General durability criteria for exposed timber structures based on the analysis and inspection of bridges and observation towers

This article analyses 14 timber bridges and 5 observation towers, all located in Switzerland and constructed between 1990 and 2020. The study aims to organize and formalize a comprehensive set of durability criteria and associated operational sub-criteria to describe the longevity of exposed timber structures. The inspected case studies include a variety of structural systems and durability design approaches. The bridges span from arch to complex truss systems, while the towers range from traditional log-based structures to modern CLT constructions. The investigations included historical research, visual inspections and the cataloguing of construction details most relevant to durability. To improve the understanding of decay mechanisms, a condition index was developed, which considers the presence, extent and timing of decay. Explanatory details and photographs of the case studies are also presented, illustrating the influence of moisture design on observed decay patterns. Based on the results, three general durability criteria for exposed timber structures are proposed: (i) intrinsic element and environmental risks, (ii) water and moisture input, and (iii) long-term quality and care, with related operational sub-criteria. Future research will focus on developing a quantitative framework to assign relative weights to each durability principle, allowing a systematic prioritization based on their actual impact on timber longevity, and on providing a methodological tool to support informed decisions in the evaluation, design, and management of timber structures.