Wireless Structural Monitoring of the Cathedral of L'Aquila Based on Remote Reading
"The PC hosts the management software we developed in LabVIEW. With the software, we can set the parameters and acquisition rate, to read and process raw data, view temporal trends in different graphical modes, store the generated files locally, communicate with appropriate authentication logic, and remotely upload data to another PC."
- G. Sforza,
Creating an integrated hardware and software remote diagnostic system to help rehabilitate and partially reconstruct the Cathedral of L'Aquila, which was seriously damaged by a devastating earthquake in April 2009.
Using National Instruments wireless sensor network (WSN) modules and wired sensors to measure structural variables and wirelessly communicate data to a central hub so a user with remote access can take appropriate corrective action during the reconstruction process.
G. Sforza - Essebi
The Cathedral of L'Aquila, built in the 13th century and dedicated to St. Massimo, has suffered from numerous disasters through the centuries, which were regularly followed by restoration. The neoclassical façade is characterized by two massive bell towers on each side and an entrance doorway surmounted by a triangular pediment supported by four columns. The part of the building that faces the side street retains traits of the original building, such as the ogive windows. The interior consists of a nave, whose ends, due to the dome of the main altar, almost completely collapsed during the April 2009 earthquake. This disastrous event spared stumps of the pillars supporting the dome, portions of the perimeter masonry walls, and a large part of the ogive apse.
We implemented provisional security interventions, but due to their inherent instability we decided to implement structural static monitoring to assess changes in the most significant physical variables during the renovation and restoration of the cathedral. Our goal is to protect and preserve the remaining parts of the structure, as well as facilitate future reconstruction so visitors can fully enjoy the monument.
We needed to monitor portions of the cathedral that are structurally isolated from each other because of the collapse and are often outdoors because of the complete lack of coverage from the end of the nave to the apse. We adopted a modular solution based on groups of sensors to measure the physical variables of interest. We connected the sensors to WSN nodes that are linked to a centralized gateway. The individual transducers are wired to the nodes, and they wirelessly transmit measurement data to the gateway. This minimizes the use of cables that could be damaged during the restoration, which might disrupt construction activities.
The system includes several 4-channel, 16-bit NI WSN-3202 analog input nodes, located mainly in the most damaged areas. The following sensors are wired to each WSN-3202:
- Direct linear slide potentiometers for lesions and disconnection between masonry walls (linearity = 0.2 percent of the FS ±25 mm)
- Electrolytic inclinometers to apply to masonry walls and arc columns (linearity = 0.5 percent of the FS ± 0.5°)
- Pressure cells at the base of masonry columns (linearity = 0.25 percent of FS of 40 bar).
The fourth channel of each module is connected to a temperature sensor with voltage output (sensitivity of 0.1 °C). This measures primary variables, but it also has the critical task of checking temperature to separate the effects of seasonal and daily temperature masking.
The gateway collects data from distributed NI WSN measurement nodes and is connected via Ethernet to a PC that users can remotely access via a GSM modem. The PC hosts the management software we developed in LabVIEW. With the software, we can set the parameters and acquisition rate, read and process raw data, view temporal trends in different graphical modes, store the generated files locally, communicate (with appropriate authentication logic) and remotely upload data to another PC.
Currently, the connection is point to point and uses a GSM phone call to log in. We plan to add a fast Internet connection to the system for continuous remote management using the dedicated console of the Civil Protection’s Project Direction (in this case, as part of the general monitoring of all the ruined monuments of L'Aquila) with access to the dedicated Web page and the ability to activate alert thresholds on trigger events.
The architecture of the monitoring system is modular and inherently flexible so we can significantly alter the system as new requirements arise. We implemented the basic system to meet provisional maintenance procedures. After we define the project for the consolidation and partial reconstruction, especially regarding the intervention modes, we may need to change the location and type of sensors. For example, the contrast slider potentiometers applied to the lesions in the middle of the concrete beams of the roof should be removed if the beams are replaced with another structural system. We can connect the channels left free by the related nodes to other sensors, whose type and location we can assess if necessary (depending on the intervention). We can make adjustments to meet operational requirements and future structural improvements. Once the interventions are complete, the monuments located in an area with high seismic vulnerability will be safely and effectively available.
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