Microstrain monitoring method for micro local damage detection in large-scale steel structures

Introduction The large earthquake anticipated to occur in the Nankai Trough in the near future poses a risk of severe damage to infrastructure along the Japanese Pacific coast. In recent years, structural health monitoring (SHM) has been increasingly used to identify building damage, support post-earthquake occupancy decisions, and determine areas requiring repair. However, conventional SHM systems mainly rely on floor acceleration responses, which limits their ability to assess damage in individual structural elements, especially micro local. This study proposes a cost-effective technique for assessing micro local damage in large-scale steel structures using microtremor-induced acceleration and strain measurements. Methods Local structural characteristics were evaluated from measured microstrain and acceleration responses. Micro local damage was identified by comparing changes in these structural characteristics before and after seismic events. The bending-moment distribution of the structure was investigated to observe changes associated with seismic loading. An index termed “local stiffness,” defined as the ratio of bending moment amplitude at a given section to the structural deformation amplitude in a given vibration mode, was introduced to quantify changes in local structural characteristics. In addition, strain data were used to track changes in the neutral-axis position of structural cross-sections and examine its potential for damage identification. The proposed approach was validated through large-scale shaking table tests of a ten-story steel structure at the E-Defense center. Structural characteristics obtained from microtremors were then compared with those from the main earthquake to examine the applicability of the proposed microstrain-based method. Results The microtremor-based results exhibited greater stability than the corresponding main-earthquake-based results under the present experimental conditions. Changes in bending-moment distribution helped identify shifts in beam or column inflection points, providing useful indicators for inferring local damage. Discussion These results demonstrate the feasibility of using microtremor-based strain and acceleration measurements to detect damage-related changes in full-scale steel structures. Although the proposed method is affected by signal quality and measurement uncertainty, the results suggest that it has practical potential as a cost-effective approach for monitoring local damage in steel structures. Overall, the findings confirm that the proposed method is a promising technique for detecting micro local damage in steel structures.