Vol.:(0123456789) Journal of Civil Structural Health Monitoring https://doi.org/10.1007/s13349-019-00373-4 123 ORIGINAL PAPER Critical measurement issues in the use of wire potentiometers for the structural health monitoring of slender structures: the case of the Duomo di Milano main spire A. Cigada 1  · M. Scaccabarozzi 1  · E. Zappa 1  · Benigno Mörlin Visconti Castiglione 2 Received: 29 May 2019 / Revised: 8 October 2019 / Accepted: 21 December 2019 © Springer-Verlag GmbH Germany, part of Springer Nature 2020 Abstract During the restoration of the Duomo di Milano main spire, a monitoring system has been set up in order to have an early detection of potential risks of the spire. A scafolding for the restoration activities was mounted around the spire, and it had a section exposed to the wind actions much wider than the spire itself. Any possible contact between the spire and the scaf- folding had to be prevented, even under the strongest winds expected in Milan, since the stability of the extremely slender marble spire could be compromised in case of contact. A safety gap between the two substructures was granted during the construction phase; however, the defection of the spire could not be reliably predicted by numerical models. Therefore, it was decided to continuously monitor the actual gap value, together with other measurements, helping in a thorough under- standing of the spire and scafolding movements. The gap monitoring was measured by three-wire potentiometers, mounted to measure the relative position between the scafolding and the spire in the section where the gap was the narrowest: the top balcony. This measurement was considered one of the most critical for the safety of the spire; therefore, every choice has been aimed at redundancy and safety. Moreover, any systematic efect has to be compensated for. A main issue related to the use of wire potentiometers is that these sensors estimate the amplitude of the displacement but does not provide any information on its direction. Since the horizontal displacement of slender structures can be in any direction, according to the wind and sun radiation conditions, the output of a wire potentiometer is also afected by displacements in the direction normal to its sensitivity axis, leading to a systematic efect in the measurements (cross-talk efect). This aspect hardly fnds literature attention, but it can play a major role if the safety of the structure is monitored with wire potentiometers. This paper focuses on the estimation and compensation of this cross-talk efect, to provide reliable measurements in the monitoring of slender structures. Numerical and experimental validation of the proposed approach demonstrates the need to compensate for the cross-talk efects for a proper uncertainty estimation and then the validity of the proposed method. Keywords Displacement measurements · Linear wire potentiometer · Structural health monitoring · Orbit measurement · Systematic efect 1 Introduction The need to protect cultural heritage having social or archi- tectural relevance is leading the growing request to develop structural health monitoring systems for ancient buildings [1, 2]. Moreover, a new approach to maintenance strategies and a thorough control in case of extreme events is also required. In this scenario, the need of accurate and reliable measurement systems is a key point for the development of automatic monitoring systems capable to early detect poten- tial damage of structures. The need of continuous monitor- ing is particularly relevant for historical cathedrals, since this type of structures is often characterized by slender spires or * A. Cigada alfredo.cigada@polimi.it M. Scaccabarozzi matteo.scaccabarozzi@polimi.it E. Zappa emanuele.zappa@polimi.it Benigno Mörlin Visconti Castiglione benigno.morlinvisconti@fastwebnet.it 1 Department of Mechanical Engineering, Politecnico di Milano, Via La Masa 1, 20156 Milan, Italy 2 Architetto del Duomo di Milano, Veneranda Fabbrica del Duomo di Milano, Via dell’Arcivescovado 1, 20121 Milan, Italy