The dynamic identification process allows to obtain a characterization of the structure of a building. The possibilities for using the data resulting from this survey are manifold. More frequently, this procedure allows to estimate or integrate information included in a numerical calculation model of the structure.
ISAAC conducted adynamic identification as part of the evaluation of the effectiveness of its own active control systems on an existing fifteen-storey prefabricated reinforced concrete building. In order to avoid disturbance, even if minimal to the occupants of the building, the possibility of adopting a vibrodine or a dynamometric hammer which usually provide the input energy to perform a EMA (Experimental Modal Analysis). The choice necessarily fell on the environmental excitement (wind, traffic, footsteps of people, etc.), and therefore on a OMA (Operational Modal Analysis).
A total of 14 very high-precision accelerometers were placed: eight on the top of the building, four in the intermediate floors, and two at the base (Figures 1 - 2).
Figure 1) Top accelerometers; 2) Detail of the fastening of the accelerometers at the top
The accelerometers were connected to an acquisition system and to amplifiers by means of long range cables (Figures below).
Figure a) Acquisition system and amplifiers; b) Sensor and long range connection cables
As detailed in the article Dynamic identification of structures,'OMA it involves a lasting acquisition, which in the specific case of this building lasted for about six days. In particular, the "root of the mean of the squares" (o Root Mean Square, RMS) of the acceleration signals in 10 min intervals. The trend of the RMS data shows the increase and decrease in vehicular traffic in the urban area where the building is located between morning and evening; it also made it possible to identify two periods in which the acceleration peaks were higher, and which are highlighted with the red squares of the Figure 3.
Figure 3: a) RMS of the accelerations recorded by a sensor at the top every 10 minutes; b) Maximum accelerations in 10 minutes recorded by the same sensor
An investigation ex post made it possible to correlate these high levels of acceleration to a construction site that took place in a housing unit on the eleventh floor. These are local stresses, perceived only by some accelerometers which, as detailed in this article, risk invalidating an entire data set. For this reason the subsequent analyzes were conducted neglecting these acceleration intervals.
The power of the sampled signal in the frequency domain has been checked (function called «Power Spectral Density»). The area of interest for the modal identification of the system and the seismic analysis of the building is below 10 Hz. From the detailed analysis of the Power Spectral Density, resonance peaks just below 2 Hz are clearly identified and a natural frequency above (Figure 4).
Figure 4: Power Spectral Density and its detail for the low frequencies
The data processing that followed made it possible to identify in detail these values and other data relating to the dynamic model of the building, which can be summarized in the frequencies of the modes and in the modal deformations, together with the damping value for each individual mode (Figure 5).
Figure 5: a) Frequency and damping summary for the first 4 modes; Modal deformations for: b) first way c) second way d) third way e) fourth way
This information has proved to be extremely useful to know the conditions of the structure in its current state, for example to capture any large cracks that reduce the initial stiffness. Secondly, they made it possible to calibrate some parameters of the linear elastic model, such as the stiffness of some one-dimensional fictitious elements that model the out-of-plane stiffness of the floors which, for reasons of model size, have not been modeled with elements two-dimensional finished products.
This is just an example of how a dynamic identification can lead to a greater accuracy of the results deriving from numerical calculation models of structures for the assessment of seismic vulnerability before and after the application of vibration control systems, such as active ones. provided by ISAAC.
Author: Giorgio D'Agostino
