This paper presents experiences and lessons from the structural health monitoring practice on the Tamar Bridge in Plymouth, UK, a 335m span suspension bridge opened in 1961. After 40 years of operations the bridge was strengthened and widened in 2001 to meet a European Union requirement to carry heavy goods vehicles up to 40 tonnes weight, a process in which additional stay cables and cantilever decks were added and the composite deck was replaced with a lightweight orthotropic steel deck. At that time a structural monitoring system comprising wind, temperature, cable tension and deck level sensors was installed to monitor the bridge behaviour during and after the upgrading. In 2006 and 2009 respectively, a dynamic response monitoring system with real time modal parameter identification and a three-dimensional total positioning system were added to provide a more complete picture of the bridge behavior, and in 2006 a one day ambient vibration survey of the bridge was carried out to characterize low frequency vibration modes of the suspended structure. Practical aspects of the instrumentation and data processing & management are discussed and some key response observations are presented. The bridge is a surprisingly complex structure with a number of inter-linked load-response mechanisms evident, all of which have to be characterized as part of a long term structural health monitoring exercise. Structural temperature leading to thermal expansion of the deck, main cables and additional stays is a major factor on global deformation, while vehicle loading and wind are apparently secondary factors. Dynamic response levels and modal parameters show apparently complex relationships among themselves and with the quasi-static load and response. As well as the challenges of fusing and managing data from three distinct but parallel monitoring systems, there is a significant challenge in interpreting the load and response data firstly to diagnose the normal service behavior and secondly to identify performance anomalies.

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A 269 tonne trailer travelled across the Tamar Suspension Bridge in October 2010, and the authors monitored the response of the structure to the load. The following investigation documents the deflection of towers and the deck during the vehicle's passage, as well as the change in cable tensions. This was achieved by studying monitored data from the bridge collected by accelerometers and strain gauges attached to the stay cables, as well as two Robotic Total Stations (RTS) that measured the deflection of the mid-span and the sway of the tower saddle. These results were subsequently compared to the response predicted by a Finite Element (FE) model of the bridge, indicating an accurate match. The FE model was also used to simulate the variation of the dynamic response of the structure, which suggests the natural frequencies vary depending on the vehicle's location to each mode shape's anti-nodes.

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