Dynamic Interaction Between the Shaking Table and the Specimen During Seismic Tests

Abstract

In shaking table studies considerable attention is paid to the design of the foundation and the anchorage of the specimen. All analyses consider completely rigid shaking table (actuators and platform). However, since long interaction between the shaking table and massive specimens has been clearly observed, namely a decrease in frequency of the specimen “on the shaking table” with respect to that “on a rigid base”. This paper investigates the validity domain of the rigid “Azalée” shaking table at the CEA (Saclay) laboratory. It demonstrates that, for this large shaking table, most of the interaction is due to the platform deformation during the test. Two charts giving the reduction in frequency as a function of specimen parameters are obtained using a simplified specimen model for lateral and vertical modes, without local deformation between the specimen and the platform. They are validated in three large experimental tests performed in the past 10 years, within the CAMUS, ECOLEADER and SMART projects. Using them it is easy to estimate the minimum decrease in frequency of a specimen fixed on the platform. If this first estimation shows significant interaction, a more detailed study is needed. A simplified FE model of the platform is available for a definite evaluation of the interaction. A second evaluation takes into account the local deformations between the specimen and the platform. For large specimens these local deformations can reduce the frequency by almost 50%. Evaluations of the interaction were made after past Azalée shaking table tests, using simplified models. This paper will help experimental and numerical researchers to better take care, during the design of the test, of the boundary conditions between the platform of the shaking table and the specimen. More detailed comparisons to numerical analyses of large shaking table tests will now be possible for the Azalée table.