PROJECT OF LARGE SCALE EARTHQUAKE TESTING FACILITY
Jean-Claude Quéval1), Thierry Payen1).
1)Engineer, Seismic Mechanical Study Laboratory, CEA/DEN/DANS/DM2S/SEMT, CEA-Saclay, 91191 Gif-sur-Yvette Cedex France, (jcqueval@cea.fr, thierry.payen@cea.fr)
ABSTRACT
In the last decade, significant progress in earthquake engineering has been made. Nevertheless, the deficiencies in our understanding of induced phenomena during earthquake require an intensive research which must be based on experimental results simulating real conditions on structures during earthquake, and validating the numerical simulation tools.
Furthermore, the need for experimental studies is pushed by the increasing of the global vulnerability of our societies and the evolution of design approaches asking to performance evaluation of structures. This is applicable to existing and new constructions
In this context, CEA have launched a new project for his seismic facility TAMARIS in order to fulfil to the R&D requirements in the next decades. The objective of this paper is to describe the outline of future research topics in the seismic domain in relation to the new project of large scale earthquake testing facility.
The objective of this paper is to describe the outline of research topics in seismic domain for the next decades in relation to the new project of large scale earthquake testing facility (E_FAST). A description of the outline of the new testing facility is given.
INTRODUCTION
The earthquakes that occurred worldwide in the last decade have shown significant progress in earthquake engineering. Nevertheless, these events have all indicated important deficiencies in our understanding of induced phenomena during earthquakes. As a consequence, intensive seismic research is necessary, which must be based on some experimental results, simulating the conditions actually occurring in a real earthquake.
Indeed, the need for experimental studies is also to be found in the safeguarding of the European Cultural Heritage and in seismic retrofit of the vast stock of housing and strategic public service buildings. The increasing of the global vulnerability of our societies will push to extend seismic protection of non structural components. For societal response to earthquakes; test results are essential in these fields.
Furthermore, new construction concepts and reinforcement or retrofitting techniques, new requirements in seismic reevaluation of buildings cannot be fully assessed without disposing of appropriate testing and facilities to validate them on full-scale constructions models. For this purpose, failure mechanisms and collapse process of various kinds of full scale structures must be investigated.
Likewise, the refined numerical models reflecting the non linear inelastic material and structural behaviors, has to be verified on large scale testing facility.
In consequence, CEA have launched an extension project of his facility in order to fulfill these new requirements. This facility is expected to become a powerful tool for European or international collaboration in seismic engineering research.
The objective of this paper is to describe the outline of research topics in seismic domain for the next decades in relation to the new project of large scale earthquake testing facility. A description of the outline of the new testing facility is given
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OUTLINE OF THE RESEARCH TOPICS
EMSI Laboratory of CEA/Saclay is involved in seismic testing of civil engineering structures and mechanical equipment, with a unique testing facility, TAMARIS, with AZALEE, the largest 6 degrees of freedom table in Europe (6 m X 6 m with 100 tons payload). In parallel, EMSI Laboratory has a large experience in numerical approaches for simulation (CAST3M code). Many national and international research programs (CAMUS, ECOLEADER, …) have been performed successfully but testing and research program needs are evolving in the earthquake engineering field.
Therefore, CEA have launched a thought on a seismic research program in the next decades related to new testing needs. The different fields of earthquake engineering are presented:
Test and analysis of reinforced concrete structures
The purpose of seismic simulation tests of large scale structures is to investigate a three dimensional dynamic response up to collapse and failure mechanism of real structures, and to obtain data for establishment of three dimensional numerical simulations which can assess and predict the dynamic behavior of a structure with sufficient accuracy. For that objective it is necessary to have non linear models for each type of structures (reinforced concrete (RC) frames, masonry, walls, steel structures, etc.) associated with acceptance criteria or damage indicators for different parameters. Furthermore, the experimental result can lead to the assessment of safety margin within re-examination framework or the developments of advanced method for evaluation of the earthquake resisting capacity of structures, the new structure systems which aim at improvement of seismic performance and aseismic reinforcement of the existing structures. To perform test on real size object or large scale models of structures, it is desirable to have the large scale three dimensional shaking table, test could be performed to give new light on the mechanism of dynamic failure using real structures.
Test and analysis of soil structure interaction
Soil behavior in the vicinity of structure during earthquake is an important factor in the response of structures and facilities, first by the characteristics of subsoil that may vary during the earthquake (in case of liquefaction for instance) and by the modification of the response of the structure (Soil Structure Interaction, especially for massive structures on soft soils). SSI can have very important effects on the dynamic performance of the superstructure, and it is crucial that these effects are accounted for in Performance Based Engineering.
