Hello everyone. My name is Thierry Chaudat and I am working as tests engineer in CEA Paris-Saclay at Seismic Mechanical Studies Laboratory. The issue of my presentation concerns lab experiments that are essential to study and understand physical phenomena in various scientific domains, as for seismic one. In association with numerical simulations, seismic tests allow to better understand and improve the behavior of structures and equipments. Among the experimental devices available for earthquake engineering, shaking table is one of the most important and the only one able to reproduce seismic excitations representative of a real event in terms of frequency content, and dynamic components: acceleration, velocity, displacement, and force. Among the most significant shaking tables, CEA Paris-Saclay center operates since 1991, the largest one in France. "Azalée", this is its name, allows seismic tests to be carried out in all three directions of excitation, horizontal and vertical, on structures of significant mass (up to 100 tons) and size (six meters large and 7-8 meters high). This table was designed in order to apply high acceleration level, necessary to study the nonlinear or ultimate behavior of structures. What is a shaking table? Even if various technical solutions can be found in the design of a shaking table, the main components are the same for all of them. First is the plate on which the specimen to be tested is anchored, mainly using screws or threaded rods. Anchorages must be correctly designed in order to ensure the transmission in the input signals without any modification to the base of the specimen. Plates can be realized in mental (steel, aluminum) or concrete. The one of Azalée is in aluminum alloy and weighs 25 tons. Second are the hydraulic actuators which impose the input signals to the table. The number, location, configuration, and dynamic performances of actuators depend on each shaking table. Eight actuators are connected to the plate of Azalée. Each actuator has a maximum dynamic force of 800 kilonewtons and a displacement amplitude of 250 millimeters. The third important and indispensable part of an experimental seismic facility is the hydraulic pool. Oil under pressure is needed to create the dynamic force necessary to perform the test. Six electrical hydraulic pressure units can be operated at CEA to perform the test, 210 bars oil pressure, around 2,000 liters per minutes maximum oil flow, 900 kilowatts electrical power, and a total volume of 9,000 liters of oil: that are the main technical characteristics. A floating reaction mass is also needed to isolate the shaking table from its environment in order to avoid vibration nuisance around. Finally, the control room contains all the equipments necessary for the general operations of the test facility and for carrying out the tests. Now, you better know how a seismic test device is done. Let me talk about the test itself and the main steps needed to prepare and realize it. As you certainly understand, a long period of time is often needed to prepare an experimental program, particularly when studying the behavior of structures. It's not so rare that two or three years are needed before performing the first test. Well define the objectives of the test is crucial. Design and manufacturing of the specimen, measuring needs, choice of input signals, and content of the test program is directly linked to those objectives. Once there are defined a preliminary phase is necessary in order to finalize the design and the drawings of the structure to be tested. Numerical simulations are of great help during this phase. Modal analysis is especially often realized to obtain natural frequencies, mode shapes, and damping ratios of the structure. The design phase being completed, the construction of the structure begins. As structures are often huge and heavy, they are realized in the testing laboratory. It can be done on the shaking table for steel or wooden structure, or outside the table for concrete structure. One of the main expectation is quality of realization of the structure: dimensions, reinforcements assembly, material properties. in order to respect the technical specifications coming from the design study. A daily and precise monitoring of production is necessary. Pictures presented concern a specimen constructed at CEA years ago: the SMART program. Three months were necessary to complete the structure. The final weight was around 20 tons. Now, the specimen is ready. Using the crane of the lab and dedicated handling device, the specimen is slowly lifted from the construction area and transferred to the shaking table. In order to ensure an optimum contact between the footing of the structure and the plate of the table, a mortar is poured before the structure is installed. Thirty four anchoring points will create during the test the better boundary conditions in order to avoid any sliding or uplift of the structure. This point is essential to submit the base of the structures (the walls) to the desired input signals. The instrumentation phase can now begin. Numerous transducers will be located on the structure to capture the global and local behavior of the specimen. Acceleration, velocity, displacement, rotation, strain on concrete or steel bars are mainly monitored during the seismic tests. A minimum of 100 channels is often installed on the specimen. In the given example, 250 channels were used during the test. Around three weeks were necessary to correctly prepare and verify the instrumentation set-up before performing the first test. After months of preparation, tests sequence can now begin. The test strategy often used consists in applying seismic input signals from low-level (elastic domain of the structure) up to high level in order to gradually damage the structure. On such concrete structures,10 to 15 seismic tests can be performed. As the evolution of natural frequency is one of the global indicator of damaging, those frequencies are measured before the first seismic test and between each test of the sequence. Cracking patterns are monitored and checked during the tests. Main cracks are identified between each test. If the total collapse of the structure is most of the time not desirable for security reason, severe damage are expected to appear in order to study the non-linear behavior of the structure. Now, months will then be necessary to analyze those results, in particular, through numerical simulations, to reach the objectives of the scientific program: assess the actual seismic response of structures from shaking table experiments.
