Measurement and Monitoring of Structural Changes in a Car park by Natural harmonics
Low Sensor Count Autonomous Wireless Structural Health Monitoring System
During routine control on corrosion and fire safety at a basement garage comprising 180 parking positions the prop head had been tested via magnetic induction and pulsed radar techniques.
During this examination it had been found that significant parts of the punching shear reinforcement had been missing on the prop heads. In this area systematic cracks had been observed on the ceiling which is overlaid by a street and further parking lots in the outside. Those cracks indicated an overload in the ceiling.
Structural Health Monitoring (SHM) installation technologies
In the last decade Wireless Sensor Networks (WSN) have been indicated as the most promising candidate to replace the existing Structural Health Monitoring (SHM) installation technologies that are based on wired sensors. Although this plausible indication moves from indubitable considerations about the installation cost of sensor cables, the commercial diffusion of WSN is at present growing only for meter-reading applications (“smart metering”) and with a general market forecastwell below that of other contemporary technologies such as wireless tags (RFID).
Some of the limitations generically affecting the diffusion of WSN are well known, such as the battery consumption associated with frequent or long range sensor interrogation required by most larger potentia ldeployments. Specifically considering the SHM applications, having common installation cases with arelatively small number of sensors introduces additional economic implications that severely affect the cost effectiveness of most commercially available WSN solutions.
Wireless Autonomous SHM devices
The SHM application as discussed below did require a number of sensors much smaller than that typical of large infrastructures. Most of the known WSN platforms, both proposed as proof-of-concept or as commercial products, addresses high sensor count applications and feature a centralized data acquisition and communication unit whose higher cost is acceptable only when scattered on a large number of peripheral sensor nodes . In response to the shortcomings of the existing SHM solutions based on WSN, a Wireless Autonomous self contained SHM (WASHM) device, also known as the “Sensobrick®” platform, has been deployed especially addressing the low sensor count applications.
The “Sensobrick®” WASHM platform
The “Sensobrick®” device embeds an extensive set of self-referenced sensors, features a multi year battery lifetime and communicates using the GSM/GPRS mobile phone network, thus eliminating the need for cables and of any other installation cost.
Sensors embedded in the device typically include environmental and substrate temperature, high-stability inclinometers on both pitch and roll axes and a tri-axial acceleration sensor. Pre-conditioned inputs for additional specific sensors are provided as well, such as direct inputs for strain gauges, crack and displacement gauges, load cells, relative humidity, wind and rainfall sensors, LASER displacement gauges, etc.
The data collected, any alarms eventually triggered and selftest information are automatically delivered to a number of recipients through SMS messages, email, and FTP file upload.
Justification of SHM
It was not clear if these cracks are static or if they are progressing. Static planning did suggest a cost intense rework of the ceiling using carbon-based reinforcing materials. Costs for this had been estimatedto reach 1M €. Civil structure consultant agency Gb&T suggested long term monitoring in order to quantify crack propagation as a function of load and environmental condition
Implementation, Analysis and Interpretation of the Data
Several potentiometric crack gages had been mounted to monitor crack propagation as to Figure 1
The individual crack gages had been linked to Sensobrick mounted on the ceiling at a central position of the basement parking garage as to Figure 2.
An external antenna to enable GPRS communication has been connected to the unit for data transmission. Temperature and 3-axis accelerometers data had been recorded over time.
The data as acquired up to recently show small changes on the cracks at times when there had been vibrations recorded in parallel. The crack movements had been completely reversible. No Influence in long term temperature drift on the individual crack opening has been recorded.
First measurement results suggest that the cracks on the ceiling are caused by a one time heavy load. The cracks do not show any progressive behavior. In this circumstances a significant reinforcement of the ceiling would not be indicated.
The monitoring shall be continued for another 2-3 years in order to come to a final conclusion. Additional force load cells shall be mounted when the punching shear reinforcement is added to the prop heads. Those load cells shall be connected to the Sensobrick in order to monitor strain on the armoring.
"SENSOBRICK delivers reliable and accurate data and completes our product range for low sensor count sensing applications. The turnkey, autonomous architecture allows us to respond unprecedentedly quick to our customers on their structural health monitoring requirements with cost transparent offerings.", Dipl. Ing. M. Eng. Martin Gruber, gb&t Gebäudebestand & Technik GmbH.
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 Aktan, A.E., Catba, F.N., Grimmelsman, K.A., and Pervizpour, M. 2002. Development of a model health monitoring guide for major bridges, Report No. DTFH61-01-P-00347, Drexel Intelligent Infrastructure and Transportation Safety Institute, USA.
 Bastianini F., Sedigh S., and Harms T. 2009. Autonomous structural health monitoring with the SmartBrick platform. Proceedings of the 12th International IEEE Conference on Intelligent Transportation Systems, St. Louis, MO, USA.
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