How optical fiber sensors are used in the reconstruction of a church destroyed in an earthquake

L'Aquila is an ancient city in central Italy with a rich cultural heritage. Baroque and Renaissance buildings and churches line the streets of the historic city center. Approximately 70,000 people call the town home. And yet the city did not become well-known for its architecture or the landscape of the surrounding Apennine Mountains, but for a catastrophe.

In 2009, an earthquake shook the region and destroyed countless buildings including family homes and historical landmarks. The 18th century church of Santa Maria del Suffragio became the symbol of this earthquake.

Seven years have passed since then. Santa Maria del Suffragio was one of the first landmark buildings to be partially restored. The use of provisional supports allowed its partial opening to the public one year after the tragedy. Full reconstruction of the church started in 2013 and is still ongoing. The earthquake did not only cause the collapse of the dome but also resulted in structural deficiencies in the whole building.

The structure is not only being reconstructed, but also seismically reinforced to guarantee the stability and safety of this important landmark in the years to come. In order to continuously monitor and control the structure’s behavior, the reinforcements are being tested by I.A.T. Ingegneria A&T and Earth System, companies focus on measurements for geotechnical, environmental and structural monitoring. Optical measurement technology by HBM FiberSensing supports monitoring of strain and temperature in the pre-stressed tie rods used for reinforcement (see figure 1 & 2 for an installation plan).

Fiber Bragg Grating (FBG) sensors were glued to the Dywidag 32WR tie rods (Figure 3) to measure strain and temperature at the same time. Each rod connector includes one array consisting of two bare FBGs – one for strain and another one for thermal compensation. When calibrated, the thermal compensation sensors provide absolute temperature values. To make things easier on site, the sensors were designed to be pre-installed at the HBM FiberSensing facilities and then transported to the site.

The use of optical splitters simplified the installation of the system and optimize the interrogator’s capacity. Because the above mentioned arrays of FBGs are terminal (meaning that they are only connected on one side), optical splitters are used to combine the signals from several FBG arrays in one optical channel. The splitters used in this case allow 1x4 or 1x8 multiplexing (see example in Figure 4).

The BraggMETER interrogator acquires all the different optical channels simultaneously at a rate of one sample per second (S/s). The 19’’ rack-mountable device used has eight optical channels. The FS22 Industrial BraggMETERs can be connected to a standard computer by Ethernet cable and controlled by BraggMONITOR software from HBM FiberSensing (Figure 5) as well as through SCPI commands or via catman®.

The fact that the static interrogator can actually operate as a stand-alone device and store data internally came in handy in this application. The interrogator was connected to a 3G router so that no computer was required on site. Data can be accessed remotely in the comfort of the office with the technician’s laptop.