Structural pathology of embedded timber tension grids in baroque vaulted corridors: Photo-based diagnosis and reinforced-concrete replacement at Vienna’s Campus Academy

Embedded timber elements in historic masonry buildings can function as concealed tension members that stabilize vaulted corridors, yet post-completion documentation of their replacement remains largely absent from engineering literature. This case study documents a rare timber-to-reinforced-concrete substitution carried out during the 2020 to 2022 renovation of Vienna’s Campus Academy, after floor demolition exposed an embedded timber grid aligned with corridor structural axes. The investigation employs a post-completion, triangulated evidence approach, combining construction-phase photographic documentation, expert testimony from the project coordination team, architectural drawings, and post-completion site inspection to reconstruct the failure mechanism and the intervention rationale. Visual indicators consistent with moisture-driven timber decay were concentrated at timber-masonry embedment zones, including pronounced discoloration, friable texture, and major cross-section loss, with the most severe damage at exterior-wall interfaces. Interpreted through qualitative load-path assessment, this deterioration was consistent with a loss of the intended tensile function required to counteract vault thrust forces at the corridor level. The implemented intervention replaced the degraded timber grid with a reinforced-concrete diaphragm configured to retain the original force-transfer concept while improving moisture robustness. Approval by the Federal Monuments Office is documented as part of the decision context for this irreversible material substitution. The case provides a transferable, photo-led diagnostic framework and documentation protocol for similar discoveries where quantitative testing and proprietary design data are unavailable, but engineering decisions must still be justified transparently. • Photo-based diagnosis for concealed timber tension grids in vaulted corridors. • Damage concentrates at timber-masonry embedment and exterior-wall interfaces. • Qualitative load-path reading links decay to loss of intended tensile function. • RC diaphragm replacement preserves original force-transfer logic. • Evidence triangulation supports decisions when test data are unavailable.