Structural monitoring

The time evolution monitoring of the cracking and deformation phenomena of existing buildings is essential for the assessment of structural safety because, due to their considerable hyperstaticity, masonry buildings often respond to stress and to imposed distortions by deformation and cracks that, where the trend does not increase over time, are not necessarily indications of incipient structural problems. For a proper evaluation of structural reliability, it is necessary to identify which movements still increase and which have instead reached a stationary state: this requires precise detection of the correlation between the magnitude of the events that cause deformations and cracking phenomena and the magnitudes of their effects.

The idea of monitoring crack patterns and structural movements is not a new one: in the past, the most important monuments have been monitored by installing probes made of plaster through the cracks or placing long pendulums inside the towers and the bell towers, whose measurements were taken periodically. Current developments in instrumentation allow to monitor the deformation and cracking phenomena in a discrete manner, but with time continuity suitable to detect the fundamental period time of the movements to be monitored.

A structural monitoring system can be implemented by the use of automatic or removable equipment, related to the importance and to the velocity of the evolution of the movement to be detected.

In the case of automatic equipment, a number of fixed sensors are installed in order to convert the variation of a physical or mechanical quantity (temperature, distance, inclination) into an electrical signal that can be sent to a control unit (data logger) and can be subsequently available for storage or further processing.

In the case of removable instrumentation, only the measuring bases are fixed on the structure; the operator takes data connecting the mechanical or electrical instruments to these bases according to a scheduled program.

These two different settings of the monitoring systems have different purposes and costs, but they are not always alternatives: for example, it is clear that the removable instrumentation cannot be used to perform measurements of vibrational phenomena, as well as measurements to be carried out in almost inaccessible positions such as the arch key of a vault. An automatic monitoring system can also be connected to a remote processing unit to make on-line data acquisition available. However, especially in the case of extreme operating or environmental conditions, it may be still essential to use removable instrumentation.

The measurement of static quantities, which are those generally recognized in the case of structural monitoring, also involves the measurement of temperature variations. These are usually the main cause of movements of the cracks and, together with the variation of irradiation cause the movements of inclination of slender structures and displacements that can also be of a magnitude order greater than those caused by structural displacements.

The availability of data series stored with continuity for a sufficient period of time (at least 5 years), make it possible to carry out statistical processing, to identify the periodicity and the amplitude of the oscillations (thus discriminating against the effect of any periodic phenomena such as temperature or irradiation variations) and finally to identify any trend of the evolution of cracking and deformational phenomenon.

Dynamic monitoring has the purpose of detecting the time-history of mechanical vibrations. This is generally carried out with two goals: to identify the structures that are subject to natural vibrations (e.g. wind, traffic, bell swinging ringing) or vibration produced by a specific trigger or to check the effects of the vibrations on the structures. In the second case it is usually necessary to define suitable vibration levels (in terms of frequency and velocity / acceleration) to exclude the occurring of damage caused by vibration on the structure.