In parallel, there are many analytical tools to study those effects, but there is a need of experimental validation, especially for non linear behavior.
The interaction between structure and soil is rarely included in testing as it is traditionally thought to reduce forces. However, it increases displacements, affecting the displacement spectrum to be used in displacement-based design. The problem has been pointed by high level technical committees (i.e. Eurocode committee) which identified soil-structure interaction as one of the areas in need of urgent research. Full-size dynamic tests on shaking tables considering the dynamic interaction of a structure with a realistic soil model is not, at present, realistic without building new large shaking tables. Nonetheless, testing with sub-structuring method, offers a realist alternative, whereby a large-scale model of a structure can be tested with the soil sub-structured and represented by appropriate constitutive models that take into account its stiffness and damping in the dynamic range. Tests of this type have not been performed to date due to difficulties in physically reproducing the appropriate boundary conditions (rotational degrees of freedom) between the structure and the soil and in obtaining reliable constitutive models of the soil.
There are two ways to address the Soil Structure Interaction (SSI) testing: experimental approach using a laminar box or a particular application of dynamic sub-structuring, in which the formulation and implementation could be performed. Current shaking tables in Europe are incapable of simulating SSI without the use of a physical soil foundation, but the latter are necessarily of limited extent due to payload considerations. The new SSI capability will enable foundations of up to semi-infinite extent to be simulated as a numerical substructure.
In the future, prediction the dynamic response of soil structure interaction will require a large scale earthquake testing facility using a laminar box and adaptive control system for performing testing and validate computational methods in non linear field. The phenomena will be investigated are: liquefaction, uplift and effect of embedded structures.
Harmful of the seismic input signal
In the last decade, recent earthquakes have shown that their seismic level can be higher important than the reference earthquake used at the design stage for a specific site. Therefore, it is obviously important to have a good knowledge of the input signal for analyses and tests. The mains questions are related to the definition of the seismic input, the properties of the temporal input signal damaging capacity signal and the variability in the response of structures associated with the input signal characterization and consequences on the definition and the number of accelerograms. The objective of this research program was to develop criteria to choose suitable earthquake signals to be used in non linear dynamic analyses for seismic design of ductile structures such as RC structures and to investigate the parameters involved in the damaging capacity of signals. Indeed, the correlation between damage and earthquake characteristics is not well known. For example, the peak ground acceleration correlates badly with structural response.
The challenge in the future is to establish relation between the structural damage and the characteristics of the acceleration time history influence the system’s displacement or ductility demand.
Development of sub structuring method and real time control system
Coupling between test and analysis is an essential theme of the CEA Earthquake Engineering Laboratory for long time. Previous RTDs, such as NEFOREEE, allows developing first step by visiting real time control of shaking tables. The
next step needed to improve and enhance their capabilities is the dynamic sub structuring, allowing generalizing the inclusion of analytic part in the dynamic test, to imagine distributed testing in different facilities.
The developing trend towards Performance Based Engineering in seismic and other kinds of dynamics engineering has created an urgent need to improve the scope and quality of physical specimen testing over the complete strain range, from small strain elastic behavior through to large strain collapse behavior, with detailed characterization of the development of failure mechanisms, crack patterns yield zones etc. Such non-linear system behavior is difficult to reproduce accurately at small model scales, and this creates the need for testing at large or prototype scales using prototype materials and construction techniques.
Shaking tables are an essential experimental tool in earthquake engineering since they are the only means available for applying true inertia loads to a specimen. However, for cost reasons, normal shaking tables have payload capacities that preclude testing of complete large-scale or prototype specimens such as buildings and bridges, and which force researchers to resort to smaller scale models and their associated difficulties. The new NIED shaking table near Kobe in Japan, built at a cost of over $200 millions, has a payload capacity of 1200 tons so that it can shake a complete, full-scale, multi-storey building. The table1 shows the principal testing facilities in the world.
In Europe, the LNEC and CEA Saclay shaking tables, with 60 tons and 100 tons payload capacities respectively are the largest multi-degree of freedom tables in Europe. The new mono axial testing facility EUCENTRE in Pavia (Italy), allows into account for testing some large specimens at high level of excitation. The table # 2 shows the principal testing facilities in west Europe.
