Photogrammetry and mean intensity mapping as methods of low-cost structural health monitoring

Structural Health Monitoring (SHM) is critical in ensuring the safety and longevity of almost all engineering infrastructure we rely on today. Conventional methods are often costly, requiring specialised equipment and expertise. With continual reductions in computational processing times and component costs, photogrammetry - the process of extracting information from photographs - could offer a promisingly affordable alternative for such analysis. This paper delves into the utilisation of the most well-established techniques in this burgeoning field of research and proposes a novel alternative: Mean Intensity Mapping (MIM). These were experimentally tested and compared to existing (often more expensive) methods of acquiring data for dynamic analysis, such as laser vibrometers and accelerometers, to gauge accuracy, viability, and ease of use. Experimental testing included preliminary studies with varying mass, structural degradation, and tracking frame complexity; followed by in-situ testing at the Clifton Suspension Bridge (CSB). The aim was to validate a low-cost method of extracting modal data from video recordings, such that these methods could be applied to Structural Health Monitoring (SHM). An entry-level Panasonic camcorder (200) pound was used for its high optical zoom; which allowed for frequencies to be detected from a distance using both the conventional tracking algorithms and the novel method (employing brightness-based virtual sensors). These frequencies were verified by comparing them to models and past studies on the CSB. Natural frequencies recorded from the photogrammetric methods under favourable conditions were notably very similar in accuracy to conventional methods (with similar to 0.5% error), and they therefore have many practical applications. This demonstrates that photogrammetry can be performed at a lower-cost to conventional methods, allowing surveyors and engineers to make observations and detections in the space of a few minutes, in a non-contact way, and from a distance. There are still a number of barriers which must be overcome including variations in visibility due to weather, changes in the refractive index of air as a result of wind/temperature, difficulty gathering data at night time or in low light, processing/calculation times, and tripod/mounting instability. However, it is very conceivable that as the cost of optical equipment and processing is decreasing, photogrammetric methods are likely to become an indispensable mechanism in SHM of the future.