ISAAC’s test on a masonry bell tower: preliminary results
Do you want to find out how the tests conducted by ISAAC and Mapei at the EUCENTRE laboratories went?
Read the article to find out the results that emerge from the preliminary analyses.
The experimental test on a full-scale masonry bell tower
ISAAC conducted several experimental tests on a masonry bell tower, built in full scale specifically for the occasion, with the collaboration of Mapei. A combined solution was found that includes ISAAC’s innovative active mass damper, combined with MAPEI’s advanced earthquake-resistant systems. These two state-of-the-art technologies were applied to the structure of the bell tower, which was placed on a vibrating table and subjected to a series of earthquake shocks of varying intensities.
The tests were carried out at the prestigious EUCENTRE laboratories in Pavia, recognized across Europe as a centre of excellence for earthquake research.
The earthquakes examined, which we chose to reproduce with increasing intensity, were the one in Emilia in 2012 (magnitude 5.9) and the one in Montenegro in 1979 (magnitude 6.9).
The objective of the tests conducted by ISAAC and Mapei
This experiment aimed to demonstrate the effectiveness of the solutions proposed for protecting historical structures from earthquake damage, thus helping to preserve cultural heritage with state-of-the-art technologies.
The tests just carried out are in fact the starting point of a way to protect our past and build a safer future.
The preliminary results of the tests
Following the experiments in question, we can appreciate the preliminary results of the tests.
How is ISAAC’s active mass damper an improvement?
Reducing the displacement of the structure
First of all, the action of the AMD allows for significantly reducing the displacements of the bell tower structure while maintaining its response in the linear field. As can be seen from the comparison graphs (below), it is possible to observe a significant reduction in the displacement of the bell tower. In the two earthquakes given as examples below, you can notice a reduction in maximum displacements at the peak of 70% for the Emilia earthquake at 50% intensity and at the peak of 60% for the Montenegro earthquake at 100% intensity.
Graph comparing displacement of the structure during the 2012 Emilia earthquake simulation testing, at 50% of its intensity, with the AMD on (orange line) and with the AMD off (blue line).
Graph comparing displacement of the structure during the 1979 Montenegro earthquake simulation testing, at 100% of its intensity, with the AMD on (orange line) and with the AMD off (blue line).
The phenomenon of “rocking”
A parameter that is typically used to make a quick assessment of the damaged condition of a structure is the resonant frequency of the structure itself, called natural frequency. Natural frequency depends on the stiffness of structural elements and lowers when they degrade.
During the tests, the natural frequency of the bell tower was measured following each earthquake by imposing a random excitation (white noise) on the vibrating table and measuring its main response frequency. A slight reduction was observed in this value, more accentuated during the tests with the AMD off, which brought the structure from a starting value of 4 Hz to a value of 3.3 Hz at the end of the test phase.
Although useful, this data is insufficient to characterize the behaviour of a masonry structure during an earthquake. The limit of this type of analysis is observed when non-linear phenomena, such as “rocking”, intervene on the structure, which only occur for earthquakes that exceed a certain intensity.
The phenomenon of “rocking” occurs when the structure “detaches” from the support surface and behaves like a rigid body. The AMD eliminates or significantly limits the occurrence of the phenomenon of “rocking” of a structure. The “rocking” phenomenon that occurred during the tests means that during the earthquake the structure’s response frequency is lower than the one identified for small stresses during dynamic identification with white noise.
Graph comparing the displacement spectrum of the structure during the 2012 Emilia earthquake simulation testing, at 50% of its intensity, with the AMD on (orange line) and with the AMD off (blue line).
While for the uncontrolled structure, the significant onset of rocking occurs for earthquake intensity from 40% or higher with the AMD, it is observed only from 70% intensity (as demonstrated by the comparison at 50% intensity where the onset of the phenomenon is not observed with the AMD on, while low-frequency oscillations are observed with the AMD off).
For higher earthquake intensity levels (e.g., at 100% of the Emilia earthquake, shown below). The rocking phenomenon also occurs with the AMD on, albeit with a lower amplitude and above all for a much shorter length of time than with the AMD off, as it is damped after just a couple of oscillations, returning the structure to the linear field well before what naturally occurs with the AMD off (comparison in the figure below).
Graph comparing the displacement of the structure during the 2012 Emilia earthquake simulation testing, at 100% of its intensity, with the AMD on (orange line) and with the AMD off (blue line).
Conclusions
Recognizing the importance of protecting bell towers and other historic masonry structures, symbols of culture, and aware of the potential positive impact of our technology, ISAAC’s commitment goes further, aiming at spreading a cutting-edge culture for preventing earthquake damage.
ISAAC’s research is not only a simple search for innovative ways to mitigate earthquake risk for buildings and structures of historical and cultural value, but a real step forward in the direction of the prevention of earthquake damage.
