Evidence-based diagnosis of pathologies in vernacular timber covered bridges:the Xin An Bridge case

Vernacular timber covered bridges are vulnerable to coupled structural and environmental deterioration, yet diagnosis often relies on empirical visual judgement. This study develops an evidence-based diagnostic workflow for timber heritage and applies it to Xin An Bridge in Liuyang, China. Historical archives, field inspection, terrestrial laser scanning, UAV photogrammetry, settlement monitoring, moisture-content spot checks and environmental investigation were integrated to assess structural inclination and localized dampness. Point-cloud deformation, simplified structural sensitivity analysis and settlement statistics indicate that inclination is more plausibly associated with localized differential settlement and asymmetric loading than with timber ageing alone. Surface moisture measurements did not indicate persistent high-risk moisture conditions within the selected seasonal windows; visible mildew was more plausibly related to roof microenvironmental water retention caused by leaf accumulation. The workflow supports traceable diagnosis, minimal intervention and continued monitoring for comparable timber heritage.