recorded local
motion (L’Aquila April 6, 2009) kindly supplied by INGV; the aim is
to explore how the isolators behave after a quake shock real but
significantly less intense than the design earthquake.
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SET 1 TEST PROGRAM
The design frequency is: F= 0,15915 Hz
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SET 2 TEST PROGRAM
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The recorded L’Aquila earthquake the 3 components of the
recorded ground motion (Courtesy of INGV)
A concern: the composed horizontal projection is highly
directional and non-symmetric
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The «stick-slip» During the experimental campaign in San Diego
one of the isolators was broken in the early runs of protocol
Series 1 (see figure 9, De Stefano A., Chiaia B., 2012). It is an
event no more secret, newspapers and television gave public
information about it. Moreover, the broken device had been cast
with the polymeric material partially different than the most of
other devices supplied by the same Company.
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Challenge for the investigating staff : understand why that
accident happened.
• An help from the video-camera record of the runs.
• There was an absolutely evident stick-slip behavior. Not all
the samples shown it, but some of them did.
• During some particular conditions of motion the convex and
concave sliding surfaces seem to be temporarily glued together.
• Then the shear force wins and a sudden sliding motion
occurs.
• Such mechanism can happen again and again, many times.
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Why stick-slip can be
destructive?
(the destructive behaviour takes place when the spheric
hinge
below has a too small curvature.
A larger curvature allows
a much better control
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More about the technical norms • No mention of it in NTC
2008:
• the E.U. Standard (EN 15129, 2010) refers to it in the
following way: “Under all loading conditions, the movement in the
sliding surfaces shall be smooth and without producing any type of
vibrations such as those induced by the stick-slip phenomenon. The
fluctuation of the horizontal force shall be within a range of +/-
5 % of the average restoring force, at any level of bearing
displacement. …” (EN 1512, 8.3.1.2.6, Isolation
characteristics).
• In fact the stick slip phenomenon can play a really
destructive role, as shown shortly in the previous figure, role
that the excerpt of EN15129 cited above does not really
suggests.
• A further comment is important: the broken specimen, as just
reminded above, was cast with polymeric materials partially
different than the other specimen extracted to be tested.
Nevertheless the commercial name was the same. It suggests a
concern of general interest: the CE mark can be equivalent to an
homologation by the National Technical Service, but it requires
previously the ETA certification (European Technical Authorization)
qualifying the constituting materials. An intriguing question
arises at this point: in a general context can a technological
device obtain the CE mark on the base of its commercial name and an
ETA qualification for a given constituting material and then use
the CE mark while having the same name but being cast using
different materials?
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A further, conclusive, important remark
the broken specimen, as just reminded above, was cast with
polymeric materials partially different than the other specimen
extracted to be tested. Nevertheless the commercial name was the
same. It suggests a concern of general interest: the CE mark can be
equivalent to an homologation by the National Technical Service,
but it requires previously the ETA certification (European
Technical Authorization) qualifying the constituting materials. An
intriguing question arises at this point: in a general context can
a technological device obtain the CE mark on the base of its
commercial name and an ETA qualification for a given constituting
material and then take advantage of the CE mark while having the
same name but being cast using different materials?