Thermal Effects on Bridges Dynamic Behaviour

A key issue in the field of SHM is the identification of a set of key performance indicators (KPI) able to fully describe a structure behaviour and its health status. Among all available KPIs, modal parameters are often chosen as monitoring indicators, since their variation, if correctly interpreted, can help recognise the presence of damage or the beginning of an incipient degradation process. However, structural variations might not be due solely to damages and attention should be paid to properly classify them. This paper aims at discussing the influence of thermal effects on the dynamics of concrete bridges, as environmental temperature changes tend to induce structural variations that might be somehow misclassified as damages over a short observation period. A considerable dataset of vibration data collected from one-year monitoring of several highway bridges (with different static schemes, cross-sections, span lengths, etc.) is analysed to support this discussion. Data are part of a large industrial SHM framework dedicated to the monitoring of bridges through MEMS-based distributed measurement nodes. All the analysed structures show a modification in their natural frequencies due to a variation of environmental temperature. The vibration modes affected by temperature variations are both flexural and lateral modes, depending on the bridge structural characteristics. By discussing how temperature can influence the dynamic behaviour of bridges, the paper also aims at identifying those mechanisms governing frequency variations related to changes in the environmental conditions and finding possible similarities between structures behaving in the same way. Great attention is also given on how to deal with modal parameters modifications, when the monitored structure is part of a more complex Early Warning System (EWS) and, in general, when reliable thresholds must be set on the Key Performance Indicators chosen to detect the birth and growth of damage.