Analysis of long-term monitoring data to assess the efficacy of tie-bars strengthening interventions on a large historical dome

The dome-drum system of the “Regina Montis Regalis” Basilica of Vicoforte has suffered over the years from significant structural problems, partly due to settlements of the building induced progressively by newly built masses. In 1983, concerns over the severe settlement and cracking phenomena affecting the structure prompted the decision to undertake inspection, monitoring and strengthening interventions. The strengthening intervention, put in place in years 1985–1987, consists of 56 active steel tie-bars, for a total cross-sectional area of 3200 mm2, which are placed within holes drilled in the masonry at the top of the drum along 14 tangents around the perimeter. The bars were slightly tensioned right after their placing. A static monitoring system was set up to measure strains and stresses in the masonry structure, crack propagation, as well as tensile loads in the reinforcing bars. The bars were re-tensioned in 1997 to compensate for stress losses. The monitoring program of the Basilica started in 1983, in order to assess the efficacy of the interventions over time, but only in 2004 the acquisition procedure was automatized. The instruments of the monitoring system can be subdivided in two groups: i) instruments for the measurement of displacements, strains and cracks; ii) instruments for measuring boundary conditions. The first group includes: 12 Linear Variable Displacement Transducers (LVDTs) applied to the main cracks at the impost of the dome and to the biggest west crack at the extrados; 20 pressure cells to determinate the stress variation in the masonry; 56 load cells installed at the ends of the active tie-bars to control their stress condition and 2 wire gauges at the impost of the dome to assess the overall geometry of the structure. The second group of instruments includes: 24 temperature sensors (17 located inside the walls and 8 externally). Data acquired from monitoring systems are inevitably affected by the environmental conditions and many other factors. Therefore, environmental fluctuations and trends have to be separated by variations induced by possible damage in the structure or tension loss in the reinforcement system. To this aim, interpolating functions helped to interpret the data of the monitoring system. A special emphasis has been put on the investigation of the tension loss occurring in the tie-bars strengthening system, as well as on its correlation with the crack openings. The long-term evolution in time of all the monitored parameters, and their correlation analysis, are relevant to a lifecycle assessment referred to the tie-bars strengthening intervention.