PSI - Issue 64

Alex Carpenter et al. / Procedia Structural Integrity 64 (2024) 319–326 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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structure while continuing to target areas of structural concern. Phase 3 targets areas identified by the preceding phases to best utilise the budget for long-term monitoring.

Table 2. Sensor details for the Cathedral of the Immaculate Conception Sensor Sensor Type Phase Location

Range 10mm 10mm

Resolution

Wired or Bluetooth

C1 C2

Crack metre Crack metre

1 1

Northeastern tower pinnacle Base of the spire central timber mast Mid-point of spire central timber mast Base of spire central timber mast Southeastern tower pinnacle Interior bell tower masonry wall

1μm 1μm

Wired Wired

T1

Tilt metre

1

360 arc degrees

0.001 arc degrees Bluetooth

T2 C3 T3 T4

Tilt metre Crack metre Tilt metre Tilt metre

1 2 2 2 2

360 arc degrees

0.001 arc degrees Bluetooth

10mm

1μm

Wired

360 arc degrees 360 arc degrees -40ºC to 50ºC 1%RH to 99%RH 0km/h to 160 km/h -40ºC to 60ºC 1%RH to 99%RH

0.001 arc degrees Bluetooth 0.001 arc degrees Wired

Interior front façade wall Spire central timber mast

TH1

Temperature & humidity sensor Weather station (temp., humidity, anemometer)

0.1ºC 1%RH 0.1km/h 0.1ºC 1%RH

Wired

WS1

2

Atop a neighbouring building (rectory)

Bluetooth

S1-S8 Bluetooth Phase 1 installations took place in December 2023 and January 2024 to provide a warning system of the structural movements before the stabilisation measures in May 2025. The selection of crack metres to monitor cracks developing on the bell tower’s northeastern pinnacle and a split on the spire’s timber central mast was based on their reported benefits for long-term monitoring of developing cracks noted in Section 2 , with displacement sensors having the highest reported use (18 sites) for long-term monitoring. While Table 1 notes the necessity of temperature data for crack metres, a separate temperature sensor was not installed as the crack metres also report ambient temperature. Preliminary data obtained from crack metre C1 showed a widening trend, confirming the necessity of the May 2024 construction works to secure the northeastern pinnacle. Two tilt metres (T1 and T2) were installed at the base and the midpoint of the spire’s central timber mast. As Table 1 illustrates, these sensors are beneficial for long-term monitoring, with T1 and T2 providing data on the global response of the spire. Based on preliminary data obtained from the Phase 1, previous site surveys, and the review in Section 2 , Phase 2 implemented 13 sensors. One crack metre was installed to monitor the exis ting crack on the bell tower’s southeastern pinnacle, chosen for the same reasons as C1. Preliminary data from T1 and T2 reported a bending of the spire’s central mast and necessitated the installation of a tilt metre on the bell tower’s masonry wall to determine if the spire’s tilt is occurring independently of the stone masonry. Another tilt metre was mounted on the exterior attic wall above the rose window to provide a better understanding of the movements of the cathedral’s front façade, allowing for comparisons between movements in the tower and the nave. As indicated by the information displayed in Table 1, all sites, except those exclusively performing AVT, implemented temperature sensors. Humidity sensors were also prevalent, present in 13 sites. These sensors are beneficial for understanding the environmental conditions, baseline corrections, and regression analysis. Therefore, a temperature and humidity sensor was installed on the spire’s central timber mast in Phase 2 to verify the temperature readings built into the tilt and crack metres and to provide relative humidity measurements. Table 1 shows a prevalent use of anemometers to measure wind data during long-term monitoring, noting a necessity for the analysis of inclinometer data. This project uses a weather station that functions as an ambient temperature sensor, relative humidity sensor, and an anemometer to allow for regression analysis and damage detection of the tilt and crack metre data. It is located atop a neighbouring building owned by the cathedral for ease of access and to minimise interference with the wind currents. Included in the sensor selection, Phases 1 and 2 considered the costs, limitations, and benefits of hardwired vs wireless sensors. As installations, battery changes, etc. on the exterior of the tower require rope access and the Bluetooth signal cannot penetrate the copper roofing, exterior sensors have been hardwired to the data acquisition system (DAQ). The interior of the tower is easier to access and has minimal light, so the tilt metres and strain gauges were wirelessly connected to the DAQ to reduce costs and minimise tripping hazards. The attic tilt metre (T4) was hardwired to the DAQ as the Bluetooth signal can penetrate neither the stone masonry nor the copper roof. Strain gauge 2 Steel tie-rods inside the bell tower 3000με 1με