Methodologies
Dynamic surveys on structures
One of the key issues related to the stability and durability of modern buildings and such infrastructural works as masterpieces or places of historical interest, is represented by the assessment of the response to dynamic stresses they may be subjected to in the course of their lives as a result of those phenomena that can be continuous or constant over the time (such as the effect of the vehicular traffic, the wind on a bridge or the seismic events). In these cases it is necessary to accurately measure both speeds and accelerations that the structures can hold up without any significant damage. In order to measure the movements produced by dynamic stresses, it is necessary to use real-time sensors, because speed and frequency of the movements are so high to require data acquisition architectures usually made up of accelerometers with characteristics that vary depending on the investigated object and connected to high resolution acquisition systems.
Below the main targets of the dynamic surveys on structures:
- Checking the project’s validity and consequent compliance to it in the realization of the work performed
- Dynamic characterization of infrastructures, buildings, historical monuments affected by temporary or permanent vibrations induced by: urban, rail or any kind of heavy traffic
- Real response of the structures to events of hydrologic or seismic instability.
- Monitoring of damages or breaks of structural elements caused by induced vibrations
- Verification of the work’s durability prevision
Seismic vibration monitoring
The survey methodologies based on passive seismic may be of "on time" type, where the instrument performs a constant recording of the signals coming from the sensor connected to it, or of "trigger" type.
TRIGGER BASED SEISMIC MONITORING
The seismic vibrations monitoring called "trigger", or threshold, is one of the methodologies the non-destructive testing (CND) surveys are made up of and it is widely used within structural monitoring and dynamic surveys on the structures. In the trigger acquisitions such "stand alone" seismographs are used; these are specifically designed for the continuous seismic monitoring, where even if they are constantly listening, they are ready to start recording as soon as an event exceeding the trigger threshold (time 0 , start of recording) set in the instrument at the programming time. The sensors used are usually the tri-axial low frequency 1, 2 Hz type. The use of trigger seismic vibrations allows:
- to assess the consequences of seismic vibrations generated by vehicular traffic or temporary construction sites on buildings, infrastructure and surrounding houses, in order to assess their potential dangerousness on them.
- to plan eventual corrective actions in the traffic circulation, in the works progress modalities, in order to avoid breaks, irreparable structural damages or buildings collapses in case of construction sites.
- to constantly monitor (at low cost) the state of monuments and buildings of particular value
- to allow (at the design beginning of works and infrastructures) avoiding any accident during the work, coming from specific problems of the site that cannot be detected by any other methods of surveys.
TIME BASED SEISMIC MONITORING
This technique is used to obtain information concerning possible dynamic amplification effects of seismic waves in "emersion". It is based on the recording of the background noise in the time domain and following elaboration of the signal’s frequencies in the domain. It is carried out by placing a tridimensional geophone on the ground with low frequency response and by recording the seismic noise in different temporal windows. Later on, the study of the spectrums obtained from the convolution of the frequency of the signal recorded in the domain for the three components of the ground motion and the application of techniques on spectrum analysis, allows to define and measure possible local seismic amplifications and the seismic frequency of the site. The measurements of the microearthquake can also be taken in linear “arrays” for the localization of faults.
Seismic monitoring is carried out in areas subject to risks related to a seismogenic activity, by acquiring the seisms with time and recording the seismograms. Seismic stations are used able to record in threshold or continuous type, and low frequency geophones or seismic accelerometers. The recording in the long period, of the earthquakes relative to a site or fairly large area allows to configure the seismic scenario of an area and evaluate the risk and vulnerability conditions. If the monitoring activity is supported by specific knowledge of geological and geo-technicaltype, we are talking about seismic Micro-zoning.
