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

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;

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.

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)

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?

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

Packaging and sealing and carrying the devices for shipment (L’Aquila Court area)

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?

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

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

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

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.

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?

• 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

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?

SRMD: PERFORMANCE NUMBERS

PLAN AND VERTICAL SECTION OF THE TESTING MACHINE

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,

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?

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.

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.

Carrelli di aeroplani

Horizontal «clover leaf» time-history

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.

SET 1 TEST PROGRAM

The design frequency is: F= 0,15915 Hz

SET 2 TEST PROGRAM

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

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.

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.

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

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?

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?