Structural Redundancy Degradation and Critical Intervention Threshold: Conservation Decision-Making for the Nationally Protected Shunde Bridge

Abstract

In traditional heritage conservation, the principle of “minimum intervention” often ends up being applied subjectively because there are no clear quantitative benchmarks. This study introduces a “structural redundancy degradation model” along with a “critical intervention threshold” theory. Together they shift safety assessments of historic timber buildings from qualitative descriptions to quantitative evaluations based on material decay, accumulated deformation, and leftover load-bearing capacity. Using Shunde Bridge—a nationally protected heritage site—as an example, we systematically assessed the loss of redundancy in each component of its wooden arch frame. We used 3D laser scanning, resistograph testing, and manual surveys. The results show that: inside the three-segment rafter A-2, 49% of the cross-section has failed, and its load-bearing redundancy is close to zero; component A-5 has completely lost function; termite damage at the end of the bull‑head timber exceeds one quarter of the cross‑section, causing local redundancy loss; the overall tilt of the bridge’s covered corridor (110 mm) has seriously reduced the system’s structural redundancy. Based on this theory, we determined that Shunde Bridge has already passed the “local repair threshold” and reached the “system‑level dismantling threshold.” This gives a quantifiable, evidence‑based reason for deciding to fully dismantle and carry out a major repair. Our work shows that the redundancy degradation model can fill the practical gap left by the minimum intervention principle in cases of extreme deterioration. It also offers a scientific framework for deciding when and how much to intervene in similar wooden heritage structures.