Institution Payload (tons)
Peak acc (g) (bare table )
Peak velocity (m/s)
Stroke (± mm)
Degrees of freedom
Size (m) Reaction wall along the table(s)
NIED (Japan) 1200 1.7 2 1000 6 15 x 20 no
NUPEC (Japan) 1000 2 15 x 15 no
University of San Diego (USA)
400 4.7 1.8 750 2 12 x 7.6 no
Public Works Research Institute (Japan)
272 6 8 x 8 no
New Saclay testing facility (France)
100 or 200 with 2 tables
7 2 1000 6 2 time
6 x 6 or 6 x 12
L shaped 20 x 12
TAMARIS (Saclay) (France)
100 1.6 1 125 6 6 x 6 4 x 3
SEESL Buffalo (USA)
50 or 100 (2 tables)
4 1.25 150 6 2 time 3.6
x 3.6
2 walls 12.5 - 7 CHRI (Chongqing)
China
35 or 70 (2 tables)
3 0.8 150 6 2 time
3x6
no
Berkeley (USA) 45 6 6.1 x 6.1 no
Table 1
Institution Payload (tons)
Peak acc (g) (bare table condition) Peak velocity (m/s) Stroke (± mm) Degrees of freedom Size (m) Reaction wall along the table(s) New Saclay testing
facility (France)
100 or 200 with 2 tables
7 2 1000 6 2 time
6 x 6 or 6 x 12
L shaped 20 m x
12m CEA/Saclay
(France)
100 1.6 1 125 6 6 x 6 4 m x 3 m
EUCENTRE (Italy) 60 5 1.5 500 1 7 x 5.6 no
LNEC (Portugal) 40 1.8 0.2 175 3 5.6 x 4.6 no
University of St Cyril and Methodius (Macedonia)
36 3 5 x 5 no
CESI-ISMES (Italy) 30 3 0.55 100 no
Univ. of Naples (Italy) 20 1 1 250 2 3 x 3 or 3
x 6
no
U. Bristol (UK) 15 4.54 0.7 150 6 3 x 3 no
ENEA (Italy) 10 3 1 250 6 4 x 4 no
NTUA (Greece) 10 2 1 100 6 4 x 4 no
Table 2
In order to overcome some of these payload capacity limitations of normal shaking tables, researchers in Europe, the US and Japan have been researching a new test technique, known as dynamic sub structuring, in which only a physical component of a structure or system is tested, with the remainder of the structure being modeled numerically within the shaking table controller. The controller ensures compatibility of the interface tractions and displacements between the physical and numerical substructures. In principle, this technique allows larger sized physical components to be tested on a shaking table, which in turn helps to address the non linear performance issues discussed above.
In different universities (Bristol, Oxford, Trento and Kassel universities), are acknowledged world leaders in the development of the associated dynamic sub structuring control technologies. We have established the essential theoretical control frameworks.
This work is now being evaluated for adoption at various shaking table sites belonging to the $80 millions Network for Earthquake Engineering Simulation (NEES) in the US and at the NIED shaking table in Japan. These control developments have now reached a stage where robust first generation production level implementations are viable. The overall objective of CEA research during the next years is to perform the necessary development to achieve implementation on the Azalee shaking table and enhance performance on Tamaris facility in the first step before introducing in the future facility.
These enhanced facilities will impact research practice by enabling for the first time, for example, simulation of soil-structure interaction (SSI) effects without the need for physically modeling the extensive foundation domain. As another example, in combination with additional actuators reacting off reaction walls or frames, the shaking tables will be able to conduct tests on single panels of masonry in filled frames in which the out-of-plane inertia forces on the masonry panel are correctly induced by the shaking table motion while the in-plane dynamic shear forces, arising from the dynamic sway of the building frame, are provided by the additional actuators that are connected to a numerical representation of the rest of the vibrating building. These are just two examples of important classes of experimental problems that are very difficult, if not impossible, to conduct at present, but which will become viable on existing shaking tables with this new dynamic sub-structuring technology.
We will ensure that the user community is kept abreast of the sub-structuring developments and, through appropriate training opportunities, is in a position to take advantage of it as soon as it is deployed. The results of this research will improve European competitiveness in wider areas, including: robust adaptive control design and implementation for improved machine performance, advanced test-machine control systems using dynamical sub-structuring techniques and improved control of servo-hydraulic actuation systems. Thus, there will be an impact on the key industries within the automotive, aerospace, railway, materials, as well as in the civil engineering and utilities sectors.
In parallel, CEA considers necessary to enhance the performance in terms of real time control improvement and to develop and implement robust multivariable controllers that will compensate for dynamic uncertainty, changes in the dynamic parameters and non-linear behavior in the actuators and specimen, plus the dynamic coupling between the various axes of motion, which is induced by the test facility (shaking-table(s)) and test specimen.
Some contacts and collaborations are in process to investigate new testing concepts, new controllers, with some European and US Universities and hydraulic component builders.
Test and analysis of equipments
In the context of re-evaluation or construction of new installations, the Seismic Probabilistic Risk Assessment (SPRA) developed by EPRI allows the evaluation of the seismic failure mode and fragility methodology. The seismic fragility methodology is the methodology for calculating the seismic capacity of structures and equipments, and from that capacity the fragility curve of each equipment for example. For equipment, a failure mode means the inability to perform its safety function. To develop a fragility curve, there are three types of information that can be relied on: data from real earthquake, analysis and test data. For equipment, reliance on test and experience data is the common approach.
In consequence, tests on shaking table are necessary and require very high level acceleration to establish the fragility curve of equipments. Furthermore, this approach enables to assess the current safety margin between real seismic capacity and this assessed with design rules.
OUTLINE OF THE NEW TESTING FACILITY (E-FAST)
Fulfilling these new requirements in terms of testing and research program needs in the earthquake engineering field, a performance analysis of hydraulic components has been performed. Indeed, designing an earthquake simulation system to operate under a broad range of conditions is an exercise in optimization and design. These systems must operate with a variety of specimen configurations, between low and high level accelerations and over a wide frequency range. The facility will be used to reproduce dynamic behavior of reduced or full-scale structure models subjected to seismic or dynamic loadings.
The first step in the optimization is to decide on the system configuration. The basic cinematic specifications of the feasibility study are listed below. The specimen size is 100 tons per table and the table size is 6 meter X 6 meter as AZALEE shaking table. The new testing facility is composed of two tables which could be either uncoupled or coupled and moveable from 0 m to 20 m. The required performances for horizontal and vertical motions summarized in the table 3, lead to high performance actuators.
Payload 200 tons (2 x 100 tons)
Size 2 x (6 m x 6m)
Mass on one table 38 tons
Distance between tables 0 to 20 m
Electric Power 3500 KVA
Continuous flow 7000 l/min – 8000 l/min
Additional accumulators 13000 l
Shaking Directions XY horizontal Z vertical
Maximum acceleration ± 2,0 g ± 2,0 g
Maximum velocity ± 2 m/s ± 2 m/s
Maximum displacement ± 1,0 m ± 0,7 m
Number and type of actuators 2 (4 x 1850 kN-1320kN) 2 (4 or 5 1300 kN)
Oil column frequency (Table+100 tons) 6 Hz 11 Hz
Lateral frequency of actuator 22 Hz to 50 Hz 22 Hz to 50 Hz Table 3
Several configurations have been studied (figures # 1 to 2). These different studies show that the required performances are possible and the principal difficulties of the project are:
- the length of the actuators and the associated lateral frequencies (22 Hz to 50 Hz), - the maximum rotation for the swivels,
- the oil column frequencies (8 Hz to 11 Hz),
- the large electric power needed to obtain the required flow for the maximum velocity.
The high displacement requirements and the need to make the tables moveable lead to the optimum solution for the system with single ended actuators mounted in pairs at a 45 degree angle to the principal driving axis (figures # 1 to 2). The second difficulty is solved with the controller and the last by using additional accumulators which allow reaching 2 m/s during 5 seconds and hydraulic pumps (3500 KVA) which give a continuous flow rate (7000 l/min) for sine sweep and random excitation (figure # 3).
Figure # 1 – View of one of the 6 DDL shaking table
Figure # 2 – View of different configurations
Figure # 3 – View of the hydraulic pack
The limit performances for horizontal and vertical axis of the project are shown in figure # 4.
Figure #4 - Maximum Horizontal and Vertical performances of the two shaking tables
A view of the new project is shown in figure #5. A modular reaction wall (10 m x 30 m+10m x 15) and a strong floor (15m X 35 m) are added to the shaking tables to perform dynamic tests and hybrid tests using additional actuators which will represent non tested part of mockup. Just the part of interest of the mock up will be built and the other part will be modeled in a computer. This part will be take into account during the test and the effect will be applied to the tested part by additional actuators in real time.
Figure #6 – Global view of the new project E-FAST
CONCLUSIONS
This paper presents the main fields of investigation for the next decades about a new testing facility with large performances and a high flexibility (hybrid tests and distributed tests) to investigate in new domains and to eliminate problems met by the past on shaking tables. The studies performed show that the required performances are feasible (2 moveable shaking tables, ± 1 m, ± 2m/s, 200 tons). The new testing facility (E-FAST) is expected to become a powerful tool for European and international collaboration in seismic engineering research (distributed testing, exchange of data and development of new testing concepts).
